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.

Running in the heat similarly reduces lipid oxidation and peak oxygen consumption in trained runners and inactive individuals

This study compared oxygen consumption and substrate oxidation while exercising in hot and temperate conditions in individuals with different physical activity statuses (i.e., inactive individuals vs. trained runners). Ten inactive individuals (IA: 26 ± 6 yr; 79.1 ± 14.1 kg; 40.7 ± 5.1 mL·kg-1·min-1) and 10 trained runners (TR: 25 ± 6 yr; 69.5 ± 9.1 kg; 63.1 ± 5.1 mL·kg-1·min-1) completed two incremental exercise tests (4-min stages) until exhaustion in temperate (TEMP: 18.7 ± 0.1°C; 43.2 ± 4.1% relative humidity) and hot (HOT: 34.4 ± 0.2°C and 42.6 ± 1.6% relative humidity) conditions. Expired gas and blood lactate concentrations were measured at the end of each stage. Peak oxygen consumption similarly decreased in HOT compared with TEMP for IA and TR [-13.2 ± 4.5% vs. -15.2 ± 7%; P = 0.571; effect size (ES) = 0.25]. In HOT compared with TEMP, lipid oxidation, from 30% to 70% of peak oxygen consumption (V̇o2peak), was reduced for both groups (IA: P = 0.023, ES = 0.43; TR: P < 0.001, ES = 0.72), whereas carbohydrate oxidation was increased for TR (P = 0.011; ES = 0.45) but not for IA (P = 0.268; ES = 0.21). Core temperature was different between conditions for TR (higher in HOT, P = 0.017; ES = 0.66) but not for IA (P = 0.901; ES = 0.25). Despite reduced physiological capacities in IA, both populations demonstrated reductions in lipid utilization and peak oxygen consumption in hot compared with temperate conditions. However, the increased carbohydrate oxidation in HOT for TR was not observed in IA, potentially explained by lower thermal strain. NEW & NOTEWORTHY This study shows that lipid oxidation and oxygen consumption are similarly affected by heat exposure in trained runners and inactive individuals. Carbohydrate oxidation and core temperature are greater in hot conditions in trained runners but not in inactive individuals. A lower metabolic heat production in inactive individuals for a similar relative intensity compared with trained runners could explain these differences in core temperature.

Resting and exercise metabolic characteristics in obese children with insulin resistance

Purpose: This study aimed to explore the characteristics of resting energy expenditure (REE) and lipid metabolism during incremental load exercise in obese children and adolescents with insulin resistance (IR) to provide evidence for exercise intervention in obese children and adolescents with IR.

Method: From July 2019 to August 2021, 195 obese children and adolescents aged 13–17 were recruited through a summer camp. The participants were divided into IR (n = 67) and no-IR (without insulin resistance, n = 128) groups and underwent morphology, blood indicators, body composition, and resting energy consumption gas metabolism tests. Thirty participants each were randomly selected from the IR and no-IR groups to carry out the incremental treadmill test.

Results: Significant metabolic differences in resting and exercise duration were found between the IR and no-IR groups. In the resting state, the resting metabolic equivalents (4.33 ± 0.94 ml/min/kg vs. 3.91 ± 0.73 ml/min/kg, p = 0.001) and REE (2464.03 ± 462.29 kcal/d vs. 2143.88 ± 380.07 kcal/d, p &lt; 0.001) in the IR group were significantly higher than in the no-IR group. During exercise, the absolute maximal fat oxidation (0.33 ± 0.07 g/min vs. 0.36 ± 0.09 g/min, p = 0.002) in the IR group was significantly lower than in the no-IR group; maximal fat oxidation intensity (130.9 ± 8.9 bpm vs. 139.9 ± 7.4 bpm, p = 0.040) was significantly lower in the IR group.

Conclusion: Significant resting and exercise metabolic differences were found between obese IR and no-IR children and adolescents. Obese IR children and adolescents have higher REE and lower maximal fat oxidation intensity than obese no-IR children and adolescents.

Effects of high-intensity interval training (HIIT) and sprint interval training (SIT) on fat oxidation during exercise: a systematic review and meta-analysis

OBJECTIVE

To investigate the effects of high-intensity interval training (HIIT) and sprint interval training (SIT) on fat oxidation during exercise (FatOx) and how they compare with the effects of moderate-intensity continuous training (MICT).

DESIGN

Systematic review and meta-analysis.

DATA SOURCES

Academic Search Ultimate, CINAHL, Networked Digital Library of Theses and Dissertations, Open Access Theses and Dissertations, OpenDissertations, PubMed/MEDLINE, Scopus, SPORTDiscus and Web of Science.

ELIGIBILITY CRITERIA FOR SELECTING STUDIES

Studies using a between-group design, involving adult participants who were not trained athletes, and evaluating effects of HIIT or SIT on FatOx (vs no exercise or MICT) were included.

RESULTS

Eighteen studies of fair-to-good quality were included; nine comparing HIIT or SIT with no exercise and eleven comparing HIIT or SIT with MICT. A significant pooled effect of these types of interval training on FatOx was found (mean difference in g/min (MD)=0.08; 95% confidence interval (CI) 0.04 to 0.12; p<0.001). Significant effects were found for exercise regimens lasting ≥4 weeks, and they increased with every additional week of training (β=0.01; 95% CI 0.00 to 0.02; p=0.003). HIIT and/or SIT were slightly more effective than MICT (MD=0.03; 95% CI 0.01 to 0.05; p=0.005). The effects on FatOx were larger among individuals with overweight/obesity.

CONCLUSION

Engaging in HIIT or SIT can improve FatOx, with larger effects expected for longer training regimens and individuals with overweight/obesity. While some effects seem small, they may be important in holistic approaches to enhance metabolic health and manage obesity.

Beyond the Calorie Paradigm: Taking into Account in Practice the Balance of Fat and Carbohydrate Oxidation during Exercise?

Recent literature shows that exercise is not simply a way to generate a calorie deficit as an add-on to restrictive diets but exerts powerful additional biological effects via its impact on mitochondrial function, the release of chemical messengers induced by muscular activity, and its ability to reverse epigenetic alterations. This review aims to summarize the current literature dealing with the hypothesis that some of these effects of exercise unexplained by an energy deficit are related to the balance of substrates used as fuel by the exercising muscle. This balance of substrates can be measured with reliable techniques, which provide information about metabolic disturbances associated with sedentarity and obesity, as well as adaptations of fuel metabolism in trained individuals. The exercise intensity that elicits maximal oxidation of lipids, termed LIPOXmax, FATOXmax, or FATmax, provides a marker of the mitochondrial ability to oxidize fatty acids and predicts how much fat will be oxidized over 45–60 min of low- to moderate-intensity training performed at the corresponding intensity. LIPOXmax is a reproducible parameter that can be modified by many physiological and lifestyle influences (exercise, diet, gender, age, hormones such as catecholamines, and the growth hormone-Insulin-like growth factor I axis). Individuals told to select an exercise intensity to maintain for 45 min or more spontaneously select a level close to this intensity. There is increasing evidence that training targeted at this level is efficient for reducing fat mass, sparing muscle mass, increasing the ability to oxidize lipids during exercise, lowering blood pressure and low-grade inflammation, improving insulin secretion and insulin sensitivity, reducing blood glucose and HbA_1c in type 2 diabetes, and decreasing the circulating cholesterol level. Training protocols based on this concept are easy to implement and accept in very sedentary patients and have shown an unexpected efficacy over the long term. They also represent a useful add-on to bariatric surgery in order to maintain and improve its weight-lowering effect. Additional studies are required to confirm and more precisely analyze the determinants of LIPOXmax and the long-term effects of training at this level on body composition, metabolism, and health.

The day-to-day reliability of peak fat oxidation and FATMAX

PURPOSE

Prior studies exploring the reliability of peak fat oxidation (PFO) and the intensity that elicits PFO (FATMAX) are often limited by small samples. This study characterised the reliability of PFO and FATMAX in a large cohort of healthy men and women.

METHODS

Ninety-nine adults [49 women; age: 35 (11) years; [Formula: see text]O2peak: 42.2 (10.3) mL·kg BM-1·min-1; mean (SD)] completed two identical exercise tests (7-28 days apart) to determine PFO (g·min-1) and FATMAX (%[Formula: see text]O2peak) by indirect calorimetry. Systematic bias and the absolute and relative reliability of PFO and FATMAX were explored in the whole sample and sub-categories of: cardiorespiratory fitness, biological sex, objectively measured physical activity levels, fat mass index (derived by dual-energy X-ray absorptiometry) and menstrual cycle status.

RESULTS

No systematic bias in PFO or FATMAX was found between exercise tests in the entire sample (- 0.01 g·min-1 and 0%[Formula: see text]O2peak, respectively; p > 0.05). Absolute reliability was poor [within-subject coefficient of variation: 21% and 26%; typical errors: ± 0.06 g·min-1 and × / ÷ 1.26%[Formula: see text]O2peak; 95% limits of agreement: ± 0.17 g·min-1 and × / ÷ 1.90%[Formula: see text]O2peak, respectively), despite high (r = 0.75) and moderate (r = 0.45) relative reliability for PFO and FATMAX, respectively. These findings were consistent across all sub-groups.

CONCLUSION

Repeated assessments are required to more accurately determine PFO and FATMAX.

Frequent Carbohydrate Ingestion Reduces Muscle Glycogen Depletion and Postpones Fatigue Relative to a Single Bolus

The timing of carbohydrate ingestion and how this influences net muscle glycogen utilization and fatigue has only been investigated in prolonged cycling. Past findings may not translate to running because each exercise mode is distinct both in the metabolic response to carbohydrate ingestion and in the practicalities of carbohydrate ingestion. To this end, a randomized, cross-over design was employed to contrast ingestion of the same sucrose dose either at frequent intervals (15 × 5 g every 5 min) or at a late bolus (1 × 75 g after 75 min) during prolonged treadmill running to exhaustion in six well-trained runners ( 61 ± 4 ml·kg−1·min−1). The muscle glycogen utilization rate was lower in every participant over the first 75 min of running (Δ 0.51 mmol·kg dm−1·min−1; 95% confidence interval [−0.02, 1.04] mmol·kg dm−1·min−1) and, subsequently, all were able to run for longer when carbohydrate had been ingested frequently from the start of exercise compared with when carbohydrate was ingested as a single bolus toward the end of exercise (105.6 ± 3.0 vs. 96.4 ± 5.0 min, respectively; Δ 9.3 min, 95% confidence interval [2.8, 15.8] min). A moderate positive correlation was apparent between the magnitude of glycogen sparing over the first 75 min and the improvement in running capacity (r = .58), with no significant difference in muscle glycogen concentrations at the point of exhaustion. This study indicates that failure to ingest carbohydrates from the outset of prolonged running increases reliance on limited endogenous muscle glycogen stores—the ergolytic effects of which cannot be rectified by subsequent carbohydrate ingestion late in exercise.

Dietary supplementation with New Zealand blackcurrant extract enhances fat oxidation during submaximal exercise in the heat

OBJECTIVES

This study investigated the effect of 7 days' supplementation with New Zealand blackcurrant extract on thermoregulation and substrate metabolism during running in the heat.

DESIGN

Randomized, double-blind, cross-over study.

METHODS

Twelve men and six women (mean±SD: Age 27±6 years, height 1.76±0.10m, mass 74±12kg, V̇O_2max 53.4±7.0mLkg^-1min^-1) completed one assessment of maximal aerobic capacity and one familiarisation trial (18°C, 40% relative humidity, RH), before ingesting 2×300mgday^-1 capsules of CurraNZ™ (each containing 105mg anthocyanin) or a visually matched placebo (2×300mg microcrystalline cellulose M102) for 7 days (washout 14 days). On day 7 of each supplementation period, participants completed 60min of fasted running at 65% V̇O_2max in hot ambient conditions (34°C and 40% relative humidity).

RESULTS

Carbohydrate oxidation was decreased in the NZBC trial [by 0.24gmin^-1 (95% CI: 0.21-0.27gmin^-1)] compared to placebo (p= 0.014, d=0.46), and fat oxidation was increased in the NZBC trial [by 0.12gmin^-1 (95% CI: 0.10 to 0.15gmin^-1)], compared to placebo (p=0.008, d=0.57). NZBC did not influence heart rate (p=0.963), rectal temperature (p=0.380), skin temperature (p=0.955), body temperature (p=0.214) or physiological strain index (p=0.705) during exercise.

CONCLUSIONS

Seven-days intake of 600mg NZBC extract increased fat oxidation without influencing cardiorespiratory or thermoregulatory variables during prolonged moderate intensity running in hot conditions.

Fat oxidation at rest and during exercise in male monozygotic twins

PURPOSE

We aimed to investigate if hereditary factors, leisure-time physical activity (LTPA) and metabolic health interact with resting fat oxidation (RFO) and peak fat oxidation (PFO) during ergometer cycling.

METHODS

We recruited 23 male monozygotic twin pairs (aged 32-37 years) and determined their RFO and PFO with indirect calorimetry for 21 and 19 twin pairs and for 43 and 41 twin individuals, respectively. Using physical activity interviews and the Baecke questionnaire, we identified 10 twin pairs as LTPA discordant for the past 3 years. Of the twin pairs, 8 pairs participated in both RFO and PFO measurements, and 2 pairs participated in either of the measurements. We quantified the participants' metabolic health with a 2-h oral glucose tolerance test.

RESULTS

Fat oxidation within co-twins was correlated at rest [intraclass correlation coefficient (ICC) = 0.54, 95% confidence interval (CI) 0.15-0.78] and during exercise (ICC = 0.67, 95% CI 0.33-0.86). The LTPA-discordant pairs had no pairwise differences in RFO or PFO. In the twin individual-based analysis, PFO was positively correlated with the past 12-month LTPA (r = 0.26, p = 0.034) and the Baecke score (r = 0.40, p = 0.022) and negatively correlated with the area under the curve of insulin (r = - 0.42, p = 0.015) and glucose (r = - 0.31, p = 0.050) during the oral glucose tolerance test.

CONCLUSIONS

Hereditary factors were more important than LTPA for determining fat oxidation at rest and during exercise. Additionally, PFO, but not RFO, was associated with better metabolic health.

Assessment of maximal fat oxidation during exercise: A systematic review

Maximal fat oxidation during exercise (MFO) and the exercise intensity eliciting MFO (Fat_max ) are considered biological markers of metabolic health and performance. A wide range of studies have been performed to increase our knowledge about their regulation by exercise and/or nutritional intervention. However, numerous data collection and analysis approaches have been applied, which may have affected the MFO and Fat_max estimation. We aimed to systematically review the available studies describing and/or comparing different data collection and analysis approach factors that could affect MFO and Fat_max estimation in healthy individuals and patients. Two independent researchers performed the search. We included all original studies in which MFO and/or Fat_max were estimated by indirect calorimetry through an incremental graded exercise protocol published from 2002 to 2019. This systematic review provides key information about the factors that could affect MFO and Fat_max estimation: ergometer type, metabolic cart used, warm-up duration and intensity, stage duration and intensities imposed in the graded exercise protocol, time interval selected for data analysis, stoichiometric equation selected to estimate fat oxidation, data analysis approach, time of the day when the test was performed, fasting time/previous meal before the test, and testing days for MFO/Fat_max and maximal oxygen uptake assessment. We suggest that researchers measuring MFO and Fat_max should take into account these key methodological issues that can considerably affect the accuracy, validity, and reliability of the measurement. Likewise, when comparing different studies, it is important to check whether the above-mentioned key methodological issues are similar in such studies to avoid ambiguous and unacceptable comparisons.