Effect of training status on regional disposal of circulating free fatty acids in the liver and skeletal muscle during physiological hyperinsulinemia

Mechanisms underpinning the effect of exercise on the non-alcoholic fatty liver disease: review

Non-alcoholic Fatty Liver Disease (NAFLD) - whose terminology was recently replaced by metabolic liver disease (MAFLD) - is an accumulation of triglycerides in the liver of >5 % of its weight. Epidemiological studies indicated an association between NAFLD and reduced physical activity. In addition, exercise has been shown to improve NAFLD independently of weight loss. In this paper, we aim to systematically review molecular changes in sedentary experimental NAFLD models vs. those subjected to exercise. We utilized the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) checklist and standard review techniques. Studies were considered for inclusion if they addressed the primary question: the mechanisms by which exercise influenced NAFLD. This review summarized experimental evidence of improvements in NAFLD with exercise in the absence of weight loss. The pathways involved appeared to have AMPK as a common denominator. See also the graphical abstract(Fig. 1).

β-Cell function during a high-fat meal in young versus old adults: role of exercise

The acute effect of exercise on β-cell function during a high-fat meal (HFM) in young adults (YA) versus old adults (OA) is unclear. In this randomized crossover trial, YA (n = 5 M/7 F, 23.3 ± 3.9 yr) and OA (n = 8 M/4 F, 67.7 ± 6.0 yr) underwent a 180-min HFM (12 kcal/kg body wt; 57% fat, 37% CHO) after a rest or exercise [∼65% heart rate peak (HRpeak)] condition ∼12 h earlier. After an overnight fast, plasma lipids, glucose, insulin, and free fatty acid (FFA) were determined to estimate peripheral, or skeletal muscle, insulin sensitivity (Matsuda index) as well as hepatic [homeostatic model assessment of insulin resistance (HOMA-IR)] and adipose insulin resistance (adipose-IR). β-Cell function was derived from C-peptide and defined as early-phase (0-30 min) and total-phase (0-180 min) disposition index [DI, glucose-stimulated insulin secretion (GSIS) adjusted for insulin sensitivity/resistance]. Hepatic insulin extraction (HIE), body composition [dual-energy X-ray absorptiometry (DXA)], and peak oxygen consumption (V̇o2peak) were also assessed. OA had higher total cholesterol (TC), LDL, HIE, and DI across organs as well as lower adipose-IR (all, P < 0.05) and V̇o2peak (P = 0.056) despite similar body composition and glucose tolerance. Exercise lowered early-phase TC and LDL in OA versus YA (P < 0.05). However, C-peptide area under the curve (AUC), total phase GSIS, and adipose-IR were reduced postexercise in YA versus OA (P < 0.05). Skeletal muscle DI increased in YA and OA after exercise (P < 0.05), whereas adipose DI tended to decline in OA (P = 0.06 and P = 0.08). Exercise-induced skeletal muscle insulin sensitivity (r = -0.44, P = 0.02) and total-phase DI (r = -0.65, P = 0.005) correlated with reduced glucose AUC180min. Together, exercise improved skeletal muscle insulin sensitivity/DI in relation to glucose tolerance in YA and OA, but only raised adipose-IR and reduced adipose-DI in OA.NEW & NOTEWORTHY High-fat diets may induce β-cell dysfunction. This study compared how young and older adults responded to a high-fat meal with regard to β-cell function and whether exercise comparably impacted glucose regulation. Older adults secreted more insulin during the high-fat meal than younger adults. Although exercise increased β-cell function adjusted for skeletal muscle insulin sensitivity in relation to glucose tolerance, it raised adipose insulin resistance and reduced pancreatic β-cell function relative to adipose tissue in older adults. Additional work is needed to discern nutrient-exercise interactions across age to mitigate chronic disease risk.

Multisystem physiological perspective of human frailty and its modulation by physical activity

"Frailty" is a term used to refer to a state characterized by enhanced vulnerability to, and impaired recovery from, stressors compared with a nonfrail state, which is increasingly viewed as a loss of resilience. With increasing life expectancy and the associated rise in years spent with physical frailty, there is a need to understand the clinical and physiological features of frailty and the factors driving it. We describe the clinical definitions of age-related frailty and their limitations in allowing us to understand the pathogenesis of this prevalent condition. Given that age-related frailty manifests in the form of functional declines such as poor balance, falls, and immobility, as an alternative we view frailty from a physiological viewpoint and describe what is known of the organ-based components of frailty, including adiposity, the brain, and neuromuscular, skeletal muscle, immune, and cardiovascular systems, as individual systems and as components in multisystem dysregulation. By doing so we aim to highlight current understanding of the physiological phenotype of frailty and reveal key knowledge gaps and potential mechanistic drivers of the trajectory to frailty. We also review the studies in humans that have intervened with exercise to reduce frailty. We conclude that more longitudinal and interventional clinical studies are required in older adults. Such observational studies should interrogate the progression from a nonfrail to a frail state, assessing individual elements of frailty to produce a deep physiological phenotype of the syndrome. The findings will identify mechanistic drivers of frailty and allow targeted interventions to diminish frailty progression.

Effects of Interrupting Prolonged Sitting with Physical Activity Breaks on Blood Glucose, Insulin and Triacylglycerol Measures: A Systematic Review and Meta-analysis

BACKGROUND

Physical activity (PA) breaks in sitting time might attenuate metabolic markers relevant to the prevention of type 2 diabetes.

OBJECTIVES

The primary aim of this paper was to systematically review and meta-analyse trials that compared the effects of breaking up prolonged sitting with bouts of PA throughout the day (INT) versus continuous sitting (SIT) on glucose, insulin and triacylglycerol (TAG) measures. A second aim was to compare the effects of INT versus continuous exercise (EX) on glucose, insulin and TAG measures.

METHODS

The review followed the Preferred Reporting Items for Systematic Reviews and Meta-analyses (PRISMA) recommendations. Eligibility criteria consisted of trials comparing INT vs. SIT or INT vs. one bout of EX before or after sitting, in participants aged 18 or above, who were classified as either metabolically healthy or impaired, but not with other major health conditions such as chronic obstructive pulmonary disease or peripheral arterial disease.

RESULTS

A total of 42 studies were included in the overall review, whereas a total of 37 studies were included in the meta-analysis. There was a standardised mean difference (SMD) of - 0.54 (95% CI - 0.70, - 0.37, p = 0.00001) in favour of INT compared to SIT for glucose. With respect to insulin, there was an SMD of - 0.56 (95% CI - 0.74, - 0.38, p = 0.00001) in favour of INT. For TAG, there was an SMD of - 0.26 (95% CI - 0.44, - 0.09, p = 0.002) in favour of INT. Body mass index (BMI) was associated with glucose responses (β = - 0.05, 95% CI - 0.09, - 0.01, p = 0.01), and insulin (β = - 0.05, 95% CI - 0.10, - 0.006, p = 0.03), but not TAG (β = 0.02, 95% CI - 0.02, 0.06, p = 0.37). When energy expenditure was matched, there was an SMD of - 0.26 (95% CI - 0.50, - 0.02, p = 0.03) in favour of INT for glucose, but no statistically significant SMDs for insulin, i.e. 0.35 (95% CI - 0.37, 1.07, p = 0.35), or TAG i.e. 0.08 (95% CI - 0.22, 0.37, p = 0.62). It is worth noting that there was possible publication bias for TAG outcomes when PA breaks were compared with sitting.

CONCLUSION

The use of PA breaks during sitting moderately attenuated post-prandial glucose, insulin, and TAG, with greater glycaemic attenuation in people with higher BMI. There was a statistically significant small advantage for PA breaks over continuous exercise for attenuating glucose measures when exercise protocols were energy matched, but no statistically significant differences for insulin and TAG. PROSPERO Registration: CRD42017080982.

PROSPERO REGISTRATION

CRD42017080982.

Adipose tissue and liver

Adipose tissue and liver are central tissues in whole body energy metabolism. Their composition, structure, and function can be noninvasively imaged using a variety of measurement techniques that provide a safe alternative to an invasive biopsy. Imaging of adipose tissue is focused on quantitating the distribution of adipose tissue in subcutaneous and intra-abdominal (visceral) adipose tissue depots. Also, detailed subdivisions of adipose tissue can be distinguished with modern imaging techniques. Adipose tissue (or adipocyte) accumulation or infiltration of other organs can also be imaged, with intramuscular adipose tissue a common example. Although liver fat content is now accurately imaged using standard magnetic resonance imaging (MRI) techniques, inflammation and fibrosis are more difficult to determine noninvasively. Liver imaging efforts are therefore concerted on developing accurate imaging markers of liver fibrosis and inflammatory status. Magnetic resonance elastography (MRE) is presently the most reliable imaging technique for measuring liver fibrosis but requires an external device for introduction of shear waves to the liver. Methods using multiparametric diffusion, perfusion, relaxometry, and hepatocyte-specific MRI contrast agents may prove to be more easily implemented by clinicians, provided they reach similar accuracy as MRE. Adipose tissue imaging is experiencing a revolution with renewed interest in characterizing and identifying distinct adipose depots, among them brown adipose tissue. Magnetic resonance spectroscopy provides an interesting yet underutilized way of imaging adipose tissue metabolism through its fatty acid composition. Further studies may shed light on the role of fatty acid composition in different depots and why saturated fat in subcutaneous adipose tissue is a marker of high insulin sensitivity.

Effect of acute physiological free fatty acid elevation in the context of hyperinsulinemia on fiber type-specific IMCL accumulation

It is well described that increasing free fatty acids (FFAs) to high physiological levels reduces insulin sensitivity. In sedentary humans, intramyocellular lipid (IMCL) is inversely related to insulin sensitivity. Since muscle fiber composition affects muscle metabolism, whether FFAs induce IMCL accumulation in a fiber type-specific manner remains unknown. We hypothesized that in the setting of acute FFA elevation by lipid infusion within the context of a hyperinsulinemic-euglycemic clamp, IMCL will preferentially accumulate in type 1 fibers. Normal-weight participants (n = 57, mean ± SE: age 24 ± 0.6 yr, BMI 22.2 ± 0.3 kg/m²) who were either endurance trained or sedentary by self-report were recruited from the University of Minnesota (n = 31, n = 15 trained) and University of Pittsburgh (n = 26, n = 14 trained). All participants underwent a hyperinsulinemic-euglycemic clamp in the context of a 6-h infusion of either lipid or glycerol control. A vastus lateralis muscle biopsy was obtained at baseline and end-infusion (6 h). The muscle biopsies were processed and analyzed at the University of Pittsburgh for fiber type-specific IMCL accumulation by Oil-Red-O staining. Regardless of training status, acute elevation of FFAs to high physiological levels (~400-600 meq/l) increased IMCL preferentially in type 1 fibers (+35 ± 11% compared with baseline, +29 ± 11% compared with glycerol control: P < 0.05). The increase in IMCL correlated with a decline in insulin sensitivity as measured by the hyperinsulinemic-euglycemic clamp (r = -0.32, P < 0.01) independent of training status. Regardless of training status, increase of FFAs to a physiological range within the context of hyperinsulinemia shows preferential IMCL accumulation in type 1 fibers.NEW & NOTEWORTHY This novel human study examined the effects of FFA elevation in the setting of hyperinsulinemia on accumulation of fat in specific types of muscle fibers. Within the context of the hyperinsulinemic-euglycemic clamp, we found that an increase of FFAs to a physiological range sufficient to reduce insulin sensitivity is associated with preferential IMCL accumulation in type 1 fibers.

Effects of exercise training on intrahepatic lipid content in humans

Non-alcoholic fatty liver (NAFL) is the most common liver disorder in western society. Various factors may play a role in determining hepatic fat content, such as delivery of lipids to the liver, de novo lipogenesis, hepatic lipid oxidation, secretion of intrahepatic lipids to the circulation or a combination of these. If delivery of lipids to the liver outweighs the sum of hepatic lipid oxidation and secretion, the intrahepatic lipid (IHL) content starts to increase and NAFL may develop. NAFL is closely related to obesity and insulin resistance and a fatty liver increases the vulnerability to type 2 diabetes development. Exercise training is a cornerstone in the treatment and prevention of type 2 diabetes. There is a large body of literature describing the beneficial metabolic consequences of exercise training on skeletal muscle metabolism. Recent studies have started to investigate the effects of exercise training on liver metabolism but data is still limited. Here, first, we briefly discuss the routes by which IHL content is modulated. Second, we review whether and how these contributing routes might be modulated by long-term exercise training. Third, we focus on the effects of acute exercise on IHL metabolism, since exercise also might affect hepatic metabolism in the physically active state. This will give insight into whether the effect of exercise training on IHL could be explained by the accumulated effect of acute bouts of exercise, or whether adaptations might occur only after long-term exercise training. The primary focus of this review will be on observations made in humans. Where human data is missing, data obtained from well-accepted animal models will be used.

Organ-specific physiological responses to acute physical exercise and long-term training in humans

Virtually all tissues in the human body rely on aerobic metabolism for energy production and are therefore critically dependent on continuous supply of oxygen. Oxygen is provided by blood flow, and, in essence, changes in organ perfusion are also closely associated with alterations in tissue metabolism. In response to acute exercise, blood flow is markedly increased in contracting skeletal muscles and myocardium, but perfusion in other organs (brain and bone) is only slightly enhanced or is even reduced (visceral organs). Despite largely unchanged metabolism and perfusion, repeated exposures to altered hemodynamics and hormonal milieu produced by acute exercise, long-term exercise training appears to be capable of inducing effects also in tissues other than muscles that may yield health benefits. However, the physiological adaptations and driving-force mechanisms in organs such as brain, liver, pancreas, gut, bone, and adipose tissue, remain largely obscure in humans. Along these lines, this review integrates current information on physiological responses to acute exercise and to long-term physical training in major metabolically active human organs. Knowledge is mostly provided based on the state-of-the-art, noninvasive human imaging studies, and directions for future novel research are proposed throughout the review.

Nutrition and physical activity in NAFLD: an overview of the epidemiological evidence

Nonalcoholic fatty liver disease (NAFLD) has been recognized as a major health burden. The high prevalence of NAFLD is probably due to the contemporary epidemics of obesity, unhealthy dietary pattern, and sedentary lifestyle. The efficacy and safety profile of pharmacotherapy in the treatment of NAFLD remains uncertain and obesity is strongly associated with hepatic steatosis; therefore, the first line of treatment is lifestyle modification. The usual management of NAFLD includes gradual weight reduction and increased physical activity (PA) leading to an improvement in serum liver enzymes, reduced hepatic fatty infiltration, and, in some cases, a reduced degree of hepatic inflammation and fibrosis. Nutrition has been demonstrated to be associated with NAFLD and Non-alcoholic steatohepatitis (NASH) in both animals and humans, and thus serves as a major route of prevention and treatment. However, most human studies are observational and retrospective, allowing limited inference about causal associations. Large prospective studies and clinical trials are now needed to establish a causal relationship. Based on available data, patients should optimally achieve a 5%-10% weight reduction. Setting realistic goals is essential for long-term successful lifestyle modification and more effort must be devoted to informing NAFLD patients of the health benefits of even a modest weight reduction. Furthermore, all NAFLD patients, whether obese or of normal weight, should be informed that a healthy diet has benefits beyond weight reduction. They should be advised to reduce saturated/trans fat and increase polyunsaturated fat, with special emphasize on omega-3 fatty acids. They should reduce added sugar to its minimum, try to avoid soft drinks containing sugar, including fruit juices that contain a lot of fructose, and increase their fiber intake. For the heavy meat eaters, especially those of red and processed meats, less meat and increased fish intake should be recommended. Minimizing fast food intake will also help maintain a healthy diet. PA should be integrated into behavioral therapy in NAFLD, as even small gains in PA and fitness may have significant health benefits. Potentially therapeutic dietary supplements are vitamin E and vitamin D, but both warrant further research. Unbalanced nutrition is not only strongly associated with NAFLD, but is also a risk factor that a large portion of the population is exposed to. Therefore, it is important to identify dietary patterns that will serve as modifiable risk factors for the prevention of NAFLD and its complications.

Does physical inactivity cause nonalcoholic fatty liver disease?

While physical activity represents a key element in the prevention and management of many chronic diseases, we and others believe that physical inactivity is a primary cause of obesity and associated metabolic disorders. Unfortunately, accumulating evidence suggests that we have engineered physical activity out of our normal daily living activity. One such consequence of our sedentary and excessive lifestyle is nonalcoholic fatty liver disease (NAFLD), which is now considered the most common cause of chronic liver disease in Westernized societies. In this review, we will present evidence that physical inactivity, low aerobic fitness, and overnutrition, either separately or in combination, are an underlying cause of NAFLD.

Exercise and fat accumulation in the human liver

PURPOSE OF REVIEW

Fat accumulation in the liver is strongly associated with metabolic dysfunction. Regular exercise improves many cardiometabolic risks factors; however, its effect on intrahepatic triglyceride (IHTG) content remains elusive. This article summarizes available data regarding the effects of exercise on IHTG.

RECENT FINDINGS

Several but not all observational studies report negative associations of habitual physical activity and cardiorespiratory fitness with IHTG and the prevalence of fatty liver. Aerobic exercise training in combination with hypocaloric diet reduces IHTG by a considerable amount (20-60%), even when weight loss is mild (<5%); hence weight loss per se may not be a critical factor. Longitudinal studies involving exercise training without dietary restriction and no weight loss demonstrate that increased cardiorespiratory fitness and reduced intra-abdominal adiposity are not invariably associated with liver fat depletion, whereas relatively large exercise-induced reductions in IHTG content (20-40%) can occur even in the absence of changes in body weight, body composition, or visceral adipose tissue. Although the majority of studies have examined aerobic training, resistance exercise has also been shown to be inversely associated with the prevalence of fatty liver in humans and effectively reduces IHTG content in animals.

SUMMARY

Exercise does hold promise as an effective treatment for hepatic steatosis; this field of research is still in its infancy, and there is much more to be learned.

Free fatty acid-induced hepatic insulin resistance is attenuated following lifestyle intervention in obese individuals with impaired glucose tolerance

OBJECTIVE

The objective of the study was to examine the effects of an exercise/diet lifestyle intervention on free fatty acid (FFA)-induced hepatic insulin resistance in obese humans.

RESEARCH DESIGN AND METHODS

Obese men and women (n = 23) with impaired glucose tolerance were randomly assigned to either exercise training with a eucaloric (EU; approximately 1800 kcal; n = 11) or hypocaloric (HYPO; approximately 1300 kcal; n = 12) diet for 12 wk. Hepatic glucose production (HGP; milligrams per kilogram fat-free mass(-1) per minute(-1)) and hepatic insulin resistance were determined using a two-stage sequential hyperinsulinemic (40 mU/m(2) . min(-1)) euglycemic (5.0 mm) clamp with [3-(3)H]glucose. Measures were obtained at basal, during insulin infusion (INS; 120 min), and insulin plus intralipid/heparin infusion (INS/FFA; 300 min).

RESULTS

At baseline, basal HGP was similar between groups; hyperinsulinemia alone did not completely suppress HGP, whereas INS/FFA exhibited less suppression than INS (EU, 4.6 +/- 0.8, 2.0 +/- 0.5, and 2.6 +/- 0.4; HYPO, 3.8 +/- 0.5, 1.2 +/- 0.3, and 2.3 +/- 0.4, respectively). After the intervention the HYPO group lost more body weight (P < 0.05) and fat mass (P < 0.05). However, both lifestyle interventions reduced hepatic insulin resistance during basal (P = 0.005) and INS (P = 0.001) conditions, and insulin-mediated suppression of HGP during INS was equally improved in both groups (EU: -42 +/- 22%; HYPO: -50 +/- 20%, before vs. after, P = 0.02). In contrast, the ability of insulin to overcome FFA-induced hepatic insulin resistance and HGP was improved only in the HYPO group (EU: -15 +/- 24% vs. HYPO: -58 +/- 19%, P = 0.02).

CONCLUSIONS

Both lifestyle interventions are effective in reducing hepatic insulin resistance under basal and hyperinsulinemic conditions. However, the reversal of FFA-induced hepatic insulin resistance is best achieved with a combined exercise/caloric-restriction intervention.

Role of leisure-time physical activity in nonalcoholic fatty liver disease: a population-based study

Physical activity (PA) is commonly recommended for nonalchoholic fatty liver disease (NAFLD) patients. However, there is limited evidence on the independent role of PA in NAFLD. The aim of this study was to examine the association between PA and NAFLD. We conducted a cross-sectional study of a subsample (n = 375) of the Israeli National Health and Nutrition Survey. Exclusion criteria were any known etiology for liver disease. Participants underwent an abdominal ultrasound examination; biochemical tests, including leptin, adiponectin, and resistin; and the noninvasive biomarker SteatoTest and anthropometric evaluations. A semiquantitative food frequency questionnaire and a detailed PA questionnaire were administered. Three hundred forty-nine patients (52.7% men, 30.9% primary NAFLD) were included. The NAFLD group engaged in less aerobic, resistance, or other kinds of PA (P </= 0.03). The SteatoTest was significantly lower among subjects engaging in any PA or resistance PA at least once a week (P </= 0.01). PA at least once a week in all categories was associated with a reduced risk for abdominal obesity. Adjusting for sex, engaging in any kind of sports (odds ratio [OR] 0.66, 95% confidence interval [CI] 0.44-0.96 per 1 standard deviation increment in PA score) and resistance exercise (OR 0.61, 95% CI 0.38-0.85) were inversely associated with NAFLD. These associations remained unchanged after adjusting for homeostasis model assessment, most nutritional factors, adiponectin, and resistin. Only the association with resistance PA remained significant with further adjustment for body mass index (OR 0.61, 95% CI 0.44-0.85). Adding leptin or waist circumference to the model eliminated the statistical significance.

CONCLUSION

Habitual leisure-time PA, especially anaerobic, may play a protective role in NAFLD. This association appears to be mediated by a reduced rate of abdominal obesity.

Prospective study of body size and risk for stroke amongst women below age 60

The relation between obesity, particularly abdominal obesity, and risk of stroke amongst women remains unclear. In 1991-1992, a prospective study was initiated in Sweden amongst women who returned a self-administered questionnaire. Through linkage with nation-wide registries, 45,449 women, free of stroke at entry, were followed up until diagnosis of first incident stroke, death, or the end of follow-up in 2002. We estimated multivariate relative risks (RRs) with 95% confidence intervals (CIs) from Cox proportional hazards regression models. A total of 170 incident stroke cases occurred during an average of 11 years of follow-up. The RR of stroke amongst women in the highest compared with the lowest quintile was 2.4 (95% CI 1.3-4.2; P for trend 0.04) for waist-to-hip ratio, 2.5 (95% CI 1.5-4.3; P for trend 0.01) for waist-to-height ratio and 2.3 (95% CI 1.2-4.3; P for trend 0.02) for waist circumference. Adjustment for hypertension and diabetes attenuated these risk estimates. In contrast, birth weight, body mass index (BMI) at age 18, BMI at entry, weight change in adulthood and adult height were not significantly associated with risk of stroke. This study provides evidence that, in contrast to BMI, several different measures of abdominal obesity are strong predictors of stroke in women.

Increased physical activity decreases hepatic free fatty acid uptake: a study in human monozygotic twins

Exercise is considered to be beneficial for free fatty acid (FFA) metabolism, although reports of the effects of increased physical activity on FFA uptake and oxidation in different tissues in vivo in humans have been inconsistent. To investigate the heredity-independent effects of physical activity and fitness on FFA uptake in skeletal muscle, the myocardium, and liver we used positron emission tomography (PET) in nine healthy young male monozygotic twin pairs discordant for physical activity and fitness. The cotwins with higher physical activity constituting the more active group had a similar body mass index but less body fat and 18 +/- 10% higher (P < 0.001) compared to the less active brothers with lower physical activity. Low-intensity knee-extension exercise increased skeletal muscle FFA and oxygen uptake six to 10 times compared to resting values but no differences were observed between the groups at rest or during exercise. At rest the more active group had lower hepatic FFA uptake compared to the less active group (5.5 +/- 4.3 versus 9.0 +/- 6.1 micromol (100 ml)(-1) min(-1), P = 0.04). Hepatic FFA uptake associated significantly with body fat percentage (P = 0.05). Myocardial FFA uptake was similar between the groups. In conclusion, in the absence of the confounding effects of genetic factors, moderately increased physical activity and aerobic fitness decrease body adiposity even in normal-weighted healthy young adult men. Further, increased physical activity together with decreased intra-abdominal adiposity seems to decrease hepatic FFA uptake but has no effects on skeletal muscle or myocardial FFA uptake.