Fat and carbohydrate metabolism during exercise in elderly and young subjects

Exercise Testing for Metabolic Flexibility: Time for Protocol Standardization

Metabolic syndrome (MetS) is a combination of risk factors that contribute to the development of many of today's chronic diseases. Rates of MetS continue to increase and it is now considered a worldwide epidemic. As with many chronic diseases it may take years for symptoms and the effects of MetS to manifest into severe health problems. Therefore, early detection is paramount A recently proposed method for the early detection of MetS is the assessment of an individual's metabolic flexibility during exercise. Metabolic flexibility is defined as the ability of the body to switch between energy substrates, primarily fats and carbohydrates, to produce energy and meet metabolic demand. This provides an indication of mitochondrial health, the possible beginning point of early insulin resistance and the development of MetS.Although there is widespread use of exercise and expired gas analysis to determine metabolic flexibility, there is no consensus on the appropriate guidelines, protocol, or interpretation of the subsequent data. Studies have used a variety of different protocols involving maximal and submaximal tests with step protocols ranging from 2 to 10 min, differences in data averaging, analysis, and stoichiometric equations, as well as variations in nutritional status of participants, and mode of exercise. This has led to considerable variation in reported results. Although the use of exercise to determine metabolic flexibility and act as a possible marker of early mitochondrial dysfunction holds significant promise, more work is required to determine the optimal protocol for clinical and research purposes.

Aging delays the suppression of lipolysis and fatty acid oxidation in the postprandial period

Plasma glycerol and free fatty acid concentrations decrease following oral glucose consumption, but changes in the rate of lipolysis during an oral glucose tolerance test (OGTT) have not been documented in conjunction with changes in fatty acid (FA) oxidation or reesterification rates in healthy individuals. After a 12-h overnight fast, 15 young (21-35 yr; 7 men and 8 women) and 14 older (60-80 yr; 7 men and 7 women) participants had the forearm vein catheterized for primed continuous infusion of [1,1,2,3,3-2H]glycerol. A contralateral hand vein was catheterized for arterialized blood sampling. Indirect calorimetry was performed simultaneously to determine total FA and carbohydrate (CHO) oxidation rates (Rox). Total FA reesterification rates (Rs) were estimated from tracer-measured lipolytic and FA oxidation rates. After a 90-min equilibration period, participants underwent a 120-min, 75-g OGTT. Glycerol rate of appearance (Ra), an index of lipolysis, decreased significantly from baseline 5 min postchallenge in young participants and 30 min in older participants. At 60 min, FA Rox decreased in both groups, but was significantly higher in older participants. Between 5 and 90 min, CHO Rox was significantly lower in older participants. In addition, FA Rs was significantly lower in older participants at 60 and 90 min. The area under the curve (AUC) for FA Rox was greater than that for FA Rs in older, but not in young participants. Our results indicate that, in aging, the postprandial suppression of lipolysis and FA oxidation are delayed such that FA oxidation is favored over CHO oxidation and FA reesterification.NEW & NOTEWORTHY To our knowledge, our investigation is the first to demonstrate changes in lipolysis during an oral glucose tolerance test (OGTT) in healthy young and older individuals. Plasma glycerol and free fatty acid concentrations changed after glycerol rate of appearance (Ra), indicating that plasma concentrations are incomplete surrogates of the lipolytic rate. Moreover, simultaneous determinations of substrate oxidation rates are interpreted to indicate that metabolic inflexibility in aging is characterized by delayed changes in postprandial substrate utilization related to the lipolytic rate.

Metabolic dysfunction and the development of physical frailty: an aging war of attrition

The World Health Organization recently declared 2021-2030 the decade of healthy aging. Such emphasis on healthy aging requires an understanding of the biologic challenges aging populations face. Physical frailty is a syndrome of vulnerability that puts a subset of older adults at high risk for adverse health outcomes including functional and cognitive decline, falls, hospitalization, and mortality. The physiology driving physical frailty is complex with age-related biological changes, dysregulated stress response systems, chronic inflammatory pathway activation, and altered energy metabolism all likely contributing. Indeed, a series of recent studies suggests circulating metabolomic distinctions can be made between frail and non-frail older adults. For example, marked restrictions on glycolytic and mitochondrial energy production have been independently observed in frail older adults and collectively appear to yield a reliance on the highly fatigable ATP-phosphocreatine (PCr) energy system. Further, there is evidence that age-associated impairments in the primary ATP generating systems (glycolysis, TCA cycle, electron transport) yield cumulative deficits and fail to adequately support the ATP-PCr system. This in turn may acutely contribute to several major components of the physical frailty phenotype including muscular fatigue, weakness, slow walking speed and, over time, result in low physical activity and accelerate reductions in lean body mass. This review describes specific age-associated metabolic declines and how they can collectively lead to metabolic inflexibility, ATP-PCr reliance, and the development of physical frailty. Further investigation remains necessary to understand the etiology of age-associated metabolic deficits and develop targeted preventive strategies that maintain robust metabolic health in older adults.

Altered glucose kinetics occurs with aging: a new outlook on metabolic flexibility

Our purpose was to determine how age affects metabolic flexibility and underlying glucose kinetics in healthy young and older adults. Therefore, glucose and lactate tracers along with pulmonary gas exchange data were used to determine glucose kinetics and respiratory exchange ratios [RER = carbon dioxide production (V̇co2)/oxygen consumption (V̇o2)] during a 2-h 75-g oral glucose tolerance test (OGTT). After an 12-h overnight fast, 28 participants, 15 young (21-35 yr; 7 men and 8 women) and 13 older (60-80 yr; 7 men and 6 women), received venous primed-continuous infusions of [6,6-2H]glucose and [3-13C]lactate with a [Formula: see text] bolus. After a 90-min metabolic stabilization and tracer equilibration period, volunteers underwent an OGTT. Arterialized glucose concentrations ([glucose]) started to rise 15 min post glucose consumption, peaked at 60 min, and remained elevated. As assessed by rates of appearance (Ra) and disposal (Rd) and metabolic clearance rate (MCR), glucose kinetics were suppressed in older compared to young individuals. As well, unlike in young individuals, fractional gluconeogenesis (fGNG) remained elevated in the older population after the oral glucose challenge. Finally, there were no differences in 12-h fasting baseline or peak RER values following an oral glucose challenge in older compared to young men and women, making RER an incomplete measure of metabolic flexibility in the volunteers we evaluated. Our study revealed that glucose kinetics are significantly altered in a healthy aged population after a glucose challenge. Furthermore, those physiological deficits are not detected from changes in RER during an OGTT.NEW & NOTEWORTHY To determine metabolic flexibility in response to an OGTT, we studied healthy young and older men and women to determine glucose kinetics and changes in RER. Compared to young subjects, glucose kinetics were suppressed in older healthy individuals during an OGTT. Surprisingly, the age-related changes in glucose flux were not reflected in RER measurements; thus, RER measurements do not give a complete view of metabolic flexibility in healthy individuals.

Association between dietary phosphate intake and skeletal muscle energetics in adults without cardiovascular disease

Highly bioavailable inorganic phosphate (Pi) is present in large quantities in the typical Western diet and represents a large fraction of total phosphate intake. Dietary Pi excess induces exercise intolerance and skeletal muscle mitochondrial dysfunction in normal mice. However, the relevance of this to humans remains unknown. The study was conducted on 13 individuals without a history of cardiopulmonary disease (46% female, 15% Black participants) enrolled in the pilot-phase of the Dallas Heart and Mind Study. Total dietary phosphate was estimated from 24-h dietary recall (ASA24). Muscle ATP synthesis was measured at rest, and phosphocreatinine (PCr) dynamics was measured during plantar flexion exercise using 7-T 31P magnetic resonance (MR) spectroscopy in the calf muscle. Correlation was assessed between dietary phosphate intake normalized to total caloric intake, resting ATP synthesis, and PCr depletion during exercise. Higher dietary phosphate intake was associated with lower resting ATP synthesis (r = -0.62, P = 0.03), and with higher levels of PCr depletion during plantar flexion exercise relative to the resting period (r = -0.72; P = 0.004). These associations remain significant after adjustment for age and estimated glomerular filtration rate (both P < 0.05). High dietary phosphate intake was also associated with lower serum Klotho levels, and Klotho levels are in turn associated with PCr depletion and higher ADP accumulation post exercise. Our study suggests that higher dietary phosphate is associated with reduced skeletal muscle mitochondrial function at rest and exercise in humans providing new insight into potential mechanisms linking the Western diet to impaired energy metabolism.NEW & NOTEWORTHY This is the first translational research study directly demonstrating the adverse effects of dietary phosphate on muscle energy metabolism in humans. Importantly, our data show that dietary phosphate is associated with impaired muscle ATP synthesis at rest and during exercise, independent of age and renal function. This is a new biologic paradigm with significant clinical dietary implications.

Substrate oxidation during exercise in childhood acute lymphoblastic leukemia survivors

Children with acute lymphoblastic leukemia (ALL) are at high risk of developing long-term cardiometabolic complications during their survivorship. Maximal fat oxidation (MFO) is a marker during exercise of cardiometabolic health, and is associated with metabolic risk factors. Our aim was to characterize the carbohydrate and fat oxidation during exercise in childhood ALL survivors. Indirect calorimetry was measured in 250 childhood ALL survivors to quantify substrate oxidation rates during a cardiopulmonary exercise test. A best-fit third-order polynomial curve was computed for fat oxidation rate (mg/min) against exercise intensity (%V ̇ O2peak) and was used to determine the MFO and the peak fat oxidation (Fatmax). The crossover point was also identified. Differences between prognostic risk groups were assessed (ie, standard risk [SR], high risk with and without cardio-protective agent dexrazoxane [HR + DEX and HR]). MFO, Fatmax and crossover point were not different between the groups (p = .078; p = .765; p = .726). Fatmax and crossover point were achieved at low exercise intensities. A higher MFO was achieved by men in the SR group (287.8 ± 111.2 mg/min) compared to those in HR + DEX (239.8 ± 97.0 mg/min) and HR groups (229.3 ± 98.9 mg/min) (p = .04). Childhood ALL survivors have low fat oxidation during exercise and oxidize carbohydrates at low exercise intensities, independently of the cumulative doses of doxorubicin they received. These findings alert clinicians on the long-term impact of cancer treatments on childhood ALL survivors' substrate oxidation.

Factors Influencing Substrate Oxidation During Submaximal Cycling: A Modelling Analysis

BACKGROUND

Multiple factors influence substrate oxidation during exercise including exercise duration and intensity, sex, and dietary intake before and during exercise. However, the relative influence and interaction between these factors is unclear.

OBJECTIVES

Our aim was to investigate factors influencing the respiratory exchange ratio (RER) during continuous exercise and formulate multivariable regression models to determine which factors best explain RER during exercise, as well as their relative influence.

METHODS

Data were extracted from 434 studies reporting RER during continuous cycling exercise. General linear mixed-effect models were used to determine relationships between RER and factors purported to influence RER (e.g., exercise duration and intensity, muscle glycogen, dietary intake, age, and sex), and to examine which factors influenced RER, with standardized coefficients used to assess their relative influence.

RESULTS

The RER decreases with exercise duration, dietary fat intake, age, VO2max, and percentage of type I muscle fibers, and increases with dietary carbohydrate intake, exercise intensity, male sex, and carbohydrate intake before and during exercise. The modelling could explain up to 59% of the variation in RER, and a model using exclusively easily modified factors (exercise duration and intensity, and dietary intake before and during exercise) could only explain 36% of the variation in RER. Variables with the largest effect on RER were sex, dietary intake, and exercise duration. Among the diet-related factors, daily fat and carbohydrate intake have a larger influence than carbohydrate ingestion during exercise.

CONCLUSION

Variability in RER during exercise cannot be fully accounted for by models incorporating a range of participant, diet, exercise, and physiological characteristics. To better understand what influences substrate oxidation during exercise further research is required on older subjects and females, and on other factors that could explain additional variability in RER.

Effects of metabolic syndrome on fuel utilization during exercise on middle-aged moderately trained individuals

People with the metabolic syndrome (MetS) may have blunted exercise stimulation of metabolism explaining their resistance to lower blood glucose and triglycerides with exercise training. Glycerol and glucose rate of appearance (Ra) in plasma and substrate oxidation were determined at rest and during cycle ergometer exercise at three increasing intensities (55, 80 and 95% of maximal heart rate) in 9 middle-aged (61±7 yr) individuals with MetS. Data were compared to 8 healthy-younger (29±10 yr) individuals matched for habitual exercise training and fat free mass (Healthy-young). At rest, fasting plasma triglycerides (TG), blood glucose and insulin were higher in MetS than in Healthy-young (38%, 42% and 85%, respectively; all p<0.05). At rest, and during low intensity exercise (32-43% VO2MAX), plasma glycerol Ra (index of whole-body lipolysis) and glucose Ra and Rd (index of glucose appearance and disposal) were similar in MetS and Healthy-young. Fat oxidation peaked at low intensity exercise similarly in MetS and Healthy-young (0.273±0.082 vs 0.272±0.078 g·min-1, respectively; p = 0.961). Ra glycerol increased with exercise intensity but was lower in MetS at moderate and high exercise intensities (i.e., 60-100% VO2MAX; p<0.05). Metabolic clearance rate of glucose at high intensity (85-100% VO2MAX) was lower in MetS compared to Healthy-young (p = 0.029). The MetS that develops in middle adulthood, reduces exercise lipolysis and plasma glucose clearance at high exercise intensities, but does not blunt fat or carbohydrate metabolism at low exercise intensity.

Contribution of skeletal muscle and serum lipids to muscle contraction induced by neuromuscular electrical stimulation in older individuals

Intramyocellular lipids (IMCL) stored in droplets in muscle cells and free fatty acids (FFA) from fat cells in the blood are the main substrates of adenosine triphosphate during continuous muscle contractions of relatively lower intensity. Although it is known that the lipid oxidative capacity decreases with aging, the effect of IMCL and FFA on muscle contraction in older individuals remains unclear. The purpose of this study was to investigate the contribution of skeletal muscle lipids and blood lipids as energy sources for muscle contraction in older individuals. Eighteen older individuals (mean age: 70.4 ± 3.5 years) underwent muscle contraction intervention induced by intermittent neuromuscular electrical stimulation (NMES) to the vastus lateralis for 30 min. Fasting blood samples were obtained and proton magnetic resonance spectroscopy (1 H-MRS) was performed before and after NMES, and the parameters (including IMCL and extramyocellular lipid [EMCL]) from 1 H-MRS, along with FFA and adiponectin levels, were analyzed using the blood samples of all participants. Levels of IMCL and EMCL did not change (p > 0.05); however, FFA and adiponectin levels decreased from 1.1 ± 0.5 mEq/L to 0.8 ± 0.2 mEq/L and 12.0 ± 5.3 μg/ml to 11.4 ± 5.0 μg/ml, after NMES (p < 0.05), respectively. These findings indicate that serum lipids, but not skeletal muscle lipids, are the energy substrate utilized during involuntary muscle contraction in older individuals.

Relationship between physical activity time and intramuscular adipose tissue content of the thigh muscle groups of younger and older men

We investigated the effect of physical activity on muscle tissue size and intramuscular adipose tissue (IntraMAT) content in the thigh muscle groups of younger and older men. Twenty younger and 20 older men participated in this study. The muscle tissue cross-sectional area (CSA) and the IntraMAT content in the quadriceps femoris (QF), hamstrings (HM), hip adductors (AD), and mid-thigh total were measured using magnetic resonance imaging. The physical activity time was measured using a triaxial accelerometer, and four levels of physical activity were determined depending on the metabolic equivalent of task (METs), including sedentary (≤ 1.5 METs), light intensity (≤ 2.9 METs), moderate intensity (3.0–5.9 METs), and vigorous intensity (≥ 6.0 METs). No significant correlation was observed between the physical activity parameters and muscle tissue CSA in both groups. The IntraMAT content of the three muscle groups (QF, AD, and HM) and the total thigh was inversely correlated with the time of moderate-intensity physical activity (r_s =  − 0.625 to − 0.489, P < 0.05, for all comparisons) in the young group, but not in the older group. These results indicate that IntraMAT accumulation was associated with the amount of moderate-physical activity in younger men.

Effects of Exercise Training on Cardiopulmonary Function and Quality of Life in Elderly Patients with Pulmonary Fibrosis: A Meta-Analysis

(1) Objective: Our objective was to conduct a meta-analysis of randomized controlled trials that have evaluated the benefits of exercise training for elderly pulmonary fibrosis (PF) patients. (2) Methods: Studies in either English or Chinese were retrieved from the China National Knowledge Infrastructure (CNKI) and the Wanfang, PubMed, Web of Science and SPORTDiscus databases from inception until the first week of April 2021. Age, body mass index (BMI), and exercise frequency, intensity, type, and duration were considered for each participant. The specific data recorded were the six-minute walk distance (6MWD), maximal rate of oxygen consumption (peak VO2), predicted forced vital capacity (FVC% pred), predicted diffusing capacity of the lung for carbon monoxide (DLCO% pred), predicted total lung capacity (TLC% pred), St. George's respiratory questionnaire (SGRQ) total score and a modified medical research council score (mMRC). (3) Results: Thirteen studies comprised this meta-analysis (eleven randomized controlled trials and two prospective studies design), wherein 335 patients were exercised and 334 were controls. The results showed that exercise training increased the 6MWD (Cohen's d = 0.77, MD = 34.04 (95% CI, 26.50-41.58), p < 0.01), peak VO2 (Cohen's d = 0.45, MD = 1.13 (95% CI, 0.45-1.82), p = 0.0001) and FVC% pred (Cohen's d = 0.42, MD = 3.94 (95% CI, 0.91-6.96), p = 0.01). However, exercise training reduced scores for the SGRQ (Cohen's d = 0.89, MD = -8.79 (95% CI, -10.37 to -7.21), p < 0.01) and the mMRC (Cohen's d = 0.64, MD = -0.58 (95% CI, -0.79 to -0.36), p < 0.01). In contrast, exercise training could not increase DLCO% pred (Cohen's d = 0.16, MD = 1.86 (95% CI, -0.37-4.09), p = 0.10) and TLC% pred (Cohen's d = 0.02, MD = 0.07 (95% CI, -6.53-6.67), p = 0.98). Subgroup analysis showed significant differences in frequency, intensity, type, and age in the 6MWD results (p < 0.05), which were higher with low frequency, moderate intensity, aerobic-resistance-flexibility-breathing exercises and age ≤ 70. Meanwhile, the subgroup analysis showed significant differences in exercise intensity and types in the mMRC results (p < 0.05), which were lower with moderate intensity and aerobic-resistance exercises. (4) Conclusions: Exercise training during pulmonary rehabilitation can improved cardiopulmonary endurance and quality of life in elderly patients with PF. The 6MWDs were more noticeable with moderate exercise intensity, combined aerobic-resistance-flexibility-breathing exercises and in younger patients, which all were not affected by BMI levels or exercise durations. As to pulmonary function, exercise training can improve FVC% pred, but has no effect on DLCO% pred and TLC% pred.

The influence of age, sex and cardiorespiratory fitness on maximal fat oxidation rate

Fat oxidation decreases with age, yet no studies have previously investigated if aging affects the maximal fat oxidation rate (MFO) during exercise in men and women differently. We hypothesized that increased age would be associated with a decline in MFO and this would be more pronounced in women due to menopause, compared with men. In this cross-sectional study design, 435 (247/188, male/female) subjects of varying ages performed a DXA scan, a submaximal graded exercise test and a maximal oxygen uptake test, to measure MFO and cardiorespiratory fitness (CRF) by indirect calorimetry. Subjects were stratified into 12 groups according to sex (male/female), age (<45, 45-55 and >55 years), CRF (below average and above average). Women aged <45 years had a higher MFO relative to fat free mass (FFM) (mg/min/kg) compared with men, regardless of CRF. However, there were no differences in MFO (mg/min/kg FFM) between men and women, in the groups aged between 45-55 and >55 years. In summary, we found that women aged <45 years display a higher MFO (mg/min/kg FFM) compared with men and that this sexual divergence is abolished after the age of 45 years. Novelty: Maximal fat oxidation rate is higher in young women compared with men. This sex-related difference is attenuated after the age of 45 years. Cardiorespiratory fitness does not influence this sex-related difference.

Association of Mitochondrial Function, Substrate Utilization, and Anaerobic Metabolism With Age-Related Perceived Fatigability

Previous work has shown that poorer mitochondrial function is associated with age-related perceived fatigability. However, whether glucose oxidation and anaerobic metabolism are intermediate factors underlying this association remains unclear. We examined the total cross-sectional association between mitochondrial function and perceived fatigability in 554 adults aged 22-99 years. Mitochondrial function was assessed by skeletal muscle oxidative capacity (kPCr) using 31P magnetic resonance spectroscopy. Perceived fatigability was measured by rating of perceived exertion after a 5-minute (0.67 m/s) treadmill walk. The intermediate role of glucose oxidation (measured by the rate of change of respiratory exchange ratio [RER change rate] during the 5-minute treadmill walk) and anaerobic metabolism (measured by ventilatory threshold [VeT] during a maximal treadmill test) was evaluated by examining their cross-sectional associations with kPCr and perceived exertion. For each 0.01/s lower kPCr, perceived fatigability was 0.47 points higher (p = .002). A 0.01/s lower kPCr was also associated with 8.3 L/min lower VeT (p < .001). Lower VeT was associated with higher fatigability at lower levels of kPCr but not at higher kPCr levels (β for interaction = 0.017, p = .002). kPCr and RER change rate were not significantly associated (p = .341), but a 0.01/min higher RER change rate was associated with 0.12-point higher fatigability (p = .001). Poorer mitochondrial function potentially contributes to higher perceived fatigability through higher glucose oxidation and higher anaerobic metabolism. Future studies to further explore the longitudinal mechanisms between these metabolic changes and fatigability are warranted.

Increasing skeletal muscle carnitine content in older individuals increases whole-body fat oxidation during moderate-intensity exercise

Intramyocellular lipid (IMCL) utilization is impaired in older individuals, and IMCL accumulation is associated with insulin resistance. We hypothesized that increasing muscle total carnitine content in older men would increase fat oxidation and IMCL utilization during exercise, and improve insulin sensitivity. Fourteen healthy older men (69 ± 1 year, BMI 26.5 ± 0.8 kg/m² ) performed 1 h of cycling at 50% VO₂ max and, on a separate occasion, underwent a 60 mU/m² /min euglycaemic hyperinsulinaemic clamp before and after 25 weeks of daily ingestion of a 220 ml insulinogenic beverage (44.4 g carbohydrate, 13.8 g protein) containing 4.5 g placebo (n = 7) or L-carnitine L-tartrate (n = 7). During supplementation, participants performed twice-weekly cycling for 1 h at 50% VO₂ max. Placebo ingestion had no effect on muscle carnitine content or total fat oxidation during exercise at 50% VO₂ max. L-carnitine supplementation resulted in a 20% increase in muscle total carnitine content (20.1 ± 1.2 to 23.9 ± 1.7 mmol/kg/dm; p < 0.01) and a 20% increase in total fat oxidation (181.1 ± 15.0 to 220.4 ± 19.6 J/kg lbm/min; p < 0.01), predominantly due to increased IMCL utilization. These changes were associated with increased expression of genes involved in fat metabolism (ACAT1, DGKD & PLIN2; p < 0.05). There was no change in resting insulin-stimulated whole-body or skeletal muscle glucose disposal after supplementation. This is the first study to demonstrate that a carnitine-mediated increase in fat oxidation is achievable in older individuals. This warrants further investigation given reduced lipid turnover is associated with poor metabolic health in older adults.

Body Fat Is Related to Sedentary Behavior and Light Physical Activity but Not to Moderate-Vigorous Physical Activity in Type 2 Diabetes Mellitus

BACKGROUND

Sedentary behavior (SB) has emerged as a new risk factor for cardiovascular accidents. We investigated whether physical activity levels or SB were related to percent body fat (%BF) in type 2 diabetes mellitus (T2DM).

METHODS

In this cross sectional study, we measured the duration of SB, light physical activity (LPA), moderate to vigorous physical activity (MVPA), total energy expenditure, and step counts using a wireless activity tracker (Fitbit HR; FB) for 7 days in free-living conditions, along with %BF using a bio impedance analyzer (Inbody; Biospace) in 120 smartphone users with T2DM. Subjects were divided into exercise (Exe, n=68) and non-exercise (nonExe, n=52) groups based on self-reports of whether the recommended exercises (30 min/day, 3 days/week for 3 months) were performed. SBt, LPAt, MVPAt were transformed from SB, LPA, MVPA for normally distributed variables.

RESULTS

Participants were: female, 59.2%; age, 59.3±8.4 years; body mass index, 25.5±3.4 kg/m²; glycosylated hemoglobin (HbA1c), 7.6%±1.2%; %BF, 30.4%±7.1%. They performed SB for 15.7±3.7 hr/day, LPA for 4.4±1.7 hr/day, and MVPA for 0.9±0.8 hr/day. The %BF was related to SBt and LPAt, but not to MVPA after adjustments for age, gender, and HbA1c. VPA was significantly higher in the Exe group than in the nonExe group, but SB, LPA, and moderate physical activity were not different. Predicted %BF was 89.494 to 0.105 (age), -13.047 (gender), -0.507 (HbA1c), -7.655 (LPAt) (F[4, 64]=62.929, P<0.001), with an R² of 0.785 in multiple linear regression analysis.

CONCLUSION

Reduced body fat in elderly diabetic patients might be associated with reduced inactivity and increased LPA.

High-Phosphate Diet Induces Exercise Intolerance and Impairs Fatty Acid Metabolism in Mice

BACKGROUND

Inorganic phosphate (Pi) is used extensively as a preservative and a flavor enhancer in the Western diet. Physical inactivity, a common feature of Western societies, is associated with increased cardiovascular morbidity and mortality. It is unknown whether dietary Pi excess contributes to exercise intolerance and physical inactivity.

METHODS

To determine an association between Pi excess and physical activity in humans, we assessed the relationship between serum Pi and actigraphy-determined physical activity level, as well as left ventricular function by cardiac magnetic resonance imaging, in DHS-2 (Dallas Heart Study phase 2) participants after adjusting for relevant variables. To determine direct effects of dietary Pi on exercise capacity, oxygen uptake, serum nonesterified fatty acid, and glucose were measured during exercise treadmill test in C57/BL6 mice fed either a high-Pi (2%) or normal-Pi (0.6%) diet for 12 weeks. To determine the direct effect of Pi on muscle metabolism and expression of genes involved in fatty acid metabolism, additional studies in differentiated C2C12 myotubes were conducted after subjecting to media containing 1 to 3 mmol/L Pi (pH 7.0) to simulate in vivo phosphate conditions.

RESULTS

In participants of the DHS-2 (n=1603), higher serum Pi was independently associated with reduced time spent in moderate to vigorous physical activity ( P=0.01) and increased sedentary time ( P=0.004). There was no association between serum Pi and left ventricular ejection fraction or volumes. In animal studies, compared with the control diet, consumption of high-Pi diet for 12 weeks did not alter body weight or left ventricular function but reduced maximal oxygen uptake, treadmill duration, spontaneous locomotor activity, fat oxidation, and fatty acid levels and led to downregulation of genes involved in fatty acid synthesis, release, and oxidation, including Fabp4, Hsl, Fasn, and Pparγ, in muscle. Similar results were recapitulated in vitro by incubating C2C12 myotubes with high-Pi media.

CONCLUSIONS

Our data demonstrate a detrimental effect of dietary Pi excess on skeletal muscle fatty acid metabolism and exercise capacity that is independent of obesity and cardiac contractile function. Dietary Pi may represent a novel and modifiable target to reduce physical inactivity associated with the Western diet.

Lipotoxicity plays a key role in the development of both insulin resistance and muscle atrophy in patients with type 2 diabetes

Insulin resistance and muscle mass loss often coincide in individuals with type 2 diabetes. Most patients with type 2 diabetes are overweight, and it is well established that obesity and derangements in lipid metabolism play an important role in the development of insulin resistance in these individuals. Specifically, increased adipose tissue mass and dysfunctional adipose tissue lead to systemic lipid overflow and to low-grade inflammation via altered secretion of adipokines and cytokines. Furthermore, an increased flux of fatty acids from the adipose tissue may contribute to increased fat storage in the liver and in skeletal muscle, resulting in an altered secretion of hepatokines, mitochondrial dysfunction, and impaired insulin signalling in skeletal muscle. Recent studies suggest that obesity and lipid derangements in adipose tissue can also lead to the development of muscle atrophy, which would make insulin resistance and muscle atrophy two sides of the same coin. Unfortunately, the exact relationship between lipid accumulation, type 2 diabetes, and muscle atrophy remains largely unexplored. The aim of this review is to discuss the relationship between type 2 diabetes and muscle loss and to discuss some of the joint pathways through which lipid accumulation in organs may affect peripheral insulin sensitivity and muscle mass.

Carbohydrate oxidation and glucose utilisation under hyperglycaemia in aged and young males during exercise at the same relative exercise intensity

PURPOSE

The purpose of the present study was to investigate the age-related carbohydrate oxidation and glucose utilisation rate response during exercise at the same relative intensity under hyperglycaemia in aged and young males.

METHODS

16 endurance-trained aged (n = 8; 69.1 ± 5.2 year) and young (n = 8; 22.4 ± 2.9 year) males were studied during 40 min of cycling exercise (60% [Formula: see text]) under both hyperglycaemic and euglycaemic (control) conditions. Venous blood samples were collected at baseline, post-infusion, mid- and post-exercise. Carbohydrate and fat oxidation rates were determined at both 15 and 35 min during exercise, and glucose utilisation rates were calculated.

RESULTS

The aged group displayed significantly lower rates of carbohydrate oxidation during exercise during maintained hyperglycemia (15 min = 2.3 ± 0.4 vs. 1.6 ± 0.5 g min^-1; 35 min = 2.3 ± 0.5 vs. 1.5 ± 0.5 g min^-1) and control (15 min = 2.2 ± 0.4 vs. 1.6 ± 0.7 g min^-1; 35 min = 1.9 ± 0.7 vs. 1.3 ± 0.7 g min^-1) conditions (P = 0.01). The rate of glucose utilisation during exercise was also significantly reduced (85.76 ± 23.95 vs. 56.67 ± 15.09 μM kg^-1 min^-1). There were no differences between age groups for anthropometric measures, fat oxidation, insulin, glucose, NEFA, glycerol and lactate (P > 0.05) although hyperglycemia resulted in elevated glucose and insulin, and attenuated fat metabolite levels.

CONCLUSION

Our findings highlight that ageing results in a reduction in carbohydrate oxidation and utilisation rates during exercise at the same relative exercise intensity.

Relationship between physical activity and intramyocellular lipid content is different between young and older adults

PURPOSE

Intramyocellular lipid (IMCL) is influenced by physical exercise; however, whether the habitual level of physical activity affects resting IMCL content remains unclear. The purpose of this study was to determine the relationship between physical activity levels and resting IMCL content in young and older adults.

METHODS

In total, 15 nonobese young adults (21.0 ± 0.0 years) and 15 older adults (70.7 ± 3.8 years) were recruited. Time spent performing physical activities for 10 days was assessed using a three-dimensional ambulatory accelerometer, and intensity was categorized as light [< 3.0 metabolic equivalents (METs)], moderate (3.0-6.0 METs), or vigorous (> 6.0 METs). Physical activity level was calculated as the product of METs and time spent performing physical activities (MET h) at each intensity level. The IMCL content in the vastus lateralis was determined using ¹H-magnetic resonance spectroscopy after overnight fasting.

RESULTS

No significant differences in IMCL content were observed between young and older adults. Vigorous intensity physical activity (time and MET h) was significantly lower in older than young adults (p < 0.01); this difference was not observed for light and moderate intensity physical activity. Light intensity physical activity (time and MET h) was significantly and inversely correlated with IMCL content in young adults (r = - 0.59 and r = - 0.58; both p < 0.05), but not in older adults.

CONCLUSIONS

These results suggest that daily light intensity physical activity reduces resting IMCL content in young adults, whereas no significant relationship was seen between daily physical activity and resting IMCL content in older adults.

Muscle Characteristics and Substrate Energetics in Lifelong Endurance Athletes

PURPOSE

The goal of this study was to explore the effect of lifelong aerobic exercise (i.e., chronic training) on skeletal muscle substrate stores (intramyocellular triglyceride [IMTG] and glycogen), skeletal muscle phenotypes, and oxidative capacity (ox), in older endurance-trained master athletes (OA) compared with noncompetitive recreational younger (YA) athletes matched by frequency and mode of training.

METHODS

Thirteen OA (64.8 ± 4.9 yr) exercising 5 times per week or more were compared with 14 YA (27.8 ± 4.9 yr) males and females. IMTG, glycogen, fiber types, succinate dehydrogenase, and capillarization were measured by immunohistochemistry in vastus lateralis biopsies. Fat-ox and carbohydrate (CHO)-ox were measured by indirect calorimetry before and after an insulin clamp and during a cycle ergometer graded maximal test.

RESULTS

V˙O2peak was lower in OA than YA. The OA had greater IMTG in all fiber types and lower glycogen stores than YA. This was reflected in greater proportion of type I and less type II fibers in OA. Type I fibers were similar in size, whereas type II fibers were smaller in OA compared with YA. Both groups had similar succinate dehydrogenase content. Numbers of capillaries per fiber were reduced in OA but with a higher number of capillaries per area. Metabolic flexibility and insulin sensitivity were similar in both groups. Exercise metabolic efficiency was higher in OA. At moderate exercise intensities, carbohydrate-ox was lower in OA but with similar Fat-ox.

CONCLUSIONS

Lifelong exercise is associated with higher IMTG content in all muscle fibers and higher metabolic efficiency during exercise that are not explained by differences in muscle fibers types and other muscle characteristics when comparing older with younger athletes matched by exercise mode and frequency.

Relative Contribution of Intramyocellular Lipid to Whole-Body Fat Oxidation Is Reduced With Age but Subsarcolemmal Lipid Accumulation and Insulin Resistance Are Only Associated With Overweight Individuals

Insulin resistance is closely related to intramyocellular lipid (IMCL) accumulation, and both are associated with increasing age. It remains to be determined to what extent perturbations in IMCL metabolism are related to the aging process per se. On two separate occasions, whole-body and muscle insulin sensitivity (euglycemic-hyperinsulinemic clamp with 2-deoxyglucose) and fat utilization during 1 h of exercise at 50% VO2max ([U-(13)C]palmitate infusion combined with electron microscopy of IMCL) were determined in young lean (YL), old lean (OL), and old overweight (OO) males. OL displayed IMCL content and insulin sensitivity comparable with those in YL, whereas OO were markedly insulin resistant and had more than twofold greater IMCL in the subsarcolemmal (SSL) region. Indeed, whereas the plasma free fatty acid Ra and Rd were twice those of YL in both OL and OO, SSL area only increased during exercise in OO. Thus, skeletal muscle insulin resistance and lipid accumulation often observed in older individuals are likely due to lifestyle factors rather than inherent aging of skeletal muscle as usually reported. However, age per se appears to cause exacerbated adipose tissue lipolysis, suggesting that strategies to reduce muscle lipid delivery and improve adipose tissue function may be warranted in older overweight individuals.

The effect of age and unilateral leg immobilization for 2 weeks on substrate utilization during moderate-intensity exercise in human skeletal muscle

KEY POINTS

This study aimed to provide molecular insight into the differential effects of age and physical inactivity on the regulation of substrate metabolism during moderate-intensity exercise. Using the arteriovenous balance technique, we studied the effect of immobilization of one leg for 2 weeks on leg substrate utilization in young and older men during two-legged dynamic knee-extensor moderate-intensity exercise, as well as changes in key proteins in muscle metabolism before and after exercise. Age and immobilization did not affect relative carbohydrate and fat utilization during exercise, but the older men had higher uptake of exogenous fatty acids, whereas the young men relied more on endogenous fatty acids during exercise. Using a combined whole-leg and molecular approach, we provide evidence that both age and physical inactivity result in intramuscular lipid accumulation, but this occurs only in part through the same mechanisms.

ABSTRACT

Age and inactivity have been associated with intramuscular triglyceride (IMTG) accumulation. Here, we attempt to disentangle these factors by studying the effect of 2 weeks of unilateral leg immobilization on substrate utilization across the legs during moderate-intensity exercise in young (n = 17; 23 ± 1 years old) and older men (n = 15; 68 ± 1 years old), while the contralateral leg served as the control. After immobilization, the participants performed two-legged isolated knee-extensor exercise at 20 ± 1 W (∼50% maximal work capacity) for 45 min with catheters inserted in the brachial artery and both femoral veins. Biopsy samples obtained from vastus lateralis muscles of both legs before and after exercise were used for analysis of substrates, protein content and enzyme activities. During exercise, leg substrate utilization (respiratory quotient) did not differ between groups or legs. Leg fatty acid uptake was greater in older than in young men, and although young men demonstrated net leg glycerol release during exercise, older men showed net glycerol uptake. At baseline, IMTG, muscle pyruvate dehydrogenase complex activity and the protein content of adipose triglyceride lipase, acetyl-CoA carboxylase 2 and AMP-activated protein kinase (AMPK)γ3 were higher in young than in older men. Furthermore, adipose triglyceride lipase, plasma membrane-associated fatty acid binding protein and AMPKγ3 subunit protein contents were lower and IMTG was higher in the immobilized than the contralateral leg in young and older men. Thus, immobilization and age did not affect substrate choice (respiratory quotient) during moderate exercise, but the whole-leg and molecular differences in fatty acid mobilization could explain the age- and immobilization-induced IMTG accumulation.

Age-related changes in basal substrate oxidation and visceral adiposity and their association with metabolic syndrome

PURPOSE

Ageing is directly associated with visceral fat (VAT) deposition and decline of metabolically active cellular mass, which may determine age-related shifts in substrate oxidation and increased cardiometabolic risk. We tested whether VAT and fasting respiratory quotient (RQ, an index of macronutrient oxidation) changed with age and if they were associated with increased risk of metabolic syndrome (MetSyn).

METHODS

A total of 2819 adult participants (age range: 18-81 years; men/women: 894/1925) were included; we collected history, anthropometric measures, biochemistry, smoking habits, and physical activity. The body mass index range was 18.5-60.2 kg/m(2). Gas exchanges (VO2 and VCO2) were measured by indirect calorimetry in fasting conditions, and RQ was calculated. Body composition was measured by bioelectrical impedance. Abdominal subcutaneous fat and VAT were measured by ultrasonography. MetSyn was diagnosed using harmonised international criteria. Multivariate linear and logistic regression models were utilised.

RESULTS

VAT increased with age in both men (r = 0.31, p < 0.001) and women (r = 0.37, p < 0.001). Basal RQ was not significantly associated with age (p = 0.49) and VAT (p = 0.20); in addition, basal RQ was not a significant predictor of MetSyn (OR 3.31, 0.57-19.08, p = 0.27). VAT was the primary predictor of MetSyn risk in a fully adjusted logistic model (OR 4.25, 3.01-5.99, p < 0.001).

CONCLUSIONS

Visceral adiposity remains one of the most important risk factors for cardiometabolic risk and is a significant predictor of MetSyn. Post-absorptive substrate oxidation does not appear to play a significant role in age-related changes in body composition and cardiometabolic risk, except for a correlation with triglyceride concentration.

Skeletal muscle as a regulator of the longevity protein, Klotho

Klotho is a powerful longevity protein that has been linked to the prevention of muscle atrophy, osteopenia, and cardiovascular disease. Similar anti-aging effects have also been ascribed to exercise and physical activity. While an association between muscle function and Klotho expression has been previously suggested from longitudinal cohort studies, a direct relationship between circulating Klotho and skeletal muscle has not been investigated. In this paper, we present a review of the literature and preliminary evidence that, together, suggests Klotho expression may be modulated by skeletal muscle activity. Our pilot clinical findings performed in young and aged individuals suggest that circulating Klotho levels are upregulated in response to an acute exercise bout, but that the response may be dependent on fitness level. A similar upregulation of circulating Klotho is also observed in response to an acute exercise in young and old mice, suggesting that this may be a good model for mechanistically probing the role of physical activity on Klotho expression. Finally, we highlight overlapping signaling pathways that are modulated by both Klotho and skeletal muscle and propose potential mechanisms for cross-talk between the two. It is hoped that this review will stimulate further consideration of the relationship between skeletal muscle activity and Klotho expression, potentially leading to important insights into the well-documented systemic anti-aging effects of exercise.

The effects of age and muscle contraction on AMPK activity and heterotrimer composition

Sarcopenia is characterized by increased skeletal muscle atrophy due in part to alterations in muscle metabolism. AMP-activated protein kinase (AMPK) is a master regulator of skeletal muscle metabolic pathways which regulate many cellular processes that are disrupted in old-age. Functional AMPK is a heterotrimer composed of α, β and γ subunits, and each subunit can be represented in the heterotrimer by one of two (α1/α2, β1/β2) or three (γ1/γ2/γ3) isoforms. Altered isoform composition affects AMPK localization and function. Previous work has shown that overall AMPK activation with endurance-type exercise is blunted in old vs. young skeletal muscle. However, details regarding the activation of the specific isoforms of AMPK, as well as the heterotrimeric composition of AMPK in old skeletal muscle, are unknown. Our purpose here, therefore, was to determine the effect of old-age on 1) the activation of the α1 and α2 catalytic subunits of AMPK in skeletal muscle by a continuous contraction bout, and 2) the heterotrimeric composition of skeletal muscle AMPK. We studied gastrocnemius (GAST) and tibialis anterior (TA) muscles from young adult (YA; 8months old) and old (O; 30months old) male Fischer344×Brown Norway F1 hybrid rats after an in situ bout of endurance-type contractions produced via electrical stimulation of the sciatic nerve (STIM). AMPKα phosphorylation and AMPKα1 and α2 activities were unaffected by age at rest. However, AMPKα phosphorylation and AMPKα2 protein content and activity were lower in O vs. YA after STIM. Conversely, AMPKα1 content was greater in O vs. YA muscle, and α1 activity increased with STIM in O but not YA muscles. AMPKγ3 overall concentration and its association with AMPKα1 and α2 were lower in O vs. YA GAST. We conclude that activation of AMPKα1 is enhanced, while activation of α2 is suppressed immediately after repeated skeletal muscle contractions in O vs. YA skeletal muscle. These changes are associated with changes in the AMPK heterotrimer composition. Given the known roles of AMPK α1, α2 and γ3, this may contribute to sarcopenia and associated muscle metabolic dysfunction.

Physical activity and exercise in the regulation of human adipose tissue physiology

Physical activity and exercise are key components of energy expenditure and therefore of energy balance. Changes in energy balance alter fat mass. It is therefore reasonable to ask: What are the links between physical activity and adipose tissue function? There are many complexities. Physical activity is a multifaceted behavior of which exercise is just one component. Physical activity influences adipose tissue both acutely and in the longer term. A single bout of exercise stimulates adipose tissue blood flow and fat mobilization, resulting in delivery of fatty acids to skeletal muscles at a rate well-matched to metabolic requirements, except perhaps in vigorous intensity exercise. The stimuli include adrenergic and other circulating factors. There is a period following an exercise bout when fatty acids are directed away from adipose tissue to other tissues such as skeletal muscle, reducing dietary fat storage in adipose. With chronic exercise (training), there are changes in adipose tissue physiology, particularly an enhanced fat mobilization during acute exercise. It is difficult, however, to distinguish chronic "structural" changes from those associated with the last exercise bout. In addition, it is difficult to distinguish between the effects of training per se and negative energy balance. Epidemiological observations support the idea that physically active people have relatively low fat mass, and intervention studies tend to show that exercise training reduces fat mass. A much-discussed effect of exercise versus calorie restriction in preferentially reducing visceral fat is not borne out by meta-analyses. We conclude that, in addition to the regulation of fat mass, physical activity may contribute to metabolic health through beneficial dynamic changes within adipose tissue in response to each activity bout.

Lipogenic regulators are elevated with age and chronic overload in rat skeletal muscle

AIM

Both muscle mass and strength decline with ageing, but the loss of strength far surpasses what is projected based on the decline in mass. Interestingly, the accumulation of fat mass has been shown to be a strong predictor of functional loss and disability. Furthermore, there is a known attenuated hypertrophic response to skeletal muscle overload with ageing. The purpose of this study was to determine the effect of 28 days of overload on the storage of intramuscular triglycerides (IMTG) and metabolic regulators of lipid synthesis in young and old skeletal muscle.

METHODS

The phosphorylation and expression of essential lipogenic regulators were determined in the plantaris of young (YNG; 6-month-old) and aged (OLD; 30-month-old) rats subjected to bilateral synergist ablation (SA) of two-thirds of the gastrocnemius muscle or sham surgery.

RESULTS

We demonstrate that age-induced increases in IMTG are associated with enhancements in the expression of lipogenic regulators in muscle. We also show that the phosphorylation and concentration of the 5'AMP-activated protein kinase (AMPK) isoforms are altered in OLD. We observed increases in the expression of lipogenic regulators and AMPK signalling after SA in YNG, despite no increase in IMTG. Markers of oxidative capacity were increased in YNG after SA. These overload-induced effects were blunted in OLD.

CONCLUSION

These data suggest that lipid metabolism may be altered in ageing skeletal muscle and is unaffected by mechanical overload via SA. By determining the role of increased lipid storage on skeletal muscle mass during ageing, possible gene targets for the treatment of sarcopenia may be identified.

Dyslipidemia in the elderly: should it be treated?

Elderly or older adults constitute a rapidly growing segment of the United States population, thus resulting in an increase in morbidity and mortality related to cardiovascular disease-an increase that is reaching epidemic proportions. Dyslipidemia is a well established risk factor for cardiovascular disease and is estimated to account for more than half of the global cases of coronary artery disease. Despite the increased prevalence of dyslipidemia in the older adult population, controversy persists regarding the benefits of treatment in this group. Epidemiologic studies have shown that dyslipidemia is often underdiagnosed and under treated in this population probably as a result of a paucity of evidence regarding the impact of treatment in delaying the progression of atherosclerotic disease, concerns involving increased likelihood of adverse events or drug interactions, or doubts regarding the cost effectiveness of lipid-lowering therapy in older adults. In conclusion, despite the proven efficacy of lipid-lowering therapy in decreasing cardiovascular morbidity and mortality, these therapies have been underutilized in older patients.

Aging, resistance training, and diabetes prevention

With the aging of the baby-boom generation and increases in life expectancy, the American population is growing older. Aging is associated with adverse changes in glucose tolerance and increased risk of diabetes; the increasing prevalence of diabetes among older adults suggests a clear need for effective diabetes prevention approaches for this population. The purpose of paper is to review what is known about changes in glucose tolerance with advancing age and the potential utility of resistance training (RT) as an intervention to prevent diabetes among middle-aged and older adults. Age-related factors contributing to glucose intolerance, which may be improved with RT, include improvements in insulin signaling defects, reductions in tumor necrosis factor-α, increases in adiponectin and insulin-like growth factor-1 concentrations, and reductions in total and abdominal visceral fat. Current RT recommendations and future areas for investigation are presented.

Influence of exercise on nutritional requirements

There is no consensus on the best diet for exercise, as many variables influence it. We propose an approach that is based on the total energy expenditure of exercise and the specific macro- and micronutrients used. di Prampero quantified the impact of intensity and duration on the energy cost of exercise. This can be used to determine the total energy needs and the balance of fats and carbohydrates (CHO). There are metabolic differences between sedentary and trained persons, thus the total energy intake to prevent overfeeding of sedentary persons and underfeeding athletes is important. During submaximal sustained exercise, fat oxidation (FO) plays an important role. This role is diminished and CHO's role increases as exercise intensity increases. At super-maximal exercise intensities, anaerobic glycolysis dominates. In the case of protein and micronutrients, specific recommendations are required. We propose that for submaximal exercise, the balance of CHO and fat favors fat for longer exercise and CHO for shorter exercise, while always maintaining the minimal requirements of each (CHO: 40% and fat: 30%). A case for higher protein (above 15%) as well as creatine supplementation for resistance exercise has been proposed. One may also consider increasing bicarbonate intake for exercise that relies on anaerobic glycolysis, whereas there appears to be little support for antioxidant supplementation. Insuring minimal levels of substrate will prevent exercise intolerance, while increasing some components may increase exercise tolerance.

Twelve weeks of endurance training increases FFA mobilization and reesterification in postmenopausal women

We examined the effects of exercise intensity and training on rates of lipolysis, plasma free fatty acid (FFA) appearance (R(a)), disappearance (R(d)), reesterification (R(s)), and oxidation (R(oxP)) in postmenopausal (PM) women. Ten sedentary but healthy women (55 ± 0.6 yr) completed 12 wk of supervised endurance exercise training on a cycle ergometer [5 days/wk, 1 h/day, 65% peak oxygen consumption (Vo(2peak))]. Flux rates were determined by continuous infusion of [1-(13)C]palmitate and [1,1,2,3,3-(2)H(5)]glycerol during 90 min of rest and 60 min of cycle ergometer exercise during one pretraining exercise trial [65% Vo(2peak) (PRE)] and two posttraining exercise trials [at power outputs that elicited 65% pretraining Vo(2peak) (absolute training; ABT) and 65% posttraining Vo(2peak) (relative training; RLT)]. Initial body weights (68.2 ± 4.5 kg) were maintained over the course of study. Training increased Vo(2peak) by 16.3 ± 3.9% (P < 0.05) (Zarins ZA, Wallis GA, Faghihnia N, Johnson ML, Fattor JA, Horning MA and Brooks GA. Metabolism 58: 9: 1338-1346, 2009). Glycerol R(a) and R(d) were elevated in the RLT trial (P < 0.05), but not the ABT trial after training. Rates of plasma FFA R(a), R(d), and R(oxP) were elevated during the ABT compared with PRE trial (P < 0.05). FFA R(s) accounted for most (50-70%) of R(d) during exercise; training reduced FFA R(s) during ABT, but not RLT compared with PRE. We conclude that, despite the large age-related decrease in metabolic scope in PM women, endurance training increases the capacities for FFA mobilization and oxidation during exercises of a given power output. However, after menopause, total lipid oxidation capacity remains low, with reesterification accounting for most of FFA R(d).

Effects of endurance training on cardiorespiratory fitness and substrate partitioning in postmenopausal women

We examined the effect of endurance training on energy substrate partitioning during rest and exercise in postmenopausal women. Ten healthy sedentary (55 +/- 1 years old) subjects completed 12 weeks of endurance exercise training on a cycle ergometer (5 d/wk, 1 h/d, 65% peak oxygen consumption [Vo(2)peak]). Whole-body energy substrate oxidation was determined by indirect calorimetry during 90 minutes of rest and 60 minutes of cycle ergometer exercise. Subjects were studied at 65% Vo(2)peak before training and after training at the same absolute exercise intensity (same absolute workload as 65% of pretraining Vo(2)peak) and same relative exercise intensity (65% of posttraining Vo(2)peak). After training, Vo(2)peak increased by 16.3% +/- 3.9% and resting heart rate decreased by 4 beats per minute (P < .05). During exercise at same absolute intensity, mean arterial pressure decreased by 8 mm Hg (P < .05), heart rate decreased by 19 beats per minute (P < .05), energy derived from carbohydrate decreased by 9.6%, and the energy derived from lipid increased by 9.2% (P < .05). Lactate concentration was lower at the same absolute and relative exercise intensities (P < .05). Changes in substrate partitioning during exercise were accomplished without changes in dietary composition, body weight, or body composition. We conclude that endurance training in healthy postmenopausal women who remain in energy balance results in many of the classic cardiopulmonary training effects, decreases the reliance on carbohydrate, and increases lipid oxidation during a given submaximal exercise task without a reduction in body weight.

Training improves the response in glucose flux to exercise in postmenopausal women

We examined the effects of endurance training on parameters of glucose flux during rest and exercise in postmenopausal women. Ten sedentary, but healthy women (55 +/- 1 yr) completed 12 wk of endurance exercise training on a cycle ergometer [5 days/wk, 1 h/day, 65% peak oxygen consumption (Vo(2peak))]. Flux rates were determined by primed continuous infusion of [6,6-(2)H]glucose (D(2)-glucose) during 90 min of rest and 60 min of cycle ergometer exercise during one pretraining exercise trial [65% Vo(2peak) (PRE)] and two posttraining exercise trials [the power output that elicited 65% pretraining Vo(2peak) (ABT) and 65% posttraining Vo(2peak) (RLT)]. Training increased Vo(2peak) by 16.3 +/- 3.9% (P < 0.05). Epinephrine and glucagon were lower during ABT and lactate was lower during ABT and RLT (P < 0.05), but the apparent insulin response was unchanged. Whole body glucose rate of appearance decreased posttraining during exercise at a given power output (4.58 +/- 0.39 mg.kg(-1).min(-1) during ABT compared with 5.21 +/- 0.48 mg.kg(-1).min(-1) PRE, P < 0.05), but not at the same relative workload (5.85 +/- 0.36 mg.kg(-1).min(-1)). Training resulted in a 35% increase in glucose MCR during exercise at the same relative intensity (7.16 +/- 0.42 ml.kg(-1).min(-1) during RLT compared with 5.28 +/- 0.42 ml.kg(-1).min(-1) PRE, P < 0.05). Changes in parameters of glucose kinetics during exercise were accomplished without changes in dietary composition, body weight, or body composition. We conclude that despite changes in the hormonal milieu that occur at menopause, endurance training results in a similar magnitude in training-induced alterations of glucose flux as seen previously in younger women.

Effects of exercise intensity and duration on fat metabolism in trained and untrained older males

Advancing age is associated with changes in fat and carbohydrate (CHO) metabolism, which is considered a risk factor for cardiovascular disease and diabetes. The effects of exercise intensity and duration on fat and CHO metabolism in elderly male subjects were investigated in the present study. Seven trained (63.7+/-4.7 years) and six untrained (63.5+/-4.5 years) healthy males performed three 30 min trials on a cycle ergometer at 50, 60 and 70% VO2max and two other trials at 60 and 70% VO2max in which the total energy expenditure was equal to that for 30 min at 50% VO2max Respiratory measures were undertaken throughout the exercise and blood samples taken before and immediately after each trial. Statistical analyses revealed a significant effect of exercise intensity on fat oxidation when the exercise durations were equated as well as when the energy expenditure was held constant for the three trials, though no training effect was noted. Total carbohydrate oxidation increased significantly with exercise intensity (P<0.05) and with training. Significantly higher levels of non-esterified free fatty acid (NEFA) and glycerol were observed for trained compared with untrained though not for B-hydroxybutyrate (3-OH) or insulin. No differences in NEFA, glycerol, 3-OH were evident for increases in exercise intensity. Carbohydrate and fat oxidation are significantly affected by exercise intensity in elderly males, although only CHO oxidation is influenced by training. Furthermore, training-induced increases in the availability of NEFA and glycerol are not associated with an increase in fat oxidation, rather an increase in CHO oxidation.

Changes in 24-h substrate oxidation in older and younger men in response to exercise

The purpose of this study was to compare 24-h substrate oxidation in older (OM; 60-75 yr, n = 7) and younger (YM; 20-30 yr, n = 7) men studied on sedentary day (Con) and on a day with exercise (Ex; net energy expenditure = 300 kcal). Plasma glucose and free fatty acids were also measured at several time points during the 24-h measurement. Weight was not different in OM and YM (means +/- SD; 84.8 +/- 16.9 vs. 81.4 +/- 10.4 kg, respectively), although percent body fat was slightly higher in OM (25.9 +/- 3.5 vs. 21.9 +/- 9.7%; P = 0.17). Values of 24-h energy expenditure did not differ in OM and YM on the Con (means +/- SE; 2,449 +/- 162 vs. 2,484 +/- 104 kcal/day, respectively) or Ex (2,902 +/- 154 vs. 2,978 +/- 122 kcal/day) days. Under both conditions, 24-h respiratory quotient was significantly lower and fat oxidation significantly higher in OM. Glucose concentrations were not different at any time point, but plasma free fatty acid concentrations were higher in OM, particularly following meals. Thus, under these controlled conditions, 24-h fat oxidation was not reduced and was in fact greater in OM. We speculate that differences in the availability of circulating free fatty acids in the postprandial state contributed to the observed differences in 24-h fat oxidation in OM and YM.

The underappreciated role of muscle in health and disease

Muscle plays a central role in whole-body protein metabolism by serving as the principal reservoir for amino acids to maintain protein synthesis in vital tissues and organs in the absence of amino acid absorption from the gut and by providing hepatic gluconeogenic precursors. Furthermore, altered muscle metabolism plays a key role in the genesis, and therefore the prevention, of many common pathologic conditions and chronic diseases. Nonetheless, the maintenance of adequate muscle mass, strength, and metabolic function has rarely, if ever, been targeted as a relevant endpoint of recommendations for dietary intake. It is therefore imperative that factors directly related to muscle mass, strength, and metabolic function be included in future studies designed to demonstrate optimal lifestyle behaviors throughout the life span, including physical activity and diet.

Ageing, oxidative stress, and mitochondrial uncoupling

Mitochondria are a cell's single greatest source of reactive oxygen species. Reactive oxygen species are important for many life sustaining processes of cells and tissues, but they can also induce cell damage and death. If their production and levels within cells is not effectively controlled, then the detrimental effects of oxidative stress can accumulate. Oxidative stress is widely thought to underpin many ageing processes, and the oxidative stress theory of ageing is one of the most widely acknowledged theories of ageing. As well as being the major source of reactive oxygen species, mitochondria are also a major site of oxidative damage. The purpose of this review is a concise and current review of the effects of oxidative stress and ageing on mitochondrial function. Emphasis is placed upon the roles of mitochondrial proton leak, the uncoupling proteins, and the anti-ageing effects of caloric restriction.

Muscle tissue changes with aging

PURPOSE OF REVIEW

This review article focuses on the changes that occur in muscle with age, specifically the involuntary loss of muscle mass, strength and function, termed sarcopenia. Particular emphasis is given to the metabolic alterations that characterize sarcopenia, and to the potentially treatable causes of this condition, including age-related endocrine and nutritional changes, and inactivity.

RECENT FINDINGS

Recent data reported include those regarding the potential role of insulin resistance in the development of sarcopenia, the potential role of androgens and growth hormone in the treatment of this condition, the usefulness of exercise including both resistance and aerobic training to improve muscle growth and function, and, finally, the possible use of nutritional manipulations to improve muscle mass.

SUMMARY

Sarcopenia is likely a multifactorial condition that impairs physical function and predisposes to disability. It may be prevented or treated with lifestyle interventions and pharmacological treatment. Further long-term investigations are needed, however, to ascertain what type and combinations of interventions are the most efficacious in improving muscle mass and function in older people.

Human skeletal muscle mitochondrial metabolism in youth and senescence: no signs of functional changes in ATP formation and mitochondrial oxidative capacity

The mitochondrial theory of ageing was tested. Isolated mitochondria from the quadriceps muscle from normal, healthy, young (age 20+ years, n=12) and elderly (70+ years, n=11) humans were studied in respiratory experiments and the data expressed as activities of the muscle. In each group, the subjects exhibited a variation of physical activity but, on average, the groups were representative for their age with maximum O(2) consumption rate of 50+/-9 and 34+/-13 ml min(-1) kg(-1) (mean+/-SD), respectively. Thirteen different activities were assayed. alpha-Glycerophosphate oxidation was lower in the 70+ group (38%, P~0.001), as was the respiratory capacity for fatty acids (19%, P~0.03). The remaining eleven activities, including those of the central bioenergetic reactions, were not lower in the 70+ group. Pyruvate and alpha-ketoglutarate dehydrogenase activities (i.e. the tricarboxylic acid cycle turnover) and the respiratory chain activity could all account for ~14 mmol O(2) min(-1) kg(-1) muscle (37 degrees C). The capacity for aerobic ATP synthesis was ~35 mmol ATP min(-1) kg(-1). The mitochondrial capacities were far in excess of whole-body performance. They were related to physical activity, but not to age. The mitochondrial theory of ageing, which attributes the age-related decline of muscle performance to decreased mitochondrial function, is incompatible with these results.

Aging skeletal muscle mitochondria in the rat: decreased uncoupling protein-3 content

The goal of the present study was to discern the cellular mechanism(s) that contributes to the age-associated decrease in skeletal muscle aerobic capacity. Skeletal muscle mitochondrial content, a parameter of oxidative capacity, was significantly lower (25 and 20% calculated on the basis of citrate synthase and succinate dehydrogenase activities, respectively) in 24-mo-old Fischer 344 rats compared with 6-mo-old adult rats. Mitochondria isolated from skeletal muscle of both age groups had identical state 3 (ADP-stimulated) and ADP-stimulated maximal respiratory rates and phosphorylation potential (ADP-to-O ratios) with both nonlipid and lipid substrates. In contrast, mitochondria from 24-mo-old rats displayed significantly lower state 4 (ADP-limited) respiratory rates and, consequently, higher respiratory control ratios. Consistent with the tighter coupling, there was a 68% reduction in uncoupling protein-3 (UCP-3) abundance in mitochondria from elderly compared with adult rats. Congruent with the respiratory studies, there was no age-associated decrease in carnitine palmitoyltransferase I and carnitine palmitoyltransferase II activities in isolated skeletal muscle mitochondria. However, there was a small, significant decrease in tissue total carnitine content. It is concluded that the in vivo observed decrease in skeletal muscle aerobic capacity with advanced age is a consequence of the decreased mitochondrial density. On the basis of the dramatic reduction of UCP-3 content associated with decreased state 4 respiration of skeletal muscle mitochondria from elderly rats, we propose that an increased free radical production might contribute to the metabolic compromise in aging.

Gender related differences in the effect of aging on blood amino acid compartmentation*

This work has been focused on the study of the variations in blood amino acid compartmentation (plasma and blood cells) with aging, both in men and women. Aging is a situation which, under the influence of gender, involves a decline in body weight functions and variations in energy metabolism with a deterioration of muscular metabolism leading to changes in amino acid handling. We determined the blood levels of individual amino acids in whole blood, plasma compartment and blood cell compartment of 51 men and 51 women. Subjects were classified in three age groups-AG1 (18 to 35 y), AG2 (35-50 y) and AG3 (more than 50 y). Aging was accompanied by significant changes in blood levels of amino acids showing gender-linked differences which were distinct for both blood compartments (plasma and blood cells). In men, aging was accompanied by a drop in blood levels of several amino acids, due mainly to the plasma compartment, whereas in women aging brought about a rise in blood levels of various amino acids mainly in blood cell compartment. This paper contributes to enhancing the physiological importance of the blood cell pool in the handling of amino acids.

Endurance training and GH administration in elderly women: effects on abdominal adipose tissue lipolysis

In the present study, the effect of endurance training alone and endurance training combined with recombinant human growth hormone (rhGH) administration on subcutaneous abdominal adipose tissue lipolysis was investigated. Sixteen healthy women [age 75 +/- 2 yr (mean +/- SE)] underwent a 12-wk endurance training program on a cycle ergometer. rhGH was administered in a randomized, double-blinded, placebo-controlled design in addition to the training program. Subcutaneous abdominal adipose tissue lipolysis was estimated by means of microdialysis combined with measurements of subcutaneous abdominal adipose tissue blood flow (ATBF; (133)Xe washout). Whole body fat oxidation was estimated simultaneously by indirect calorimetry. Before and after completion of the training program, measurements were performed both at rest and during 60 min of continuous cycling at a workload corresponding to 60% of pretraining peak oxygen uptake. Endurance training alone did not affect subcutaneous abdominal adipose tissue lipolysis either at rest or during exercise, as reflected by identical levels of interstitial adipose tissue glycerol, subcutaneous abdominal ATBF, and plasma nonesterified fatty acids before and after completion of the training program. Similarly, no effect on subcutaneous abdominal adipose tissue lipolysis was observed when combining endurance training with rhGH administration. However, in both the placebo and the GH groups, fat oxidation was significantly increased during exercise performed at the same absolute workload after completion of the training program. We conclude that the changed lipid metabolism during exercise observed after endurance training alone or after endurance training combined with rhGH administration is not due to alterations in subcutaneous abdominal adipose tissue metabolism in elderly women.

Muscle fatty acid oxidative capacity is a determinant of whole body fat oxidation in elderly people

In sedentary elderly people, a reduced muscle fatty acid oxidative capacity (MFOC) may explain a decrease in whole body fat oxidation. Eleven sedentary and seven regularly exercising subjects (65.6 +/- 4. 5 yr) were characterized for their aerobic fitness [maximal O(2) uptake (VO(2 max))/kg fat free mass (FFM)] and their habitual daily physical activity level [free-living daily energy expenditure divided by sleeping metabolic rate (DEE(FLC)/SMR)]. MFOC was determined by incubating homogenates of vastus lateralis muscle with [1-(14)C]palmitate. Whole body fat oxidation was measured by indirect calorimetry over 24 h. MFOC was 40.4 +/- 14.7 and 44.3 +/- 16.3 nmol palmitate. g wet tissue(-1). min(-1) in the sedentary and regularly exercising individuals, respectively (P = nonsignificant). MFOC was positively correlated with DEE(FLC)/SMR (r = 0.58, P < 0. 05) but not with VO(2 max)/kg FFM (r = 0.35, P = nonsignificant). MFOC was the main determinant of fat oxidation during all time periods including physical activity. Indeed, MFOC explained 19.7 and 30.5% of the variance in fat oxidation during walking and during the alert period, respectively (P < 0.05). Furthermore, MFOC explained 23.0% of the variance in fat oxidation over 24 h (P < 0.05). It was concluded that, in elderly people, MFOC may be influenced more by overall daily physical activity than by regular exercising. MFOC is a major determinant of whole body fat oxidation during physical activities and, consequently, over 24 h.