The effect of a 3-month low-intensity endurance training program on fat oxidation and acetyl-CoA carboxylase-2 expression

Substrate source use in older, trained males after decades of endurance training

PURPOSE

The purpose of this study was to compare substrate source use in older, long-term exercising, endurance-trained males with sedentary controls.

METHODS

[U-C]palmitate and [6,6-H2]glucose tracers were applied to assess plasma free fatty acid (FFA) and glucose oxidation rates, and to estimate muscle- and/or lipoprotein-derived triacylglycerol (TG) and muscle glycogen use. Subjects were 10 long-term exercising, endurance-trained males and 10 sedentary controls (age 57 +/- 1 and 60 +/- 2 yr, respectively). Muscle biopsy samples were collected before and after exercise to assess muscle fiber type-specific intramyocellular lipid and glycogen content.

RESULTS

During exercise, plasma palmitate Ra, Rd, and Rox were significantly greater in the trained subjects compared with the controls (Ra: 0.36 +/- 0.02 and 0.25 +/- 0.02; Rd: 0.36 +/- 0.03 and 0.24 +/- 0.02; Rox: 0.31 +/- 0.02 and 0.20 +/- 0.02 mmol.min, respectively, P < 0.01). This resulted in greater plasma FFA and total fat oxidation rates in the trained versus sedentary subjects (P < 0.001). Muscle- and/or lipoprotein-derived TG use contributed 10 +/- 2 and 11 +/- 3% in the trained and control groups, respectively (NS). No significant net changes in muscle fiber lipid content were observed.

CONCLUSIONS

Older, endurance-trained males oxidize more fat during moderate-intensity exercise than do sedentary controls. This greater total fat oxidation rate is attributed to a higher plasma FFA release, uptake, and oxidation rate. In contrast, intramyocellular triacylglycerol does not seem to represent a major substrate source during 1 h of moderate-intensity exercise in older trained or sedentary men.

Effect of training in the fasted state on metabolic responses during exercise with carbohydrate intake

Skeletal muscle gene response to exercise depends on nutritional status during and after exercise, but it is unknown whether muscle adaptations to endurance training are affected by nutritional status during training sessions. Therefore, this study investigated the effect of an endurance training program (6 wk, 3 day/wk, 1-2 h, 75% of peak Vo(2)) in moderately active males. They trained in the fasted (F; n = 10) or carbohydrate-fed state (CHO; n = 10) while receiving a standardized diet [65 percent of total energy intake (En) from carbohydrates, 20%En fat, 15%En protein]. Before and after the training period, substrate use during a 2-h exercise bout was determined. During these experimental sessions, all subjects were in a fed condition and received extra carbohydrates (1 g.kg body wt(-1) .h(-1)). Peak Vo(2) (+7%), succinate dehydrogenase activity, GLUT4, and hexokinase II content were similarly increased between F and CHO. Fatty acid binding protein (FABPm) content increased significantly in F (P = 0.007). Intramyocellular triglyceride content (IMCL) remained unchanged in both groups. After training, pre-exercise glycogen content was higher in CHO (545 +/- 19 mmol/kg dry wt; P = 0.02), but not in F (434 +/- 32 mmol/kg dry wt; P = 0.23). For a given initial glycogen content, F blunted exercise-induced glycogen breakdown when compared with CHO (P = 0.04). Neither IMCL breakdown (P = 0.23) nor fat oxidation rates during exercise were altered by training. Thus short-term training elicits similar adaptations in peak Vo(2) whether carried out in the fasted or carbohydrate-fed state. Although there was a decrease in exercise-induced glycogen breakdown and an increase in proteins involved in fat handling after fasting training, fat oxidation during exercise with carbohydrate intake was not changed.

Optimizing fat oxidation through exercise in severely obese Caucasian adolescents

OBJECTIVE

To measure the contribution of substrate oxidation to energy expenditure during cycling at different workloads and to identify the exercise intensity that elicits the maximum fat oxidation rate in groups of severely obese or nonobese Caucasian adolescents.

DESIGN

A total of 30 severely obese subjects (mean body mass index, BMI = 34.7 kg/m2; fat-mass = 39.9%) and 30 nonobese sedentary adolescents (mean BMI = 22.7 kg/m2; fat-mass = 21.8%) aged 14-16 years, participated in this study. Body composition was assessed by bioelectrical impedance. Peak oxygen uptake (VO2peak) and maximal fat oxidation rate were determined with indirect calorimetry by using a graded exercise test on an electromagnetically braked cycle ergometer.

RESULTS

Predicted VO2max were expressed in absolute (l/min) and relative (ml/kg FFM/min) values, and maximal work rates were not significantly different between obese and nonobese adolescents, but were significantly higher in boys than in girls. No significant differences in fat oxidation rates were found in obese and nonobese sedentary adolescents during the graded exercise test. Maximal fat oxidation was observed at an exercise intensity corresponding to (mean +/- SD) 41 +/- 3%VO2max or 58 +/- 3% HRmax. At this exercise intensity, fat oxidation rates were higher in boys than in girls (0.32 +/- 0.02 g/min vs. 0.25 +/- 0.02 g/min, P < 0.001).

CONCLUSIONS

Severely obese and sedentary nonobese adolescents reached maximal fat oxidation rates at 41%VO2max, which corresponds to 58% HRmax. At this exercise intensity, fat oxidation rates were higher in boys than in girls probably due to higher VO2max and absolute workload during the exercise steps for boys compared with those for girls.

Linkage of genes to total lean body mass in normal women

BACKGROUND

Total lean body mass (LEAN-tot) is one of the three major components of body weight. Its deterioration is a risk factor for frailty. Despite this, there are few studies examining the contribution of genetic factors.

OBJECTIVE

Our objective was to examine the contribution of genetic factors for LEAN-tot variation, including a genome-wide search for the genes.

RESEARCH METHODS

Dual-energy x-ray absorptiometry measurements of LEAN-tot were obtained from each of the 3180 United Kingdom females (509 monozygotic and 1081 dizygotic twin pairs). Contribution of genetic factors was assessed using variance component analysis. A genome-wide linkage analysis was performed on the dizygotic twins using a modified version of the Haseman-Elston method.

RESULTS

Age, body height, total fat, and bone mass were correlated with LEAN-tot, and commonly explained 52% of the LEAN-tot variation. The crude heritability estimate was 74.0 +/- 4.0%, after adjustment for the aforementioned factors; 65.2 +/- 4.6% was attributable to independent genetic effects. Significant (P < 0.001) genetic correlations were found between LEAN-tot and bone mass, and LEAN-tot and total fat. Adjusted only for age, LEAN-tot showed no significant linkage. After adjustment for all covariates, significant linkage (LOD = 4.49 and 3.62) was observed at chromosome 12q24.3 and 14q22.3, respectively. Additional peaks of interest were on 7p15.3-15.1 (LOD = 2.86) and 8p22 (LOD = 2.83).

CONCLUSIONS

LEAN-tot measured by dual-energy x-ray absorptiometry is highly heritable, independent of other body measures. This first genomic search for genes associated with the lean component of body mass suggests significant linkage to quantitative trait loci on chromosomes 12 and 14.

High-fat diet, muscular lipotoxicity and insulin resistance

A high dietary fat intake and low physical activity characterize the current Western lifestyle. Dietary fatty acids do not stimulate their own oxidation and a surplus of fat is stored in white adipose tissue, liver, heart and muscle. In these organs intracellular lipids serve as a rapidly-available energy source during, for example, physical activity. However, under conditions of elevated plasma fatty acid levels and high dietary fat intake, conditions implicated in the development of modern diseases such as obesity and type 2 diabetes mellitus, fat accumulation in liver and muscle (intramyocellular lipids; IMCL) is associated with the development of insulin resistance. Recent data suggest that IMCL are specifically harmful when combined with reduced mitochondrial function, both conditions that characterize type 2 diabetes. In the (pre)diabetic state reduced expression of the transcription factor PPARgamma co-activator-1alpha (PGC-1alpha), which is involved in mitochondrial biogenesis, has been suggested to underlie the reduced mitochondrial function. Importantly, the reduction in PGC-1alpha may be a result of low physical activity, consumption of high-fat diets and high plasma fatty acid levels. Mitochondrial function can also be impaired as a result of enhanced mitochondrial damage by reactive oxygen species. Fatty acids in the vicinity of mitochondria are particularly prone to lipid peroxidation. In turn, lipid peroxides can induce oxidative damage to mitochondrial RNA, DNA and proteins. The mitochondrial protein uncoupling protein 3, which is induced under high-fat conditions, may serve to protect mitochondria against lipid-induced oxidative damage, but is reduced in the prediabetic state. Thus, muscular lipotoxicity may impair mitochondrial function and may be central to insulin resistance and type 2 diabetes mellitus.

Combining fish-oil supplements with regular aerobic exercise improves body composition and cardiovascular disease risk factors

BACKGROUND

Regular exercise and consuming long-chain n-3 fatty acids (FAs) from fish or fish oil can independently improve cardiovascular and metabolic health, but combining these lifestyle modifications may be more effective than either treatment alone.

OBJECTIVE

We examined the individual and combined effects of n-3 FA supplements and regular exercise on body composition and cardiovascular health.

DESIGN

Overweight volunteers [body mass index (BMI; in kg/m(2)): >25] with high blood pressure, cholesterol, or triacylglycerols were randomly assigned to one of the following interventions: fish oil (FO), FO and exercise (FOX), sunflower oil (SO; control), or SO and exercise (SOX). Subjects consumed 6 g tuna FO/d ( approximately 1.9 g n-3 FA) or 6 g SO/d. The exercise groups walked 3 d/wk for 45 min at 75% age-predicted maximal heart rate. Plasma lipids, blood pressure, and arterial function were assessed at 0, 6, and 12 wk. Body composition was assessed by dual-energy X-ray absorptiometry at 0 and 12 wk only.

RESULTS

FO supplementation lowered triacylglycerols, increased HDL cholesterol, and improved endothelium-dependent arterial vasodilation (P<0.05). Exercise improved arterial compliance (P<0.05). Both fish oil and exercise independently reduced body fat (P<0.05).

CONCLUSIONS

FO supplements and regular exercise both reduce body fat and improve cardiovascular and metabolic health. Increasing intake of n-3 FAs could be a useful adjunct to exercise programs aimed at improving body composition and decreasing cardiovascular disease risk.

Does regular exercise protect against air pollution-associated mortality?

OBJECTIVES

To examine whether habitual exercise would modify the effects of air pollution on mortality.

METHODS

This study included 24,053 Hong Kong Chinese who died at the age of 30 years or older in 1998. Individual information on frequency of exercise was obtained by interviewing their relatives at all four death registries. The deceased subjects were categorized as never-exercise (<once/month) and exercise (> or =once/month). Excess risks (ER) of mortality per 10 microg/m(3) increase of air pollutant were estimated separately by a Poisson regression model in the exercise and never-exercise groups. An interaction model was used to estimate the difference in ER between the two groups.

RESULTS

In people aged 65 years or older and categorized as never-exercise group, there were significant ER of mortality for all natural causes attributed to nitrogen dioxide (p<0.05), ozone (p<0.05) and particulate matter with an aerodynamic diameter of 10 mum or smaller (p<0.01). When compared to the exercise group, the ER were significantly higher by 4.31% (95% confidence interval: 2.57%, 6.03%), 1.75% (0.25%, 3.23%), and 3.06% (1.74%, 4.37%), respectively. The estimates were insensitive to adjustment for socioeconomic, smoking and health status, and were non-linear by different exercise levels.

CONCLUSIONS

The results of this study provide evidence that habitual exercise may prevent premature death attributable to air pollution.

Intramyocellular triacylglycerol as a substrate source during exercise

The role of intramyocellular triacylglycerol (IMTG) as a substrate source during exercise has recently regained much attention as a result of the proposed functional relationship between IMTG accumulation and the development of insulin resistance. It has been speculated that elevated NEFA delivery and/or impaired fatty acid (FA) oxidation result in intramyocellular accumulation of triacylglycerol and FA metabolites, which are likely to induce defects in the insulin signalling cascade, causing insulin resistance. The progressive accumulation of IMTG in sedentary patients and patients who are obese and/or have type 2 diabetes should therefore form a major therapeutic target, and efforts should be made to develop interventions that prevent excess IMTG accretion by stimulating their rate of oxidation. Although regular exercise is likely to represent such an effective means, there is much controversy about the actual contribution of the IMTG pool as a substrate source during exercise. The apparent discrepancy in the published literature might be explained by differences in the applied research protocol and the selected subject population, but most of all by the techniques that have been employed to estimate IMTG use during exercise. Data obtained in trained-endurance athletes indicate that athletes can substantially reduce their IMTG pool following a single exercise session. With the growing awareness that skeletal muscle has a tremendous potential to oxidise IMTG during prolonged moderate-intensity exercise, more research is warranted to develop combined exercise, nutritional and/or pharmacological interventions that can stimulate IMTG oxidation in sedentary patients and patients who are obese and/or have type 2 diabetes.

Uncoupling protein 3 and physical activity: the role of uncoupling protein 3 in energy metabolism revisited

Physical activity influences energy metabolism in human subjects by increasing activity-induced energy expenditure and resting metabolic rate for several hours after exercise. On the other hand, physical activity increases mechanical energy efficiency, suggesting that trained subjects would need less energy for daily activities. The underlying mechanism by which physical activity influences energy metabolism is largely unknown. The skeletal muscle-specific homologue of uncoupling protein (UCP) 1, UCP3, could possibly play a major role in energy expenditure. UCP3 is, like UCP1, able to uncouple respiration from ATP production. A strong link or association between theUCP3gene and energy metabolism was found. Furthermore, UCP3 mRNA expression is related to sleeping metabolic rate, and thyroid hormone, a powerful stimulator of energy expenditure, up regulates UCP3. Finally, mice overexpressing UCP3 are hyperphagic but lean. These findings indicated that UCP3 is related to energy metabolism and that UCP3 could have a role in the effect of physical activity on energy expenditure. Thus, acute exercise up regulates UCP3, whereas endurance training results in the down-regulation of UCP3 protein content. Only a minimal amount of physical activity is needed for down-regulation of UCP3. Moreover, there is very strong evidence that UCP3 is negatively related to mechanical energy efficiency, suggesting that the down-regulation of UCP3 with training increases mechanical energy efficiency. Taken together, although the exact function of UCP3 is still unknown, exercise and training studies clearly show that under certain circumstances UCP3 is strongly related to human energy metabolism, possibly as a secondary effect of its (yet) unknown primary function.

Basic disturbances in skeletal muscle fatty acid metabolism in obesity and type 2 diabetes mellitus

The present article addresses the hypothesis that disturbances in skeletal muscle fatty acid handling in abdominal obesity and type 2 diabetes mellitus may play a role in the aetiology of increased adipose tissue stores, increased triacylglycerol storage in skeletal muscle and skeletal muscle insulin resistance. The uptake and/or oxidation of fatty acids have been shown to be impaired during post-absorptive conditions in abdominally-obese subjects and/or subjects with type 2 diabetes. Also, human studies have shown that muscle of subjects that are (abdominally) obese and/or have type 2 diabetes is characterized by an inability to increase fatty acid uptake and/or fatty acid oxidation during β-adrenergic stimulation and exercise. This disturbance in fat oxidation may promote, on one hand, the development of increased adipose tissue stores and obesity. On the other hand, fatty acids that are taken up by muscle and not oxidized may increase triacylglycerol storage in muscle, which has been associated with skeletal-muscle insulin resistance. Disturbances in the capacity to increase fat oxidation during post-absorptive conditions, β-adrenergic stimulation and exercise in subjects who are obese and/or have type 2 diabetes persist after weight reduction, indicating that the diminished fat oxidation may be a primary factor leading to the obese and/or insulin-resistant state rather than an adaptational response. Clearly, the precise sequence of events leading to an increased adiposity and insulin resistance in obesity and type 2 diabetes mellitus is not yet fully understood.

Two weeks of high-intensity aerobic interval training increases the capacity for fat oxidation during exercise in women

Our aim was to examine the effects of seven high-intensity aerobic interval training (HIIT) sessions over 2 wk on skeletal muscle fuel content, mitochondrial enzyme activities, fatty acid transport proteins, peak O(2) consumption (Vo(2 peak)), and whole body metabolic, hormonal, and cardiovascular responses to exercise. Eight women (22.1 +/- 0.2 yr old, 65.0 +/- 2.2 kg body wt, 2.36 +/- 0.24 l/min Vo(2 peak)) performed a Vo(2 peak) test and a 60-min cycling trial at approximately 60% Vo(2 peak) before and after training. Each session consisted of ten 4-min bouts at approximately 90% Vo(2 peak) with 2 min of rest between intervals. Training increased Vo(2 peak) by 13%. After HIIT, plasma epinephrine and heart rate were lower during the final 30 min of the 60-min cycling trial at approximately 60% pretraining Vo(2 peak). Exercise whole body fat oxidation increased by 36% (from 15.0 +/- 2.4 to 20.4 +/- 2.5 g) after HIIT. Resting muscle glycogen and triacylglycerol contents were unaffected by HIIT, but net glycogen use was reduced during the posttraining 60-min cycling trial. HIIT significantly increased muscle mitochondrial beta-hydroxyacyl-CoA dehydrogenase (15.44 +/- 1.57 and 20.35 +/- 1.40 mmol.min(-1).kg wet mass(-1) before and after training, respectively) and citrate synthase (24.45 +/- 1.89 and 29.31 +/- 1.64 mmol.min(-1).kg wet mass(-1) before and after training, respectively) maximal activities by 32% and 20%, while cytoplasmic hormone-sensitive lipase protein content was not significantly increased. Total muscle plasma membrane fatty acid-binding protein content increased significantly (25%), whereas fatty acid translocase/CD36 content was unaffected after HIIT. In summary, seven sessions of HIIT over 2 wk induced marked increases in whole body and skeletal muscle capacity for fatty acid oxidation during exercise in moderately active women.

Effect of physical inactivity on the oxidation of saturated and monounsaturated dietary Fatty acids: results of a randomized trial

OBJECTIVES

Changes in the way dietary fat is metabolized can be considered causative in obesity. The role of sedentary behavior in this defect has not been determined. We hypothesized that physical inactivity partitions dietary fats toward storage and that a resistance exercise training program mitigates storage.

DESIGN

We used bed rest, with randomization to resistance training, as a model of physical inactivity.

SETTING

The trial took place at the Space Clinic (Toulouse, France).

PARTICIPANTS

A total of 18 healthy male volunteers, of mean age +/- standard deviation 32.6 +/- 4.0 y and body mass index 23.6 +/- 0.7 kg/m(2), were enrolled.

INTERVENTIONS

An initial 15 d of baseline data collection were followed by 3 mo of strict bed-rest alone (control group, n = 9) or with the addition of supine resistance exercise training every 3 d (exercise group, n = 9).

OUTCOME MEASURES

Oxidation of labeled [d(31)]palmitate (the main saturated fatty acid of human diet) and [1-(13)C]oleate (the main monounsaturated fatty acid), body composition, net substrate use, and plasma hormones and metabolites were measured.

RESULTS

Between-group comparisons showed that exercise training did not affect oxidation of both oleate (mean difference 5.6%; 95% confidence interval [95% CI], -3.3% to 14.5%; p = 0.20) and palmitate (mean difference -0.2%; 95% CI, -4.1% to 3.6%; p = 0.89). Within-group comparisons, however, showed that inactivity changed oxidation of palmitate in the control group by -11.0% (95% CI, -19.0% to -2.9%; p = 0.01) and in the exercise group by -11.3% (95% CI, -18.4% to -4.2%; p = 0.008). In contrast, bed rest did not significantly affect oleate oxidation within groups. In the control group, the mean difference in oleate oxidation was 3.2% (95% CI, -4.2% to 10.5%; p = 0.34) and 6.8% (95% CI, -1.2% to 14.7%; p = 0.08) in the exercise group.

CONCLUSIONS

Independent of changes in energy balance (intake and/or output), physical inactivity decreased the oxidation of saturated but not monounsaturated dietary fat. The effect is apparently not compensated by resistance exercise training. These results suggest that Mediterranean diets should be recommended in sedentary subjects and recumbent patients.

Exercise training decreases the concentration of malonyl-CoA and increases the expression and activity of malonyl-CoA decarboxylase in human muscle

The study was designed to evaluate whether changes in malonyl-CoA and the enzymes that govern its concentration occur in human muscle as a result of physical training. Healthy, middle-aged subjects were studied before and after a 12-wk training program that significantly increased VO2 max by 13% and decreased intra-abdominal fat by 17%. Significant decreases (25-30%) in the concentration of malonyl-CoA were observed after training, 24-36 h after the last bout of exercise. They were accompanied by increases in both the activity (88%) and mRNA (51%) of malonyl-CoA decarboxylase (MCD) in muscle but no changes in the phosphorylation of AMP kinase (AMPK, Thr172) or of acetyl-CoA carboxylase. The abundance of peroxisome proliferator-activated receptor (PPAR)gamma coactivator-1alpha (PGC-1alpha), a regulator of transcription that has been linked to the mediation of MCD expression by PPARalpha, was also increased (3-fold). In studies also conducted 24-36 h after the last bout of exercise, no evidence of increased whole body insulin sensitivity or fatty acid oxidation was observed during an euglycemic hyperinsulinemic clamp. In conclusion, the concentration of malonyl-CoA is diminished in muscle after physical training, most likely because of PGC-1alpha-mediated increases in MCD expression and activity. These changes persist after the increases in AMPK activity and whole body insulin sensitivity and fatty acid oxidation, typically caused by an acute bout of exercise in healthy individuals, have dissipated.

Glucose ingestion during exercise blunts exercise-induced gene expression of skeletal muscle fat oxidative genes

Ingestion of carbohydrate during exercise may blunt the stimulation of fat oxidative pathways by raising plasma insulin and glucose concentrations and lowering plasma free fatty acid (FFA) levels, thereby causing a marked shift in substrate oxidation. We investigated the effects of a single 2-h bout of moderate-intensity exercise on the expression of key genes involved in fat and carbohydrate metabolism with or without glucose ingestion in seven healthy untrained men (22.7 +/- 0.6 yr; body mass index: 23.8 +/- 1.0 kg/m(2); maximal O(2) consumption: 3.85 +/- 0.21 l/min). Plasma FFA concentration increased during exercise (P < 0.01) in the fasted state but remained unchanged after glucose ingestion, whereas fat oxidation (indirect calorimetry) was higher in the fasted state vs. glucose feeding (P < 0.05). Except for a significant decrease in the expression of pyruvate dehydrogenase kinase-4 (P < 0.05), glucose ingestion during exercise produced minimal effects on the expression of genes involved in carbohydrate utilization. However, glucose ingestion resulted in a decrease in the expression of genes involved in fatty acid transport and oxidation (CD36, carnitine palmitoyltransferase-1, uncoupling protein 3, and 5'-AMP-activated protein kinase-alpha(2); P < 0.05). In conclusion, glucose ingestion during exercise decreases the expression of genes involved in lipid metabolism rather than increasing genes involved in carbohydrate metabolism.

Increased intramuscular lipid storage in the insulin-resistant and endurance-trained state

Numerous studies have reported a strong correlation between intramuscular triacylglycerol (IMTG) content and insulin resistance. However, the proposed relationship between IMTG accumulation and skeletal muscle insulin resistance is not unambiguous, as trained athletes have been shown to be markedly insulin sensitive despite an elevated IMTG storage. Though the latter has often been attributed to differences in muscle fibre type composition and/or structural characteristics of the intramyocellular lipid deposits, recent studies have failed to provide such evidence. The greater insulin sensitivity despite an elevated IMTG deposition in the endurance-trained state has often been described as a metabolic paradox. However, divergent metabolic events are responsible for the greater IMTG content in the endurance-trained versus insulin-resistant states. The greater IMTG storage in the trained athlete represents an adaptive response to endurance training, allowing a greater contribution of the IMTG pool as a substrate source during exercise. In contrast, elevated IMTG stores in the obese and/or type 2 diabetes patient seem to be secondary to a structural imbalance between plasma free fatty acid availability, fatty acid (FA) storage and oxidation. Therefore, the reported correlation between IMTG content and insulin resistance does not represent a functional relationship, as it is strongly influenced by training status and/or habitual physical activity. It can be argued that the ratio between IMTG content and muscle oxidative capacity represents a more accurate marker of insulin resistance. Interventions to augment mitochondrial density and/or function are likely to improve the balance between FA uptake and oxidation and should be applied to prevent and/or treat insulin resistance.

Metabolic fluxes in skeletal muscle in relation to obesity and insulin resistance

The present article addresses the hypothesis that inefficient skeletal muscle lipid utilization may relate to the development of obesity and insulin resistance. In practice, there is evidence of impaired muscle fatty acid utilization in the obese state, and studies indicate that differences in muscle fatty acid oxidative capacity might play a role in the pathogenesis of obesity. The link between obesity and insulin resistance has been recognized for many years. There is accumulating evidence that a disturbed muscle fat oxidative capacity results in the accumulation of the lipid intermediates diacylglycerol and Fatty acid acyl coenzyme A (CoA), which may interfere with insulin signaling and result in an inhibition of insulin-mediated glucose uptake. Underlying mechanisms for disturbed fatty acid handling may relate to impaired adipose tissue lipolysis, abnormal muscle fatty acid uptake and a reduced ability to oxidize fat. Many of these abnormalities have been reported both in the obese and the type 2 diabetic state. The role of obesity per se in these disturbances is an important question that needs to be addressed.

Use of intramuscular triacylglycerol as a substrate source during exercise in humans

Fat and carbohydrate are the principal substrates that fuel aerobic ATP synthesis in skeletal muscle. Most endogenous fat is stored as triacylglycerol in subcutaneous and deep visceral adipose tissue. Smaller quantities of triacylglycerol are deposited as lipid droplets inside skeletal muscle fibers. The potential role of intramyocellular triacylglycerol (IMTG) as a substrate source during exercise in humans has recently regained much of its interest because of the proposed functional relationship between IMTG accumulation and the development of skeletal muscle insulin resistance. Exercise likely represents an effective means to prevent excess IMTG accretion by stimulating its rate of oxidation. However, there is much controversy on the actual contribution of the IMTG pool as a substrate source during exercise. The apparent discrepancy in the literature likely stems from methodological difficulties that have been associated with the methods used to estimate IMTG oxidation during exercise. However, recent studies using stable isotope methodology, 1H-magnetic resonance spectroscopy, and electron and/or immunofluorescence microscopy all support the contention that the IMTG pool can function as an important substrate source during exercise. Although more research is warranted, IMTG mobilization and/or oxidation during exercise seem to be largely determined by exercise intensity, exercise duration, macronutrient composition of the diet, training status, gender, and/or age. In addition, indirect evidence suggests that the capacity to mobilize and/or oxidize IMTG is substantially impaired in an obese and/or Type 2 diabetic state. As we now become aware that skeletal muscle has an enormous capacity to oxidize IMTG stores during exercise, more research is warranted to develop combined exercise, nutritional, and/or pharmacological interventions to effectively stimulate IMTG oxidation in sedentary, obese, and/or Type 2 diabetes patients.

Do blood cells mimic gene expression profile alterations known to occur in muscular adaptation to endurance training ?

Exercise is known to upregulate mRNA synthesis for carnitine palmitoyl transferase1 (CPT1) and possibly also other mitochondrial carnitine acyltransferases in muscle tissue. The aim of this study was to test whether such an adaptation of oxidative metabolism in skeletal muscle is a systemic process and consequently, also affects other cells. Messenger RNA levels of five genes [carnitine palmitoyl transferases 1 and 2 (CPT1 and CPT2), carnitine acetyltransferase (CRAT), carnitine palmitoyltransferase 2 (CPT2), microsomal carnitine palmitoyltransferase (GRP58) and organic cation transporter (OCTN2)] were determined with quantitative real time polymerase chain reaction (PCR) in blood cells and in muscle biopsy samples from six cross country skiers before and 6 months after a high volume/low intensity exercise training, when training had elicited a significantly slower rate of lactate accumulation. Quantitative real time PCR showed that levels of mRNA in blood cells correlated significantly (CPT1B: P< 0.001) with those in muscle tissue from the same donors. After 6-months training, there was a 15-fold upregulation of CPT1B mRNA, a six to ninefold increase of CRAT mRNA, of CPT2 mRNA, GRP58 mRNA, and of OCTN2 mRNA. The observation of a concordant stimulation of CPT1, CPT2, CRAT, GRP58 and OCTN2 transcription in blood cells and muscle tissue after 6-month-endurance training leads the hypothesis of a common stimulation mechanism other than direct mechanical stress or local chemical environment, but rather humoral factors.

Regulation of metabolic transcriptional co-activators and transcription factors with acute exercise

Endurance exercise improves insulin sensitivity and increases fat oxidation, which are partly facilitated by the induction of metabolic transcription factors. Next to exercise, increased levels of FFA's also increase the gene expression of transcription factors, hence making it difficult to discern the effects from contractile signals produced during exercise, from those produced by increased circulatory FFA's. We aimed to investigate, in human skeletal muscle, whether acute exercise affects gene expression of metabolic transcriptional co-activators and transcription factors, including PGC-1alpha, PRC, PPARalpha, beta/delta, and gamma and RXR, SREBP-1c and FKHR, and to discern the effect of exercise per se from those of elevated levels of FFA. Two hours of endurance exercise was performed either in the fasted state, or following carbohydrate ingestion prior to and during exercise, thereby blunting the fasting-induced increase in FA availability and oxidation. Of the genes measured, PGC-1alpha and PRC mRNA increased immediately after, while PPARbeta/delta and FKHR mRNA increased 1-4 h after exercise, irrespective of the increases in FFA's. Our results suggest that the induction in vivo of metabolic transcription factors implicated in mitochondrial biogenesis are under the control of inherent signals, (PGC-1alpha, PRC), while those implicated in substrate selection are under the control of associated signals (PPARbeta/delta, FKHR) stimulated from the contracting skeletal muscle that are independent of circulating FFA levels.

Acylation-stimulating protein: effect of acute exercise and endurance training

INTRODUCTION

Acylation-stimulating protein (ASP) is an adipocyte-derived protein that contributes to fatty acid clearance. Regular exercise training improves fatty acid handling.

OBJECTIVE

To examine the effect of acute exercise and short-term endurance training on ASP levels.

SUBJECTS

Eight untrained men (age: 23.5+/-3.4 y; maximal power output (Wmax): 3.7+/-0.6 W/kg body weight).

DESIGN

Subjects were trained for 2 weeks. Before and after training, blood was sampled during a 3-h exercise test, and insulin sensitivity was assessed by an insulin tolerance test.

RESULTS

Before training, ASP levels decreased during exercise (from 17.9+/-2.9 to 15.5+/-3.7 nmol/l at t=0 vs 180, P<0.05). Endurance training decreased fasting ASP levels significantly (17.9+/-2.9 vs 13.4+/-2.3 nmol/l pre- and post-training, P<0.001). Interestingly, after 2 weeks of endurance training, ASP levels tended to increase during exercise (from 13.4+/-2.3 to 17.2+/-4.5 nmol/l at t=0 vs 180, P=0.09). Baseline ASP levels correlated negatively with insulin sensitivity both before (r=-0.86, P<0.01) and after training (r=-0.82, P<0.05).

CONCLUSION

Short-term endurance training reduces baseline ASP levels. These data fit with the hypothesis that reduced ASP levels indicate improved ASP sensitivity.

Optimizing fat oxidation through exercise and diet

Interventions aimed at increasing fat metabolism could potentially reduce the symptoms of metabolic diseases such as obesity and type 2 diabetes and may have tremendous clinical relevance. Hence, an understanding of the factors that increase or decrease fat oxidation is important. Exercise intensity and duration are important determinants of fat oxidation. Fat oxidation rates increase from low to moderate intensities and then decrease when the intensity becomes high. Maximal rates of fat oxidation have been shown to be reached at intensities between 59% and 64% of maximum oxygen consumption in trained individuals and between 47% and 52% of maximum oxygen consumption in a large sample of the general population. The mode of exercise can also affect fat oxidation, with fat oxidation being higher during running than cycling. Endurance training induces a multitude of adaptations that result in increased fat oxidation. The duration and intensity of exercise training required to induce changes in fat oxidation is currently unknown. Ingestion of carbohydrate in the hours before or on commencement of exercise reduces the rate of fat oxidation significantly compared with fasted conditions, whereas fasting longer than 6 h optimizes fat oxidation. Fat oxidation rates have been shown to decrease after ingestion of high-fat diets, partly as a result of decreased glycogen stores and partly because of adaptations at the muscle level.

Nutrient oxidation during moderately intense exercise in obese prepubertal boys

The aim of this study was to measure the nutrient oxidation rate during walking at different speeds and to identify the walking speed associated with the highest fat oxidation rate in a group of prepubertal boys with different levels of adiposity. Twenty-four prepubertal boys (age, 10 +/- 1 yr) with different levels of overweight (body mass index, 25.5 +/- 3.5 kg/m(2); sd score of body mass index, 3.4 +/- 1.1) performed a treadmill test. We measured by indirect calorimetry their respiratory exchange while they walked at speeds of 4, 5, and 6 km/h as well as their maximal oxygen uptake. The fat oxidation rate did not change significantly when the speed of walking was increased, whereas carbohydrate oxidation increased significantly (P < 0.001). A significant (P < 0.05) association was found between adiposity (percent fat mass) and the fat to carbohydrate oxidation ratio during walking at 4, 5, and 6 km/h (r = 0.37, r = 0.37, and r = 0.36, respectively), adjusting for exercise intensity (maximal oxygen uptake, percentage). The lowest fat to carbohydrate oxidation ratio, i.e. the highest fat oxidation/carbohydrate oxidation rate, was found at a walking speed of 4 km/h. Moderately intense exercise promoted the highest fat to carbohydrate oxidation ratio. Increasing the exercise intensity did not promote fat oxidation. Therefore, walking at a speed of 4 km/h is recommended as practicable exercise for obese boys and, consequently, for the treatment of childhood obesity.

Determinants of fat oxidation during exercise in healthy men and women: a cross-sectional study

The aim of the present study was to establish fat oxidation rates over a range of exercise intensities in a large group of healthy men and women. It was hypothesised that exercise intensity is of primary importance to the regulation of fat oxidation and that gender, body composition, physical activity level, and training status are secondary and can explain part of the observed interindividual variation. For this purpose, 300 healthy men and women (157 men and 143 women) performed an incremental exercise test to exhaustion on a treadmill [adapted from a previous protocol (Achten J, Venables MC, and Jeukendrup AE. Metabolism 52: 747–752, 2003)]. Substrate oxidation was determined using indirect calorimetry. For each individual, maximal fat oxidation (MFO) and the intensity at which MFO occurred (Fatmax) were determined. On average, MFO was 7.8 ± 0.13 mg·kg fat-free mass (FFM)−1·min−1 and occurred at 48.3 ± 0.9% maximal oxygen uptake (V̇o2 max), equivalent to 61.5 ± 0.6% maximal heart rate. MFO (7.4 ± 0.2 vs. 8.3 ± 0.2 mg·kg·FFM−1·min−1; P < 0.01) and Fatmax (45 ± 1 vs. 52 ± 1% V̇o2 max; P < 0.01) were significantly lower in men compared with women. When corrected for FFM, MFO was predicted by physical activity (self-reported physical activity level), V̇o2 max, and gender ( R2 = 0.12) but not with fat mass. Men compared with women had lower rates of fat oxidation and an earlier shift to using carbohydrate as the dominant fuel. Physical activity, V̇o2 max, and gender explained only 12% of the interindividual variation in MFO during exercise, whereas body fatness was not a predictor. The interindividual variation in fat oxidation remains largely unexplained.

Lipotoxicity: the obese and endurance-trained paradox

The potential lipotoxic effect of intramyocellular triglyceride (IMTG) accumulation has been suggested to be a major component in the development of insulin resistance. Increased levels of IMTGs correlate with insulin resistance in both obese and diabetic patients, but this relationship does not exist in endurance trained (ETr) subjects. This may be, in part, related to differences in the gene expression and activities of key enzymes involved in fatty acid transport and oxidation as well as in the perodixation status of the IMTGs in obese/diabetic patients as compared with ETr subjects. Disruptions in fat and lipid homeostasis in skeletal muscle have been shown to activate protein kinase C (PKC), which acts on several downstream signalling pathways, including the insulin and the IkappaB kinase (IKK)/NFkappaB signalling pathways. Additionally, an increased peroxidation of IMTGs may reduce insulin sensitivity by increasing TNFalpha, which is known to increase the expression of suppressor of cytokine signalling proteins (SOCS). A common characteristic observed when activating both PKC and TNFalpha/SOCS3 is the inhibition of tyrosine phosphorylation of IRS-1 and subsequently an inhibition of its activation of downstream signalling molecules. These may be important players in the development of insulin resistance and understanding their activation and expression in both obese and ETr humans should assist in understanding how and why IMTGs become lipotoxic.

Rat myocellular and perimysial intramuscular triacylglycerol: a histological approach

PURPOSE

There is controversy as to the use of intramuscular triacylglycerol (IMTAG) during exercise and to whether endurance training increases its utilization, despite the various methodologies used to address these questions. We used a histological-morphometrical approach to study the relative contribution of the two compartments of IMTAG storage, intramyocellular, and perimysial adipocytes, during exercise in sedentary and endurance-trained rats.

METHODS

After osmium impregnation, the soleus (SOL) and gastrocnemius (GAS) were studied under light and electron microscopy. IMTAG content (after Triton WR1339 treatment or not) and 14C-oleate incorporation into the muscles were studied.

RESULTS

In GAS, training, but not exercise alone, decreased extramyocellular lipid (P < 0.001 vs sedentary), an effect not found for SOL. Both muscles presented reduced lipid inclusion number (P < 0.001) and area (P < 0.05), immediately after exercise in sedentary and trained rats. For SOL, a greater number (P < 0.001 vs sedentary) of inclusions was found 24 h after exercise in trained rats. Triton WR1339 treatment decreased IMTAG content 12 h after exercise in SOL (but not in GAS), in sedentary (33%), and trained rats (52%).

CONCLUSIONS

The multi-analytical approach adopted allowed the discernment between the IMTAG compartments and provided evidence for an effect of training upon storage of lipid in perimysial adipocytes in rat gastrocnemius, as well as clearly showed that the IMTAG mobilized during submaximal exercise in sedentary and trained rats derives from intramyocellular lipid, both in SOL and GAS. Moreover, the reposition of these stores 12 h after exercise was shown to be different in GAS and SOL, as plasma triacylglycerol clearly contributed to the process only in the latter, possibly reflecting the differences in lipoprotein lipase activity in the muscles reported by others.

Intramyocellular lipid content in type 2 diabetes patients compared with overweight sedentary men and highly trained endurance athletes

Recent evidence suggests that intramyocellular lipid (IMCL) accretion is associated with obesity and the development of insulin resistance and/or type 2 diabetes. However, trained endurance athletes are markedly insulin sensitive, despite an elevated mixed muscle lipid content. In an effort to explain this metabolic paradox, we compared muscle fiber type-specific IMCL storage between populations known to have elevated IMCL deposits. Immunofluorescence microscopy was performed on muscle biopsies obtained from eight highly trained endurance athletes, eight type 2 diabetes patients, and eight overweight, sedentary men after an overnight fast. Mixed muscle lipid content was substantially greater in the endurance athletes (4.0 +/- 0.4% area lipid stained) compared with the diabetes patients and the overweight men (2.3 +/- 0.4 and 2.2 +/- 0.5%, respectively). More than 40% of the greater mixed muscle lipid content was attributed to a higher proportion type I muscle fibers (62 +/- 8 vs. 38 +/- 3 and 33 +/- 7%, respectively), which contained 2.8 +/- 0.3-fold more lipid than the type II fibers. The remaining difference was explained by a significantly greater IMCL content in the type I muscle fibers of the trained athletes. Differences in IMCL content between groups or fiber types were accounted for by differences in lipid droplet density, not lipid droplet size. IMCL distribution showed an exponential increase in lipid content from the central region toward the sarcolemma, which was similar between groups and fiber types. In conclusion, IMCL contents can be substantially greater in trained endurance athletes compared with overweight and/or type 2 diabetes patients. Because structural characteristics and intramyocellular distribution of lipid aggregates seem to be similar between groups, we conclude that elevated IMCL deposits are unlikely to be directly responsible for inducing insulin resistance.

Sucrose-induced cardiomyocyte dysfunction is both preventable and reversible with clinically relevant treatments

We recently identified cardiomyocyte dysfunction in the early stage of type 2 diabetes (i.e., diet-induced insulin resistance). The present investigation was designed to determine whether a variety of clinically relevant interventions are sufficient to prevent and reverse cardiomyocyte dysfunction in sucrose (SU)-fed insulin-resistant rats. Subsets of animals were allowed to exercise (free access to wheel attached to cage) or were treated with bezafibrate in drinking water to determine whether these interventions would prevent the adverse effects of SU feeding on cardiomyocyte function. After 6-8 wk on diet and treatment, animals were surgically prepared to assess whole body insulin sensitivity (intravenous glucose tolerance test), and isolated ventricular myocyte mechanics were evaluated (video edge recording). SU feeding produced hyperinsulinemia and hypertriglyceridemia, with euglycemia, and induced characteristic whole body insulin resistance. Both exercise and bezafibrate treatment prevented these metabolic abnormalities. Ventricular myocyte shortening and relengthening were slower in SU-fed rats (42-63%) compared with starch (ST)-fed controls, and exercise or bezafibrate completely prevented cardiomyocyte dysfunction in SU-fed rats. In separate cohorts of animals, after 5 wk of SU feeding, animals were either switched back to an ST diet or given menhaden oil for an additional 7-9 wk to determine whether the cardiomyocyte dysfunction was reversible. Both interventions have previously been shown to have favorable metabolic effects, and both improved myocyte mechanics, but only the ST diet reversed all indications of cardiomyocyte dysfunction induced by SU feeding. Thus phenotypic changes in cardiomyocyte mechanics associated with early stages of type 2 diabetes were found to be both preventable and reversible with clinically relevant treatments, suggesting that the cellular processes contributing to this dysfunction are modifiable.

Decreased muscle acetyl-coenzyme A carboxylase 2 mRNA and insulin resistance in formerly obese subjects

OBJECTIVE

A relationship between free fatty acids, intramuscular triglycerides (TG(M)s), and insulin resistance is widely accepted. The intracellular level of malonyl-coenzyme A (CoA) was suggested to be the possible link. Acetyl-CoA carboxylase (ACC) is a key enzyme in fatty acid metabolism, catalyzing the synthesis of malonyl-CoA, a fatty acid acyl-chain elongation unit, from acetyl-CoA. We assessed ACC2 mRNA expression variations in skeletal muscle of subjects who have undergone biliopancreatic diversion (BPD) operation. BPD, in fact inducing a massive lipid malabsorption, leads to a reversion of insulin resistance.

RESEARCH METHODS AND PROCEDURES

Twelve obese women (BMI > 40 kg/m(2)) were enrolled in the study. Body composition, euglycemic-hyperinsulinemic clamp, and muscle biopsies for lipid analysis and reverse transcription-competitive polymerase chain reaction were performed before and 3 years after BPD.

RESULTS

The average weight loss was around 37%. A significant inverse linear relation was observed between glucose uptake and TG(M) (y = -5.62x - 142.82, R(2) = 0.50, p = 0.01). The reduced amount of ACC2 mRNA directly correlated with both TG(M) (y = 2.11x +69.85, R(2) = 0.70, p = 0.01) and fasting insulin (y = 1.49x + 57.17, R(2) = 0.69, p < 0.01) concentrations.

DISCUSSION

In conclusion, down-regulation of ACC2 mRNA, induced by the lowering of plasma insulin concentration, is related to improvement of insulin sensitivity. We hypothesize that reduced amount of malonyl-CoA, consequent to reduced ACC2 mRNA, enhancing fatty acid oxidation, causes lowering of the intramyocitic triglyceride depot.

Regulation of Skeletal Muscle Peroxisome Proliferator-Activated Receptor .GAMMA. Expression by Exercise and Angiotensin-Converting Enzyme Inhibition in Fructose-Fed Hypertensive Rats

The purpose of this study was to examine the effects of chronic exercise training and angiotensin-converting enzyme (ACE) inhibition on peroxisome proliferator-activated receptor gamma (PPAR gamma) expression in fat and skeletal muscle in fructose-fed spontaneously hypertensive rats (SHR). SHR were fed a fructose-rich diet over 16 weeks of either exercise training (Ex group: 20 m/min, 0% grade, 60 min/day, 5 days/week), ACE inhibitor administration (TM group: temocapril, 10 mg/kg/day), or a combination of both treatments (TM+Ex group). The systolic blood pressure was reduced exclusively in the temocapril-treated groups. Serum leptin level was positively correlated with the ratio of epididymal fat weight to body weight (p<0.001). Exercise training significantly upregulated the PPARgamma expression in all tissues, which was attenuated by temocapril. PPARgamma expression was significantly upregulated in skeletal muscles in the Ex group, and temocapril administration attenuated this effect in the Ex+TM group. The level of PPARgamma protein was significantly higher in the extensor digitorum longus muscle than in the soleus muscle. Both TM and Ex prevented the fructose diet-induced transitions of fiber type. These data suggested that PPARgamma expression is tissue-specific, and that alterations in PPARgamma expression in the skeletal muscle induced by either or both treatments may have contributed to reducing the fat mass via the regulation of metabolic homeostasis. Changes in muscle morphology were independent of PPARgamma expression, and the higher proportion of type I fiber might also explain some of the beneficial impact of exercise and ACE inhibition on energy metabolism.

Endurance training in humans leads to fiber type-specific increases in levels of peroxisome proliferator-activated receptor-gamma coactivator-1 and peroxisome proliferator-activated receptor-alpha in skeletal muscle

The peroxisome proliferator-activated receptor (PPAR)-gamma coactivator-1 (PGC-1) can induce mitochondria biogenesis and has been implicated in the development of oxidative type I muscle fibers. The PPAR isoforms alpha, beta/delta, and gamma control the transcription of genes involved in fatty acid and glucose metabolism. As endurance training increases skeletal muscle mitochondria and type I fiber content and fatty acid oxidative capacity, our aim was to determine whether these increases could be mediated by possible effects on PGC-1 or PPAR-alpha, -beta/delta, and -gamma. Seven healthy men performed 6 weeks of endurance training and the expression levels of PGC-1 and PPAR-alpha, -beta/delta, and -gamma mRNA as well as the fiber type distribution of the PGC-1 and PPAR-alpha proteins were measured in biopsies from their vastus lateralis muscle. PGC-1 and PPAR-alpha mRNA expression increased by 2.7- and 2.2-fold (P < 0.01), respectively, after endurance training. PGC-1 expression was 2.2- and 6-fold greater in the type IIa than in the type I and IIx fibers, respectively. It increased by 2.8-fold in the type IIa fibers and by 1.5-fold in both the type I and IIx fibers after endurance training (P < 0.015). PPAR-alpha was 1.9-fold greater in type I than in the II fibers and increased by 3.0-fold and 1.5-fold in these respective fibers after endurance training (P < 0.001). The increases in PGC-1 and PPAR-alpha levels reported in this study may play an important role in the changes in muscle mitochondria content, oxidative phenotype, and sensitivity to insulin known to be induced by endurance training.

Intramyocellular lipids form an important substrate source during moderate intensity exercise in endurance-trained males in a fasted state

Both stable isotope methodology and fluorescence microscopy were applied to define the use of intramuscular triglyceride (IMTG) stores as a substrate source during exercise on a whole-body as well as on a fibre type-specific intramyocellular level in trained male cyclists. Following an overnight fast, eight subjects were studied at rest, during 120 min of moderate intensity exercise (60 % maximal oxygen uptake capacity (VO2,max)) and 120 min of post-exercise recovery. Continuous infusions of [U-13C]palmitate and [6,6-2H2]glucose were administered at rest and during subsequent exercise to quantify whole-body plasma free fatty acid (FFA) and glucose oxidation rates and the contribution of other fat sources (sum of muscle- plus lipoprotein-derived TG) and muscle glycogen to total energy expenditure. Fibre type-specific intramyocellular lipid content was determined in muscle biopsy samples collected before, immediately after and 2 h after exercise. At rest, fat oxidation provided 66 +/- 5 % of total energy expenditure, with FFA and other fat sources contributing 48 +/- 6 and 17 +/- 3 %, respectively. FFA oxidation rates increased during exercise, and correlated well with the change in plasma FFA concentrations. Both the use of other fat sources and muscle glycogen declined with the duration of exercise, whereas plasma glucose production and utilisation increased (P < 0.001). On average, FFA, other fat sources, plasma glucose and muscle glycogen contributed 28 +/- 3, 15 +/- 2, 12 +/- 1 and 45 +/- 4 % to total energy expenditure during exercise, respectively. Fluorescence microscopy revealed a 62 +/- 7 % net decline in muscle lipid content following exercise in the type I fibres only, with no subsequent change during recovery. We conclude that IMTG stores form an important substrate source during moderate intensity exercise in endurance-trained male athletes following an overnight fast, with the oxidation rate of muscle- plus lipoprotein-derived TG being decreased with the duration of exercise.

Lifestyle changes and lipid metabolism gene expression and protein content in skeletal muscle of subjects with impaired glucose tolerance

AIMS/HYPOTHESIS

Skeletal muscle of pre-diabetic patients is characterised by a diminished capacity to handle fatty acids. A diminished content of several enzymes involved in fatty-acid transport and oxidation have been suggested to underlie these defects. The aim of this study was to investigate whether the combination of dietary advice, increased physical activity and weight loss improves lipid metabolic gene and protein expression in skeletal muscle of subjects with impaired glucose tolerance.

METHODS

Before and after 1 year of a lifestyle-intervention programme, expression of several genes and proteins involved in lipid metabolism were measured in vastus lateralis muscle biopsies from subjects in the intervention ( n=7) and control group ( n=6).

RESULTS

After 1 year the intervention group had an improved glycaemic control and reduced body fat compared to the control group. Significant differences were observed for acetyl CoA-carboxylase 2 and uncoupling protein 2 expression (ACC2: -16.8+/-12.4% vs +51.5+/-32.3% for the intervention and control group respectively; p<0.05) (UCP2: -26.9+/-10.3% vs +10.5+/-6.2% for the intervention and control group respectively; p<0.05). Change in 3-hydroxyacyl-CoA dehydrogenase protein content tended to be different between groups (+3.2+/-1.1 vs -0.9+/-1.9 U/mg.ww for the intervention and control group, p=0.07).

CONCLUSIONS/INTERPRETATION

Lifestyle changes leading to an improved glycaemic control and reduced adiposity, resulted in a down-regulation of ACC-2 and UCP2 expression and in an increase in HAD protein content, reflecting a better capacity to utilise fatty acids.

Marked heterogeneity of human skeletal muscle lipolysis at rest

In this study, variations in lipolysis among different muscle groups were examined by measuring local net glycerol release in vivo in healthy, normal-weight subjects (n = 11) during rested, postabsorptive conditions. Microdialysis of the gastrocnemius, deltoid, and vastus lateralis muscle regions revealed that extracellular glycerol concentrations of these three muscle regions were 84.7 +/- 6.7, 59.7 + 7.3, and 56.4 +/- 7.5 micro mol/l, respectively, and the arterial plasma glycerol concentration was 44.8 +/- 2.3 micro mol/l (P = 0.0003-0.006, gastrocnemius vs. others). Local tissue blood flow, as measured by Xe clearance, did not differ among the regions. Net glycerol release was significantly higher in gastrocnemius muscle than in the two other regions. There were no regional differences in glycerol uptake when studied during glycerol infusion. Gastrocnemius muscle showed a dominance of type 1 fibers (70%), whereas the vastus lateralis muscle had equal distribution of fiber types (P = 0.02). No differences in intramuscular triaclyceride content, perimuscular fat, or the adipocyte-specific protein perilipin were observed among the muscle regions. Triglyceride turnover in the gastrocnemius muscle was 3.3 + 1.4% over 24 h, which is about 10 times more rapid than the turnover rate in subcutaneous adipose tissue (P < 0.01). Thus there were marked differences in lipolytic activity among skeletal muscle groups at rest, possibly reflecting variations in fiber type.