Skeletal muscle fatty acid metabolism in association with insulin resistance, obesity, and weight loss

The current study was undertaken to investigate fatty acid metabolism by skeletal muscle to examine potential mechanisms that could lead to increased muscle triglyceride in obesity. Sixteen lean and 40 obese research volunteers had leg balance measurement of glucose and free fatty acid (FFA) uptake (fractional extraction of [9,10 (3)H]oleate) and indirect calorimetry across the leg to determine substrate oxidation during fasting and insulin-stimulated conditions. Muscle obtained by percutaneous biopsy had lower carnitine palmitoyl transferase (CPT) activity and oxidative enzyme activity in obesity (P < 0.05). During fasting conditions, obese subjects had an elevated leg respiratory quotient (RQ, 0.83 +/- 0.02 vs. 0.90 +/- 0.01; P < 0.01) and reduced fat oxidation but similar FFA uptake across the leg. During insulin infusions, fat oxidation by leg tissues was suppressed in lean but not obese subjects; rates of FFA uptake were similar. Fasting values for leg RQ correlated with insulin sensitivity (r = -0.57, P < 0.001). Thirty-two of the obese subjects were restudied after weight loss (WL, -14.0 +/- 0.9 kg); insulin sensitivity and insulin suppression of fat oxidation improved (P < 0.01), but fasting leg RQ (0.90 +/- 0.02 vs. 0.90 +/- 0.02, pre-WL vs. post-WL) and muscle CPT activity did not change. The findings suggest that triglyceride accumulation in skeletal muscle in obesity derives from reduced capacity for fat oxidation and that inflexibility in regulating fat oxidation, more than fatty acid uptake, is related to insulin resistance.

The acute versus the chronic response to exercise

PURPOSE

There is strong and consistent evidence that a single exercise session can acutely reduce triglycerides and increase high-density lipoprotein (HDL) cholesterol (HDL-C), reduce blood pressure, and improve insulin sensitivity and glucose homeostasis. Such observations suggest that at least some of the effects on atherosclerotic cardiovascular disease (ASCVD) risk factors attributed to exercise training may be the result of recent exercise.

RESULTS

These acute and chronic exercise effects cannot be considered in isolation. Exercise training increases the capacity for exercise, thereby permitting more vigorous and/or more prolonged individual exercise sessions and a more significant acute effect. The intensity, duration, and energy expenditure required to produce these acute exercise effects are not clearly defined. The acute effect of exercise on triglycerides and HDL-C appears to increase with overall energy expenditure possibly because the effect maybe mediated by reductions in intramuscular triglycerides. Prolonged exercise appears necessary for an acute effect of exercise on low-density lipoprotein (LDL) cholesterol (LDL-C) levels. The acute effect of exercise on blood pressure is a low threshold phenomenon and has been observed after energy expenditures requiring only 40% maximal capacity. The acute effect of exercise on glucose metabolism appears to require exercise near 70% maximal, but this issue has not been carefully examined.

CONCLUSIONS

Exercise has definite acute effects on blood lipids, blood pressure, and glucose homeostasis. Exercise also has acute effects on other factors related to atherosclerosis such as immunological function, vascular reactivity, and hemostasis. Considerable additional research is required to define the threshold of exercise required to produce these putatively beneficial effects.

Magnetic resonance imaging in human body composition research. From quantitative to qualitative tissue measurement

Incremental improvements in our knowledge of human body composition are abetted by advances in research technology. Indeed, magnetic resonance imaging (MRI) represents a technological advance that has profoundly influenced body composition research. Routine applications of MRI include the measurement of whole-body and regional adipose tissue distribution, quantification of lean tissue and its principal constituent skeletal muscle, and the measurement of visceral adipose tissue. MRI is now the method of choice for calibration of field methods designed to measure body fat and skeletal muscle in vivo. Common to these applications is the measurement of tissue quantity. More recently proton (1H) and sodium (23Na) MRI protocols have been developed that measure the quality (lipid and sodium concentration) of skeletal muscle tissue. These unique applications of MRI represent a major advance in the study of altered muscle composition in vivo, with numerous applications in both applied and clinical medicine. In this review we provide a brief overview of routine applications of MRI in body composition research, followed by a focus on more recent applications of MRI that employ fast-imaging sequences for qualitative measurement of human skeletal muscle.

The effects of L-carnitine supplementation on performance during interval swimming

To examine the effects of L-carnitine supplementation on short high-intensity exercise, twenty male collegiate swimmers completed two trials separated by seven days. Each trial consisted of five 91.4 m (100 yd) swims with a two minute rest interval between each bout. Following the first trial subjects were evenly and randomly assigned to either an L-carnitine (LC) group or a placebo (PL) group. The LC group ingested 2 grams L-carnitine in a citrus drink twice daily for 7 days, while the PL group received only the citrus drink during the same time period. Performance times were recorded for each repeat during both trials. Blood samples (5 ml) were obtained from an antecubital vein 1 minute following the interval set. Blood pH, base excess (BE), lactate (LA), carnitine and carnitine fractions were measured. Total serum carnitine was significantly (p < 0.05) elevated (75.9 +/- 2.0 vs. 106.4 +/- 3.5 mumol.l-1) in the LC group following treatment, while the PL group was unchanged (79.5 +/- 2.8 vs. 77.6 +/- 5.3 mumol.l-1). Free and short-chain serum carnitine fractions were also increased (p < 0.05) in the LC group, but were not altered in the PL group. No differences in performance times were observed between trials or between groups. Blood pH, LA and BE revealed a similar response in both groups during each trial. Despite the elevation in serum L-carnitine and carnitine fractions, these results indicate that L-carnitine supplementation does not provide an ergogenic benefit during repeated bouts of high-intensity anaerobic exercise in highly trained swimmers.

Genetic markers for ancestry are correlated with body composition traits in older African Americans

Individual-specific percent European ancestry was assessed in 1,277 African Americans. We found significant correlations between proportion of European ancestry and several musculoskeletal traits, indicating that admixture mapping may be a useful strategy for locating genes affecting these traits.

INTRODUCTION

Genotype data for admixed populations can be used to detect chromosomal regions influencing disease risk if allele frequencies at disease-related loci differ between parental populations. We assessed evidence for differentially distributed alleles affecting bone and body composition traits in African Americans.

METHODS

Bone mineral density (BMD) and body composition data were collected for 1,277 African and 1,790 European Americans (aged 70-79). Maximum likelihood methods were used to estimate individual-specific percent European ancestry for African Americans genotyped at 37 ancestry-informative genetic markers. Partial correlations between body composition traits and percent European ancestry were calculated while simultaneously adjusting for the effects of covariates.

RESULTS

Percent European ancestry (median = 18.7%) in African Americans was correlated with femoral neck BMD in women (r = -0.18, p < 10(-5)) and trabecular spine BMD in both sexes (r = -0.18, p < 10(-5)) independently of body size, fat, lean mass, and other covariates. Significant associations of European ancestry with appendicular lean mass (r = -0.19, p < 10(-10)), total lean mass (r = -0.12, p < 10(-4)), and total body fat (r = 0.09, p < 0.002) were also observed for both sexes.

CONCLUSIONS

These results indicate that some population differences in body composition may be due to population-specific allele frequencies, suggesting the utility of admixture mapping for identifying susceptibility genes for osteoporosis, sarcopenia, and obesity.

Training alters the distribution of perilipin proteins in muscle following acute free fatty acid exposure

KEY POINTS

The lipid droplet (LD)-associated perilipin (PLIN) proteins promote intramuscular triglyceride (IMTG) storage, although whether the abundance and association of the PLIN proteins with LDs is related to the diverse lipid storage in muscle between trained and sedentary individuals is unknown. We show that lipid infusion augments IMTG content in type I fibres of both trained and sedentary individuals. Most importantly, despite there being no change in PLIN protein content, lipid infusion did increase the number of LDs connected with PLIN proteins in trained individuals only. We conclude that trained individuals are able to redistribute the pre-existing pool of PLIN proteins to an expanded LD pool during lipid infusion and, via this adaptation, may support the storage of fatty acids in IMTG.

ABSTRACT

Because the lipid droplet (LD)-associated perilipin (PLIN) proteins promote intramuscular triglyceride (IMTG) storage, we investigated the hypothesis that differential protein content of PLINs and their distribution with LDs may be linked to the diverse lipid storage in muscle between trained and sedentary individuals. Trained (n = 11) and sedentary (n = 10) subjects, matched for age, sex and body mass index, received either a 6 h lipid or glycerol infusion in the setting of a concurrent hyperinsulinaemic-euglycaemic clamp. Sequential muscle biopsies (0, 2 and 6 h) were analysed using confocal immunofluorescence microscopy for fibre type-specific IMTG content and PLIN associations with LDs. In both groups, lipid infusion increased IMTG content in type I fibres (trained: +62%, sedentary: +79%; P < 0.05) but did not affect PLIN protein content. At baseline, PLIN2 (+65%), PLIN3 (+105%) and PLIN5 (+53%; all P < 0.05) protein content was higher in trained compared to sedentary individuals. In trained individuals, lipid infusion increased the number of LDs associated with PLIN2 (+27%), PLIN3 (+73%) and PLIN5 (+40%; all P < 0.05) in type I fibres. By contrast, in sedentary individuals, lipid infusion only increased the number of LDs not associated with PLIN proteins. Acute free fatty acid elevation therefore induces a redistribution of PLIN proteins to an expanded LD pool in trained individuals only and this may be part of the mechanism that enables fatty acids to be stored in IMTG.

The influence of starch structure on glycogen resynthesis and subsequent cycling performance

The present study was designed to evaluate the influence of starch structure on muscle glycogen resynthesis and cycling performance. Eight male cyclists (22 +/- 1 yr) completed an exercise protocol (DP) to decrease vastus lateralis glycogen concentration. This exercise consisted of 60 min cycling at 75% VO2max, followed by six 1-min sprints at approximately 125% VO2max with 1 min rest intervals. In the 12 hr after the exercise each subject consumed approximately 3000 kcal (65:20:15% carbohydrate, fat and protein). All of the carbohydrate (CHO) consumed was derived from one of four solutions; 1) glucose, 2) maltodextrin (glucose polymer), 3) waxy starch (100% amylopectin), or 4) resistant starch (100% amylose). Muscle biopsies were taken from the vastus lateralis muscle after DP and 24 hr later to determine glycogen concentrations. A 30 min cycling time trial (TT) was performed following the 24 hr post-DP muscle biopsy to examine the influence of the feeding regimen on total work output. The post-DP glycogen concentrations were similar among the four trials, ranging from 220.3 +/- 29.2 to 264 +/- 48.3 mmol.kg-1 dry weight (d.w.) muscle. Twenty-four hours after DP, muscle glycogen concentration had increased less (p < 0.05) in the resistant starch trial (+90.8 +/- 12.8 mmol.kg-1 d.w.) than in the glucose (+197.7 +/- 31.6 mmol.kg-1 d.w.), maltodextrin (+136.7 +/- 24.5 mmol.kg-1 d.w.) and waxy starch (+171.8 +/- 37.1 mmol.kg-1 d.w.) trials. There were no differences in total work output during the TT, or blood lactate concentration immediately following the TT in any of the CHO trials. In summary, glycogen resynthesis was attenuated following ingestion of starch with a high amylose content, relative to amylopectin or glucose; however, short duration time trial performance was unaffected.

Sarcopenia Trials in Specific Diseases: Report by the International Conference on Frailty and Sarcopenia Research Task Force

Muscle atrophy occurs as a consequence of a number of conditions, including cancer, chronic obstructive pulmonary disease (COPD), diabetes mellitus, heart failure, and other chronic diseases, where it is generally a predictor of poor survival. It also occurs as a consequence of disuse and an age-related loss of muscle mass and strength (sarcopenia). The aims of the 2016, International Conference on Frailty and Sarcopenia Research (ICFSR) Task Force were to examine how these specific chronic conditions have been employed in treatment trials thus far and how future trials using these patient groups might be designed for efficient identification of effective sarcopenia interventions. Functional limitations assessed as gait speed, distance walked over a set time period, or other attributes of physical performance have been suggested as outcome measures in sarcopenia trials. Indeed, such measures have already been used successfully in a number of trials aimed at preventing disability in older adults.

Opioid antagonism alters blood glucose homeostasis during exercise in humans

In an attempt to clarify the role of endogenous opioid peptides in substrate mobilization and hormonal responses to dynamic exercise, eight trained cyclists completed exercise trials at 90% of maximal O2 consumption (VO2max) until exhaustion and at 70% VO2max for 90 min. Trials were conducted after intravenous administration of the opiate antagonist naloxone (NAL, 0.1 mg/kg bolus + 0.1 mg.kg-1.h-1) or volume-matched saline (SAL) at each intensity. Serum glucose was maintained at significantly higher levels at 60 and 90 min of exercise in the 70%-NAL than in the 70%-SAL trial and at all points during exercise and at 30 and 60 min of recovery in the 90%-NAL than in the 90%-SAL trial. The serum insulin response to exercise was not altered by NAL administration at either intensity. Serum C-peptide was approximately 50% higher at 60 and 90 min of exercise in the 70%-NAL than in the 70%-SAL trial but was significantly lower during exercise in the 90%-NAL than in the 90%-SAL trial. The plasma glucagon response to exercise at 70% VO2max was not altered by NAL administration but was significantly elevated in the 90%-NAL vs. the 90%-SAL trial. Plasma epinephrine was 50-150% (approximately 2-3 nM) higher during exercise from 30 to 90 min of exercise in the 70%-NAL than in the 70%-SAL trial and was higher at termination (4.9 +/- 2.1 vs. 2.7 +/- 1.7 nM) in the 90%-NAL than in the 90%-SAL trial, although the difference in the 90% trial was not statistically significant.(ABSTRACT TRUNCATED AT 250 WORDS)

Determination of the lumped constant for [18F] fluorodeoxyglucose in human skeletal muscle

Quantitative [18F]fluorodeoxyglucose (FDG) PET has considerable potential for the study of the physiology of skeletal muscle glucose metabolism and for the assessment of perturbations associated with insulin resistance in skeletal muscle. This application of FDG PET imaging depends in part on the determination of the analog effects of FDG relative to true glucose with respect to skeletal muscle. Deoxyglucose has a higher affinity for transporters than glucose and a lower affinity for hexokinase. This study was undertaken to assess the lumped constant (LC) for skeletal muscle, determined empirically as the quotient of FDG metabolism to that of [3H]glucose ([3H]G), and to assess whether the LC is affected by insulin, which is the principal hormonal regulator of glucose metabolism in muscle.

METHODS

Seventeen healthy lean volunteers were randomly assigned and were studied at insulin infusion rates of 0, 20, 40 and 120 mU/min/m2 body surface area. After attaining steady-state euglycemic conditions, injections of FDG and [3H]G were given, and the fractional extraction (E) for each compound across the leg was measured by arterial and venous sampling for 90 min. The LC was calculated as the ratio of the respective fractional extractions (LC = E(FDG)/E[3H]G).

RESULTS

During fasting conditions (i.e., absence of insulin infusion), the LC for skeletal muscle was slightly greater than 1. Insulin had a robust effect to increase fractional extractions of both FDG and [3H]G. The effect was symmetrical for the two compounds, and, hence, the LC did not change significantly in response to progressive insulin stimulation. The mean value of the LC across insulin doses for human skeletal muscle was 1.23 +/- 0.05.

CONCLUSION

Direct comparison of [3H]G and FDG metabolism during insulin-stimulated conditions, across the in vivo tissue bed of skeletal muscle in the leg with both tracers given in an identical manner, yielded an LC value of 1.2, indicating that there was modest preferential uptake of FDG and that insulin did not alter the LC in skeletal muscle.

The analog free testosterone assay: are the results in men clinically useful?

Men with low testosterone concentrations are usually hypogonadal. However, because variations in the testosterone transport protein, sex hormone-binding globulin (SHBG), directly influence the total testosterone concentration, confirmation of a low testosterone with a measurement of free testosterone or "bioavailable" testosterone (BAT) is recommended. In the present study, we examined the relationship of SHBG with free testosterone (Coat-A-Count assay, Diagnostic Products) and with BAT in men (n = 29) and women (n = 28) who participated in a study of the metabolic determinants of body composition. As expected, total testosterone was strongly positively correlated with SHBG among men (r = 0.68; P <0.01). Although the BAT was independent of SHBG in men (r = 0.02), SHBG was an important predictor of free testosterone (r = 0.62; P <0.01). In contrast, in women serum concentrations of total testosterone (r = -0.26; P = 0.17), free testosterone (r = -0.30; P = 0.17), and BAT (r = -0.46; P = 0.013) all tended to be lower with increasing SHBG. Free testosterone was nearly perfectly positively correlated with total testosterone (r = 0.97) in men, among whom free testosterone represented a relatively constant percentage of the total testosterone (0.5-0.65%), and the percentage of free testosterone was unrelated to SHBG. Thus the Coat-A-Count free testosterone concentration in men, like the total testosterone concentration, is determined in part by plasma SHBG. Accordingly, androgen deficiency may be misclassified with this assay in men with low SHBG. Moreover, the previous findings of reduced free testosterone concentrations with hypertension or hyperinsulinemia or as a risk factor for developing type 2 diabetes, conditions in which SHBG is reduced, may have been methodology-related.