Fatty acid oxidation in African-American and Caucasian women during physical activity

Discrepancy between predicted and measured exercise intensity for eliciting the maximal rate of lipid oxidation

BACKGROUND AND AIMS

Ectopic lipid storage is implicated in type 2 diabetes pathogenesis; hence, exercise to deplete stores (i.e., at the intensity that allows for maximal rate of lipid oxidation; MLO) might be optimal for restoring metabolic health. This intensity ("Fatmax") is estimated during incremental exercise ("Fatmax test"). However, in "the field" general recommendations exist regarding a range of percentages of maximal heart rate (HR) to elicit MLO. The degree to which this range is aligned with measured Fatmax has not been investigated. We compared measured HR at Fatmax, with maximal HR percentages within the typically recommended range in a sample of 26 individuals (Female: n = 11, European ancestry: n = 17).

METHODS AND RESULTS

Subjects completed a modified Fatmax test with a 5-min warmup, followed by incremental stages starting at 15 W with work rate increased by 15 W every 5 min until termination criteria were reached. Pulmonary gas exchange was recorded and average values for V˙ o2 and V˙ co2 for the final minute of each stage were used to estimate substrate-oxidation rates. We modeled lipid-oxidation kinetics using a sinusoidal model and expressed MLO relative to peak V˙ o2 and HR. Bland-Altman analysis demonstrated lack of concordance between HR at Fatmax and at 50%, 70%, and 80% of age-predicted maximum with a mean difference of 23 b·min-1.

CONCLUSION

Our results indicate that estimated "fat-burning" heart rate zones are inappropriate for prescribing exercise to elicit MLO and we recommend direct individual exercise lipid oxidation measurements to elicit these values.

Greater reliance on glycolysis is associated with lower mitochondrial substrate oxidation and insulin sensitivity in infant myogenic MSCs

Individuals with insulin resistance and obesity display higher skeletal muscle production of nonoxidized glycolytic products (i.e., lactate), and lower complete mitochondrial substrate oxidation to CO2. These findings have also been observed in individuals without obesity and are associated with an increased risk for metabolic disease. The purpose of this study was to determine if substrate preference is evident at the earliest stage of life (birth) and to provide a clinical blood marker (lactate) that could be indicative of a predisposition for metabolic disease later. We used radiolabeled tracers to assess substrate oxidation and insulin sensitivity of myogenically differentiated mesenchymal stem cells (MSCs), a proxy of infant skeletal muscle tissue, derived from umbilical cords of full-term infants. We found that greater production of nonoxidized glycolytic products (lactate, pyruvate, alanine) is directly proportional to lower substrate oxidation and insulin sensitivity in MSCs. In addition, we found an inverse relationship between the ratio of complete glucose oxidation to CO2 and infant blood lactate at 1 mo of age. Collectively, considering that higher lactate was associated with lower MSC glucose oxidation and has been shown to be implicated with metabolic disease, it may be an early indicator of infant skeletal muscle phenotype.NEW & NOTEWORTHY In infant myogenically differentiated mesenchymal stem cells, greater production of nonoxidized glycolytic products was directly proportional to lower substrate oxidation and insulin resistance. Glucose oxidation was inversely correlated with infant blood lactate. This suggests that innate differences in infant substrate oxidation exist at birth and could be associated with the development of metabolic disease later in life. Clinical assessment of infant blood lactate could be used as an early indicator of skeletal muscle phenotype.

Differences in substrate metabolism between African American and Caucasian infants: evidence from mesenchymal stem cells

Type 2 diabetes is more prevalent in African American (AA) than Caucasian (C) adults. Furthermore, differential substrate utilization has been observed between AA and C adults, but data regarding metabolic differences between races at birth remains scarce. The purpose of the present study was to determine if there are racial differences in substrate metabolism evident at birth using a mesenchymal stem cells (MSCs) collected from offspring umbilical cords. Using radio-labeled tracers, MSCs from offspring of AA and C mothers were tested for glucose and fatty acid metabolism in the undifferentiated state and while undergoing myogenesis in vitro. Undifferentiated MSCs from AA exhibited greater partitioning of glucose toward nonoxidized glucose metabolites. In the myogenic state, AA displayed higher glucose oxidation, but similar fatty acid oxidation rates. In the presence of both glucose and palmitate, but not palmitate only, AA exhibit a higher rate of incomplete fatty acid oxidation evident by a greater production of acid-soluble metabolites. Myogenic differentiation of MSCs elicits an increase in glucose oxidation in AA, but not in C. Together, these data suggest that metabolic differences between AA and C races exist at birth.NEW & NOTEWORTHY African Americans, when compared with Caucasians, display greater insulin resistance in skeletal muscle. Differences in substrate utilization have been proposed as a factor for this health disparity; however, it remains unknown how early these differences manifest. Using infant umbilical cord-derived mesenchymal stem cells, we tested for in vitro glucose and fatty acid oxidation differences. Myogenically differentiated MSCs from African American offspring display higher rates of glucose oxidation and incomplete fatty acid oxidation.

Lower mitochondrial respiration does not lead to decreased fat oxidation in young African American women without obesity

OBJECTIVE

The prevalence of type 2 diabetes in African American women (AAW) is nearly twice that of White women. Lower insulin sensitivity and decreased mitochondrial function may be contributing factors. The purpose of this study was to compare fat oxidation in AAW and White women.

METHODS

Participants were 22 AAW and 22 White women, matched for age (18.7-38.3 years) and BMI (< 28 kg/m2). Participants completed two submaximal (50% VO2max) exercise tests with indirect calorimetry and stable isotope tracers to assess total, plasma, and intramyocellular triglyceride fat oxidation.

RESULTS

The respiratory quotient during the exercise test was nearly identical in AAW and White women (0.813 ± 0.008 vs. 0.810 ± 0.008, p = 0.83). Although absolute total and plasma fat oxidation was lower in AAW, adjusting for the lower workload in AAW eliminated these racial differences. There was no racial difference in plasma and intramyocellular triglyceride source of fat for oxidation. No racial differences were observed in rates of ex vivo fat oxidation. Exercise efficiency was lower in AAW when adjusted to leg fat free mass.

CONCLUSIONS

The data suggest that fat oxidation is not lower in AAW compared with White women, but additional studies are needed across exercise intensity, body weight, and age to confirm these results.

Nutrition, body composition and physical activity have differential impact on the determination of lipidemic blood profiles between young females with different blood cholesterol concentrations

INTRODUCTION

This cross-sectional study explored whether nutrition, body composition, and physical activity energy expenditure (PAΕΝ) have a differential impact on lipidemic blood profiles among young females with different blood cholesterol concentrations.

METHODS

One hundred thirty-five young female students (N = 135) were allocated into three groups according to their blood cholesterol concentrations (Chol): (A) Normal [NL; Chol: < 200 mg·dL^-1; n = 56 Age: 21.4 ± 2.6 yrs, Body Mass Index (BMI): 22.1 ± 2.0 kg·m^-2], (B) Borderline (BL; Chol: ≥200 mg·dL^-1 and <240 mg·dL^-1; n = 44 Age: 21.6 ± 2.5 yrs, BMI: 24.2 ± 3.1 kg·m^-2) and (C) High level (HL; Chol: ≥240 mg·dL^-1; n = 35 Age: 22.5 ± 2.4 yrs, BMI: 28.9 ± 2.1 kg·m^-2). Body composition [bioelectrical impedance analysis including lean body mass (LBM) and body fat mass], nutritional intake (recall questionnaire), daily physical activity energy expenditure through activity trackers and resting blood lipids concentrations were evaluated.

RESULTS

Multiple linear regression analyses revealed that in the NL group, lean mass, daily PAΕΝ and daily energy balance were the determinant parameters of blood lipidemic profiles (B: -0.815 to 0.700). In the BL group, nutrition, body composition and daily physical activity energy expenditure exhibited similar impacts (B: -0.440 to 0.478). In the HL group, nutritional intake and body fat mass determined blood lipidemic profile (B: -0.740 to 0.725).

CONCLUSION

Nutrition, body composition and daily PAΕΝ impact on blood lipids concentration is not universal among young females. In NL females, PAEN, energy expenditure and LBM are the strongest determinants of blood lipids, while in HL females, nutritional intake and body fat mass are. As PAΕΝ increases, the importance of nutrition and body fat decreases, and vice versa.

African American Females Are Less Metabolically Flexible Compared with Caucasian American Females following a Single High-Fat Meal: A Pilot Study

The relationship between metabolic flexibility (MF) and components of metabolic disease has not been well-studied among African American (AA) females and may play a role in the higher incidence of chronic disease among them compared with Caucasian American (CA) females. This pilot study aimed to compare the metabolic response of AA and CA females after a high-fat meal. Eleven AA (25.6 (5.6) y, 27.2 (6.0) kg/m², 27.5 (9.7) % body fat) and twelve CA (26.5 (1.5) y, 25.7 (5.3) kg/m², 25.0 (7.4) % body fat) women free of cardiovascular and metabolic disease and underwent a high-fat meal challenge (55.9% fat). Lipid oxidation, insulin, glucose, and interleukin (IL)-8 were measured fasted, 2 and 4 h postprandial. AA females had a significantly lower increase in lipid oxidation from baseline to 2 h postprandial (p = 0.022), and trended lower at 4 h postprandial (p = 0.081) compared with CA females, indicating worse MF. No group differences in insulin, glucose or HOMA-IR were detected. IL-8 was significantly higher in AA females compared with CA females at 2 and 4 h postprandial (p = 0.016 and p = 0.015, respectively). These findings provide evidence of metabolic and inflammatory disparities among AA females compared with CA females that could serve as a predictor of chronic disease in individuals with a disproportionately higher risk of development.

Whole body lipid oxidation during exercise is impaired with poor insulin sensitivity but not with obesity per se

Equivocal findings regarding the influence of overweight/obesity on exercise lipid-oxidizing capacity (EX-LIPOX) might reflect inadequate control of 1) acute energy balance/macronutrient composition of diet; 2) intensity/duration of exercise; and/or 3) insulin sensitivity (IS) of participant. To assess independent/combined influences of IS and overweight/obesity with other factors controlled, we recruited sedentary adults with normal weight (NW; n = 15) or overweight/obesity (O; n = 15) subdivided into metabolically healthy (MH; n = 8) and unhealthy (MU; n = 7) groups (IS; MH > MU). Participants completed a 9-day, weight-stabilizing, controlled-feeding protocol comprising measurements of resting metabolism, body composition, oral glucose tolerance, and maximal exercise capacity. We measured EX-LIPOX during the initial 45 min of "steady state" during constant-work-rate cycling at 70% and 100% of participant gas-exchange threshold (GET). At 70%, average EX-LIPOX in absolute (0.11 ± 0.02 g·min^-1) and relative (2.4 ± 0.3 mg·kgFFM^-1·min^-1) terms was lower for NW-MU than MH regardless of body composition (NW-MH, 0.19 ± 0.02 g·min^-1/3.9 ± 0.3 mg·kgFFM^-1·min^-1; O-MH, 0.19 ± 0.02 g·min^-1/3.7 ± 0.3 mg·kgFFM^-1·min^-1), whereas no difference was present for NW-MU and O-MU (0.15 ± 0.02 g·min^-1/2.8 ± 0.3 mg·kgFFM^-1·min^-1). Multiple regression confirmed that with IS-controlled, overweight/obesity was not associated with decreased EX-LIPOX, whereas decreased EX-LIPOX was associated with decreased IS independent of overweight/obesity. Overweight/obesity also did not influence EX-LIPOX across MH groups or with cohort divided by body-composition classification alone (P > 0.05). Exercise lipid-oxidizing capacity is impaired with poor IS regardless of body composition, but not with overweight/obesity per se.NEW & NOTEWORTHY In this study, we have shown that the capacity to oxidize lipid during exercise is influenced by metabolic health of the exerciser regardless of body composition, but not by body composition per se. This observation refutes the belief that a reduced capacity to oxidize lipid is an obligatory characteristic of the overweight/obese condition while supporting the contention that exercise should be prescribed with specificity based on both absence/presence of overweight/obesity and compromise/lack thereof in metabolic health.

Mitochondrial Phenotype as a Driver of the Racial Dichotomy in Obesity and Insulin Resistance

African Americans (AA) are disproportionately burdened by metabolic diseases. While largely unexplored between Caucasian (C) and AA, differences in mitochondrial bioenergetics may provide crucial insight to mechanisms for increased susceptibility to metabolic diseases. AA display lower total energy expenditure and resting metabolic rate compared to C, but paradoxically have a higher amount of skeletal muscle mass, suggestive of inherent energetic efficiency differences between these races. Such adaptations would increase the chances of overnutrition in AA; however, these disparities would not explain the racial difference in insulin resistance (IR) in healthy subjects. Hallmarks associated with insulin resistance (IR), such as reduced mitochondrial oxidative capacity and metabolic inflexibility are present even in healthy AA without a metabolic disease. These adaptations might be influential of mitochondrial “substrate preference” and could play a role in disproportionate IR rates among races. A higher glycolytic flux and provision of shuttles transferring electrons from cytosol to mitochondrial matrix could be a contributing factor in development of IR via heightened reactive oxygen species (ROS) production. This review highlights the above concepts and provides suggestions for future studies that could help delineate molecular premises behind potential impairments in insulin signaling and metabolic disease susceptibility in AA.

A Comparison of Substrate Utilization Profiles During Maximal and Submaximal Exercise Tests in Athletes

Background

Exercise is primarily sustained by energy derived from lipids (plasma free fatty acids and intramuscular triglycerides), and glucose (plasma glucose and muscle glycogen). Substrate utilization is the pattern by which these fuel sources are used during activity. There are many factors that influence substrate utilization. We aim to delineate the effect of exercise intensity and body composition on substrate utilization.

Objective

The objective of our study was to discern the differences in substrate utilization profiles during a maximal and submaximal graded exercise test, and to determine the extent to which body composition influences substrate utilization during the exercise tests.

Methods

A total of 27 male athletes, 32.5 ± 11 years of age, were recruited for this study. Body composition was analyzed using a bioelectrical impedance analyzer. Maximal and submaximal exercise tests were performed on a treadmill. A novel graded submaximal treadmill protocol was used for the submaximal test.

Results

Average percent body fat (PBF) was 15.8 ± 5%. Average maximal oxygen consumption (VO₂max) was 47.6 ± 9 mL/kg/min, while the average exercise intensity (percent VO₂max) at which participants were shifting to glucose predominance for energy during the maximal and submaximal tests were 76 ± 8.3% and 58.4 ± 21.1%, respectively. A paired-samples t-test was conducted to compare percent VO₂max at crossover point in maximal and submaximal graded exercise tests. There was a significant difference in percent VO₂max at the crossover point for maximal (76 ± 8.3%) and submaximal (58 ± 21.1%) tests (t = 4.752, p = 0.001). A linear regression was performed to elucidate the interaction between exercise intensity at the crossover point and body composition during a maximal and submaximal graded exercise test. There was a significant effect of PBF on percent VO₂max at crossover point during the maximal graded exercise test [F(1,24) = 9.10, P = 0.006] with an R² of 0.245. However, there was no significant effect of PBF on percent VO₂max at crossover point during the submaximal graded exercise test (P > 0.05).

Conclusion

Substrate utilization, represented by the crossover point, is dependent on the rate of increase in exercise intensity. At maximal efforts, the crossover to carbohydrates from fats as the predominant fuel source occurs at a significantly later stage of percent VO₂max than at submaximal efforts. Furthermore, body composition represented by PBF is a significant predictor of substrate utilization during maximal efforts. Athletes with a relatively higher PBF are more likely to have increased lipid oxidation during high intensity exercises than those with a lower body fat percentage.

Impact of frailty metabolic phenotypes on the management of older people with type 2 diabetes mellitus

AIMS

To provide a pathophysiological basis for distinguishing metabolic variants of the frailty phenotype in older adults with type 2 diabetes.

METHODS

We have made an in-depth review of the possible mechanisms in diabetes, ageing and frailty that will alter allow us to describe phenotypic changes which might assist in predicting responses to particular glucose-lowering therapy.

RESULTS

Our review has enable us to describe with some confidence a sarcopenic obese phenotype and an anorexic malnourished phenotype.

CONCLUSIONS

By identifying these two phenotypes we can predict which would be most responsive to certain classes of therapy and where therapies may be ill-advised. This represents the first novel approach in this area. Further work is being planned to develop this hypothesis. Geriatr Gerontol Int 2021; 21: 614-622.

Effect of Ethnicity on Changes in Fat and Carbohydrate Oxidation in Response to Short-Term High Intensity Interval Training (HIIT): A Pilot Study

This study compared changes in substrate metabolism with high intensity interval training (HIIT) in women of different ethnicities. Twelve Caucasian (C) and ten Hispanic women (H) (age = 24 ± 5 yr) who were inactive completed nine sessions of HIIT at 85 percent peak power output (%PPO). Pre-training, changes in fat oxidation (FOx) and carbohydrate oxidation (CHOOx) during progressive cycling were measured on two days to compute the minimum difference (MD). This test was repeated after the last training session. Between baseline tests, estimates of FOx and CHOOx were not different (p > 0.05) and were highly related (intraclass correlation coefficient equal to 0.72 to 0.88), although the coefficient of variation of maximal fat oxidation (MFO) was equal to 30%. Training significantly increased MFO (p = 0.03) in C (0.19 ± 0.06 g/min to 0.21 ± 0.06 g/min, d = 0.66) and H (0.16 ± 0.03 g/min to 0.19 ± 0.03 g/min, d = 1.3) that was similar (p = 0.92) between groups. There was a significant interaction for FOx (p = 0.003) as it was only increased in H versus C, although both groups exhibited reduced CHO oxidation (p = 0.002) with training. Use of MD revealed that only 3 of 22 women show meaningful increases in MFO (>0.08 g/min). The preliminary data reveals that a small dose of low-volume HIIT does not alter fat and CHO oxidation and there is little effect of ethnicity on the response to training.

Skeletal Muscle ACSL Isoforms Relate to Measures of Fat Metabolism in Humans

INTRODUCTION

Evidence from model systems implicates long-chain acyl-coenzyme A synthetase (ACSL) as key regulators of skeletal muscle fat oxidation and fat storage; however, such roles remain underexplored in humans.

PURPOSE

We sought to determine the protein expression of ACSL isoforms in skeletal muscle at rest and in response to acute exercise and identify relationships between skeletal muscle ACSL and measures of fat metabolism in humans.

METHODS

Sedentary adults (n = 14 [4 males and 10 females], body mass index = 22.2 ± 2.1 kg·m-2, V˙O2max = 32.2 ± 4.5 mL·kg-1⋅min-1) completed two study visits. Trials were identical other than completing 1 h of cycling exercise (65% V˙O2max) or remaining sedentary. Vastus lateralis biopsies were obtained 15-min postexercise (or rest) and 2-h postexercise to determine ACSL protein abundance. Whole-body fat oxidation was assessed at rest and during exercise using indirect calorimetry. Skeletal muscle triacylglycerol (TAG) was measured via lipidomic analysis.

RESULTS

We detected protein expression for four of the five known ACSL isoforms in human skeletal muscle. ACSL protein abundances were largely unaltered in the hours after exercise aside from a transient increase in ACSL5 15-min postexercise (P = 0.01 vs rest). Skeletal muscle ACSL1 protein abundance tended to be positively related with whole-body fat oxidation during exercise (P = 0.07, r = 0.53), when skeletal muscle accounts for the majority of energy expenditure. No such relationship between ACSL1 and fat oxidation was observed at rest. Skeletal muscle ACSL6 protein abundance was positively associated with muscle TAG content at rest (P = 0.05, r = 0.57).

CONCLUSION

Most ACSL protein isoforms can be detected in human skeletal muscle, with minimal changes in abundance after acute exercise. Our findings agree with those from model systems implicating ACSL1 and ACSL6 as possible determinants of fat oxidation and fat storage within skeletal muscle.

Increased dynamic flexibility in the medial temporal lobe network following an exercise intervention mediates generalization of prior learning

Recent work has conceptualized the brain as a network comprised of groups of sub-networks or modules. "Flexibility" of brain network(s) indexes the dynamic reconfiguration of comprising modules. Using novel techniques from dynamic network neuroscience applied to high-resolution resting-state functional magnetic resonance imaging (fMRI), the present study investigated the effects of an aerobic exercise intervention on the dynamic rearrangement of modular community structure-a measure of neural flexibility-within the medial temporal lobe (MTL) network. The MTL is one of the earliest brain regions impacted by Alzheimer's disease. It is also a major site of neuroplasticity that is sensitive to the effects of exercise. In a two-group non-randomized, repeated measures and matched control design with 34 healthy older adults, we observed an exercise-related increase in flexibility within the MTL network. Furthermore, MTL network flexibility mediated the beneficial effect aerobic exercise had on mnemonic flexibility, as measured by the ability to generalize past learning to novel task demands. Our results suggest that exercise exerts a rehabilitative and protective effect on MTL function, resulting in dynamically evolving networks of regions that interact in complex communication patterns. These reconfigurations may underlie exercise-induced improvements on cognitive measures of generalization, which are sensitive to subtle changes in the MTL.

Similar rates of fat oxidation during graded submaximal exercise in women of different body composition

BACKGROUND

Moderate intensity exercise ranging 40-60% of maximum oxygen uptake is advised to promote energy expenditure and fat oxidation in overweight and obese people. Although fat oxidation has been shown to be highly variable among individual, there is still a relative uncertainty regarding exercise prescription for women specifically. This article aimed to determine whether indicators of body composition can be used to narrow the exercise intensity range for exercise prescription in women.

METHODS

A total of 35 healthy women (age 30.8±9.5 yr) classified according to their BMI in normal weight (NOR; ≤24.9 kg·m2), overweight (OVW; 25-29.9 kg·m2) and obese groups (OBE; ≥30 kg·m2) completed a submaximal graded test (intensities eliciting ~30%, 40%, 50% and 60% of maximum oxygen uptake). Blood lactate, perceived exertion and absolute and relative substrate oxidation for fat (OXFAT) and carbohydrates (OXCHO) were measured at each stage.

RESULTS

Perceived exertion and blood lactate increased as a function of exercise but did not differ across groups. There were no significant changes in absolute and relative OXFAT across groups, or as a function of exercise intensity. Peak OXFAT occurred at the 40%, 50% and 40% stages for NOR, OVW and OBE groups, respectively, with no significant differences across groups.

CONCLUSION

We measured no differences, but considerable inter-individual variation, in fat oxidation in women of different body composition. This result is in agreement with previous research based on exercise performed at constant rate and in independent participant groups. Our findings do not support the fat oxidation hypothesis, and further emphasise the perspective that exercise prescription should be individualised and likely be based on considerations other than substrate oxidation.

No Influence of Overweight/Obesity on Exercise Lipid Oxidation: A Systematic Review

Compared to lean counterparts, overweight/obese individuals rely less on lipid during fasting. This deficiency has been implicated in the association between overweight/obesity and blunted insulin signaling via elevated intramuscular triglycerides. However, the capacity for overweight/obese individuals to use lipid during exercise is unclear. This review was conducted to formulate a consensus regarding the influence of overweight/obesity on exercise lipid use. PubMed, ProQuest, ISI Web of Science, and Cochrane Library databases were searched. Articles were included if they presented original research on the influence of overweight/obesity on exercise fuel use in generally healthy sedentary adults. Articles were excluded if they assessed older adults, individuals with chronic disease, and/or exercise limitations or physically-active individuals. The search identified 1205 articles with 729 considered for inclusion after duplicate removal. Once titles, abstracts, and/or manuscripts were assessed, 24 articles were included. The preponderance of evidence from these articles indicates that overweight/obese individuals rely on lipid to a similar extent during exercise. However, conflicting findings were found in eight articles due to the outcome measure cited, participant characteristics other than overweight/obesity and characteristics of the exercise bout(s). We also identified factors other than body fatness which can influence exercise lipid oxidation that should be controlled in future research.

Exercise as 'precision medicine' for insulin resistance and its progression to type 2 diabetes: a research review

Type 2 diabetes and obesity epidemics are in effect in the United States and the two pathologies are linked. In accordance with the growing appreciation that ‘exercise is medicine,’ it is intuitive to suggest that exercise can play an important role in the prevention and/or treatment of these conditions. However, if exercise is to truly be considered as a viable alternative to conventional healthcare prevention/treatment strategies involving pharmaceuticals, it must be prescribed with similar scrutiny. Indeed, it seems reasonable to posit that the recent initiative calling for ‘precision medicine’ in the US standard healthcare system should also be applied in the exercise setting. In this narrative review, we consider a number of explanations that have been forwarded regarding the pathological progression to type 2 diabetes both with and without the concurrent influence of overweight/obesity. Our goal is to provide insight regarding exercise strategies that might be useful as ‘precision medicine’ to prevent/treat this disease. Although the etiology of type 2 diabetes is complex and cause/consequence characteristics of associated dysfunctions have been debated, it is well established that impaired insulin action plays a critical early role. Consequently, an exercise strategy to prevent/treat this disease should be geared toward improving insulin sensitivity both from an acute and chronic standpoint. However, research suggests that a chronic improvement in insulin sensitivity only manifests when weight loss accompanies an exercise intervention. This has resonance because ectopic fat accumulation appears to represent a central component of disease progression regardless of whether obesity is also part of the equation. The cause/consequence characteristics of the relationship between insulin resistance, pathological fat deposition and/or mobilsation, elevated and/or poorly-distributed lipid within myocytes and an impaired capacity to use lipid as fuel remains to be clarified as does the role of muscle mitochondria in the metabolic decline. Until these issues are resolved, a multidimensional exercise strategy (e.g., aerobic exercise at a range of intensities and resistance training for muscular hypertrophy) could provide the best alternative for prevention/treatment.

Balanced high fat diet reduces cardiovascular risk in obese women although changes in adipose tissue, lipoproteins, and insulin resistance differ by race

BACKGROUND

We previously reported that consuming a balanced high fat diet (BHFD) wherein total saturated fat was reduced and total unsaturated fat increased by proportionately balancing the type of fat (1/3 saturated, 1/3 monounsaturated, 1/3 polyunsaturated) led to significant improvements in inflammatory burden, blood pressure, and vascular function in obese premenopausal European American (EA) and African American (AA) women.

OBJECTIVE

Here we compared changes in adipose tissue, lipoproteins, insulin resistance, and cardiovascular risk between EA and AA women.

METHODS

Dietary intakes, plasma fatty acids, lipids, apolipoproteins, lipoproteins, HOMA-IR and ASCVD risk was measured in 144 women who consumed BHFD for 16 weeks. Generalized linear modeling was performed while controlling for change in body weight.

RESULTS

EA women had greater reductions in visceral adipose tissue. Only EA women had significant reductions in fasting insulin levels (↓24.8%) and HOMA-IR (↓29%) scores. In EA women, the most significant improvements occurred in VLDL particle size (↑), apolipoprotein B levels (↑), serum TG (↓), number of plasma LDL particles (↓), and serum LDL-cholesterol (↓). In AA women, significant improvements occurred in HDL particle size (↑), number of large HDL particles (↑), and apolipoprotein AI levels (↑). Consequently, both groups had improved ASCVD risk scores (↓5.5%).

CONCLUSIONS

Consuming the balanced high fat diet led to significant reduction in cardiovascular risk factors in both groups. However, the pattern of response to BHFD differed with EA women responding more in components of the apolipoprotein B pathway versus AA women responding more in components of the apolipoprotein AI pathway.

Association of ectopic fat with abdominal aorto-illiac and coronary artery calcification in african ancestry men

BACKGROUND AND AIMS

There is strong evidence that fat accumulating in non-adipose sites, "ectopic fat", is associated with cardiovascular disease (CVD), including vascular calcification. Most previous studies of this association have assessed only a single ectopic fat depot. Therefore, our aim was to assess the association of total, regional, and ectopic fat with abdominal aorto-illiac calcification (AAC) and coronary artery calcification (CAC) in 798 African ancestry men.

METHODS

Participants (mean age 62) were from the Tobago Bone Health Study cohort. Adiposity was assessed via clinical examination, dual x-ray absorptiometry, and computed tomography (CT). Ectopic fat depots included: abdominal visceral adipose tissue (VAT), liver attenuation, and calf intermuscular adipose tissue (IMAT). Vascular calcification was assessed by CT and quantified as present versus absent. Associations were tested using multiple logistic regression adjusted for traditional cardiovascular risk factors. Models of ectopic fat were additionally adjusted for total body fat and standing height.

RESULTS

All adiposity measures, except VAT, were associated with AAC. Lower liver attenuation or greater calf IMAT was associated with 1.2-1.3-fold increased odds of AAC (p < 0.03 for both), though calf IMAT was a stronger predictor than liver attenuation (p < 0.001) when entered in a single model. No ectopic fat measure was associated with CAC.

CONCLUSIONS

Greater adiposity in the skeletal muscle and liver, but not in the visceral compartment, was associated with increased odds of AAC in African ancestry men. These results highlight the potential importance of both quantity and location of adiposity accumulation throughout the body.

Disparities and genetic risk factors in obstructive sleep apnea

Obstructive sleep apnea (OSA) is an increasingly prevalent condition. A growing body of literature supports substantial racial disparities in the prevalence, risk factors, presentation, diagnosis, and treatment of this disease. Craniofacial structure among Asians appears to confer an elevated risk of OSA despite lower rates of obesity. Among African Americans, Native Americans, and Hispanics, OSA prevalence is increased, likely due in part to obesity. The burden of symptoms, particularly excessive daytime sleepiness, is higher among African Americans, although Hispanics more often report snoring. Limited data suggest that African Americans may be more susceptible to hypertension in the setting of OSA. While differences in genetic risk factors may explain disparities in OSA burden, no definitive genetic differences have yet been identified. In addition to disparities in OSA development, disparities in OSA diagnosis and treatment have also been identified. Increased severity of disease at diagnosis among African Americans suggests a delay in diagnosis. Treatment outcomes are also suboptimal among African Americans. In children, tonsillectomy is less likely to cure OSA and more commonly associated with complications in this group. Among adults, adherence to continuous positive airway pressure (CPAP) is substantially lower in African Americans. The reasons for these disparities, particularly in outcomes, are not well understood and should be a research priority.

Effect of a 1-week, eucaloric, moderately high-fat diet on peripheral insulin sensitivity in healthy premenopausal women

OBJECTIVES

To determine whether a weight-maintaining, moderate (50%) high-fat diet is deleterious to insulin sensitivity in healthy premenopausal women.

DESIGN/SETTING/PARTICIPANTS

23 African-American and non-Hispanic white, healthy, overweight, and obese premenopausal women recruited in New York City, USA, fed either a eucaloric, 1-week long high-fat (50% of total Kcal from fat) diet or a eucaloric, 1-week long low-fat (30% of total Kcal from fat) diet, assigned in a randomized crossover design.

MAIN OUTCOME MEASURES

Peripheral insulin sensitivity and metabolic flexibility during a euglycemic hyperinsulinemic (80 mU/m(2)/min) clamp measured during the follicular phase of the menstrual cycle, at the end of each diet period.

RESULTS

Peripheral insulin sensitivity (mg kg/fat-free mass/min (µU/mL)×10(-1)) was not decreased after the high-fat diet vs the low-fat diet (0.09±0.01 vs 0.08±0.01, p=0.09, respectively) in the combined group of African-American and white women, with no significant diet by race interaction (p=0.6). Metabolic flexibility (change in substrate utilization, ΔNPRQ, in response to insulin during the clamp) was similarly unaltered by the diet (0.12±0.01 vs 0.11, p=0.48, for the high-fat diet vs the low-fat diet, respectively) in the combined group of women, with no significant diet by race interaction (p=0.9). African-American women had a lower insulin clearance compared with the white women, regardless of the diet (p<0.05).

CONCLUSIONS

We conclude that a short term (1 week), moderate (50%), eucaloric high-fat diet does not lower peripheral insulin sensitivity in healthy, overweight and obese premenopausal women.

Metabolic inflexibility during submaximal aerobic exercise is associated with glucose intolerance in obese older adults

OBJECTIVE

People with type 2 diabetes have reduced cardiorespiratory fitness and metabolic impairments that are linked to obesity and often occur prior to the development of type 2 diabetes. We hypothesized that obese, older adults with impaired glucose tolerance (IGT) have lower ability to shift from fat to carbohydrate oxidation when transitioning from rest to submaximal exercise than normal glucose tolerant (NGT) controls.

DESIGN AND METHODS

Glucose tolerance, body composition, and substrate oxidation (measured by RER:respiratory exchange ratio) during submaximal exercise (50% and 60% VO₂max ) and insulin infusion (3-hour hyperinsulinemic-euglycemic clamp) were assessed in 23 sedentary, overweight-obese, older men and women.

RESULTS

Obese subjects with NGT (n = 13) and IGT (n = 10) had similar resting RER, but during submaximal exercise those with IGT had a lower RER and less transition to carbohydrate oxidation than the NGT group (P < 0.05). The IGT group also oxidized less carbohydrate during insulin infusion than NGT (P < 0.05). RER at each exercise intensity independently correlated with 120-minute postprandial glucose (r = -0.54 to -0.58, P < 0.05), but not with body composition, VO₂max , or RER during insulin infusion.

CONCLUSIONS

Obese, older adults have metabolic inflexibility during exercise that is associated with the degree of glucose intolerance independent of age and body composition.

Obesity and african americans: physiologic and behavioral pathways

Although progress has been made to understand the association between physiological and lifestyle behaviors with regard to obesity, ethnic differences in markers of obesity and pathways towards obesity remain somewhat unexplained. However, obesity remains a serious growing concern. This paper highlights ethnic differences in African Americans and Caucasians that may contribute to the higher prevalence of obesity among African Americans. Understanding ethnic differences in metabolic syndrome criteria, functioning of the hypothalamic pituitary adrenal axis, variations in glucocorticoid sensitivity and insulin resistance, and physical activity and cardiovascular fitness levels may help to inform practical clinical and public health interventions and reduce obesity disparities.

Glucose intolerance in the West African Diaspora: a skeletal muscle fibre type distribution hypothesis

In the United States, Black Americans are largely descendants of West African slaves; they have a higher relative proportion of obesity and experience a higher prevalence of diabetes than White Americans. However, obesity rates alone cannot explain the higher prevalence of type 2 diabetes. Type 2 diabetes is characterized by insulin resistance and beta-cell dysfunction. We hypothesize that the higher prevalence of type 2 diabetes in African Americans (as compared to White Americans) is facilitated by an inherited higher percentage of skeletal muscle fibre type II and a lower percentage of skeletal muscle fibre type I. Skeletal muscle fibre type II is less oxidative and more glycolytic than skeletal muscle fibre type I. Lower oxidative capacity is associated with lower fat oxidation and a higher disposal of lipids, which are stored as muscular adipose tissue in higher amounts in Black compared to White Americans. In physically active individuals, the influence of muscle fibre composition will not be as detrimental as in physically inactive individuals. This discrepancy is caused by the plasticity in the skeletal muscle fibre characteristics towards a higher activity of oxidative enzymes as a consequence of physical activity. We suggest that a higher percentage of skeletal muscle fibre type II combined with physical inactivity has an impact on insulin sensitivity and high prevalence of type 2 diabetes in Blacks of West African ancestry.

Exercise intensity and oxygen uptake kinetics in African-American and Caucasian women

The effect of exercise intensity on the on- and off-transient kinetics of oxygen uptake (VO(2)) was investigated in African American (AA) and Caucasian (C) women. African American (n = 7) and Caucasian (n = 6) women of similar age, body mass index and weight, performed an incremental test and bouts of square-wave exercise at moderate, heavy and very heavy intensities on a cycle ergometer. Gas exchange threshold (LT(GE)) was lower in AA (13.6 ± 2.3 mL kg(-1) min(-1)) than C (18.6 ± 5.6 mL kg(-1) min(-1)). The dynamic exercise and recovery VO(2) responses were characterized by mathematical models. There were no significant differences in (1) peak oxygen uptake (VO(2peak)) between AA (28.5 ± 5 mL kg(-1) min(-1)) and C (31.1 ± 6.6 mL kg(-1) min(-1)) and (2) VO(2) kinetics at any exercise intensity. At moderate exercise, the on- and off- VO(2) kinetics was described by a monoexponential function with similar time constants τ (1,on) (39.4 ± 12.5; 38.8 ± 15 s) and τ (1,off) (52.7 ± 10.1; 40.7 ± 4.4 s) for AA and C, respectively. At heavy and very heavy exercise, the VO(2) kinetics was described by a double-exponential function. The parameter values for heavy and very heavy exercise in the AA group were, respectively: τ (1,on) (47.0 ± 10.8; 44.3 ± 10 s), τ (2,on) (289 ± 63; 219 ± 90 s), τ (1,off) (45.9 ± 6.2; 50.7 ± 10 s), τ (2,off) (259 ± 120; 243 ± 93 s) while in the C group were, respectively: τ (1,on) (41 ± 12; 43.2 ± 15 s); τ (2, on) (277 ± 81; 215 ± 36 s), τ (1,off) (40.2 ± 3.4; 42.3 ± 7.2 s), τ (2,off) (215 ± 133; 228 ± 64 s). The on- and off-transients were symmetrical with respect to model order and dependent on exercise intensity regardless of race. Despite similar VO(2) kinetics, LT(GE) and gain of the VO(2) on-kinetics at moderate intensity were lower in AA than C. However, generalization to the African American and Caucasian populations is constrained by the small subject numbers.

Protein intake induced an increase in exercise stimulated fat oxidation during stable body weight

BACKGROUND

Protein-rich weight-loss diets spare fat-free mass at the cost of fat mass. The objective was to examine if there is a change in stimulated fat oxidation related to protein intake during stable body weight.

METHODS

Subjects' (BMI 22±2kg/m(2), age 25±8 years) maximal fat oxidation (Fat(max)) was assessed during a graded bicycle test, before and after a 3-month dietary-intervention of 2MJ/day supplements exchanged with 2MJ/d of habitual energy intake. The parallel design consisted of protein-rich supplements in the protein group and an isocaloric combination of carbohydrate and fat supplements in the control group. Daily protein intake was determined according to 24-h urine nitrogen. Body composition was measured according to a 4-compartment model by a combination of underwater-weighing technique, deuterium-dilution technique and whole-body dual-energy X-ray absorptiometry (DXA).

RESULTS

Subjects were weight stable and did not change their physical activity. The protein group (n=12) increased protein intake (11±14g, P<0.05) and had significantly higher daily protein intake vs. control (n=4) (80±21 vs.59±11g, P<0.05). Fat(max) increased significantly in the protein group (0.08±0.08g/min, P<0.01). Fat-free mass increased independent of change in body weight (P<0.01), and fat mass and fat percentage decreased (P<0.05). Change in Fat(max) was a function of change in protein intake (r=0.623, P<0.05), and not of changes in body composition or VO(2)max.

CONCLUSION

Increased stimulated fat oxidation was related to increased protein intake.

Greater adipose tissue infiltration in skeletal muscle among older men of African ancestry

CONTEXT

There is substantial variability across ethnic groups in the predisposition to obesity and associated metabolic abnormalities. Skeletal muscle fat has been identified as an important depot that increases with aging and may contribute to the development of diabetes.

OBJECTIVE

We tested whether men of African ancestry have greater calf intermuscular adipose tissue (IMAT), compared to Caucasian men, and whether IMAT is associated with type 2 diabetes (T2D).

DESIGN

We measured fasting serum glucose, body mass index, total body fat by dual-energy x-ray absorptiometry, and calf skeletal muscle composition by quantitative computed tomography in 1105 Caucasian and 518 Afro-Caribbean men aged 65+.

RESULTS

Compared to Caucasian men, we found greater IMAT and lower sc adipose tissue in Afro-Caribbean men at all levels of total adiposity (P < 0.0001), including the subset of men matched on age and dual-energy x-ray absorptiometry total body fat percentage (P < 0.001). In addition, IMAT was 29 and 23% greater, whereas sc adipose tissue was 6 and 8% lower among Caucasian and Afro-Caribbean men with T2D, respectively, compared to men without T2D (P < 0.01). Observed differences in intermuscular and sc fat, both ethnic and between men with and without T2D, were independent of age, height, calf skeletal muscle and total adipose tissue, and lifestyle factors.

CONCLUSIONS

Our analyses suggest that despite lower total adiposity, skeletal muscle fat infiltration is greater among African than among Caucasian ancestry men and is associated with T2D in both ethnic groups. Additional studies are needed to determine the mechanisms contributing to ethnic differences in skeletal muscle adiposity and to define the metabolic and health implications of this fat depot.

Racial difference in Acylation Stimulating Protein (ASP) correlates to triglyceride in non-obese and obese African American and Caucasian women

Background

Acylation Stimulating Protein (ASP) has been shown to influence adipose tissue triglyceride (TG) storage. The aim was to examine ethnic differences in ASP and leptin levels in relation to lipid profiles and postprandial changes amongst African American (AA) and Caucasian American (CA) women matched for BMI.

Methods

129 women were recruited in total (age 21 – 73 y): 24 non-obese (BMI < 30 kg/m²) CA, 27 obese (BMI ≥ 30 kg/m²) CA, 13 obese diabetic CA, 25 non-obese AA, 25 obese AA, and 15 obese diabetic AA. Cholesterol, HDL-C, LDL-C, apoB, glucose and insulin were measured at baseline. TG, non-esterified fatty acids, leptin, and ASP were measured at baseline and postprandially following a fat meal.

Results

ASP, leptin, insulin and TG were significantly increased in obese subjects within each race. However, AA women had significantly lower ASP and TG than CA women at all BMI. Obese and diabetic AA women had significantly lower apoB levels than CA women when compared to their respective counterparts. For AA women, fasting ASP was positively correlated with BMI, cholesterol, apoB, LDL-C and glucose. For CA women, fasting ASP was positively correlated with BMI, leptin, glucose and insulin. However, for any given BMI, ASP was significantly reduced in AA vs CA (p = 0.0004). Similarly, for any given leptin level or TG levels, ASP was significantly lower in AA women (p = 0.041 and p = 0.003, respectively).

Conclusion

CA women have higher baseline TG levels and an earlier TG peak that is accompanied with higher ASP levels suggesting increased ASP resistance, while AA women have lower baseline TG levels and a later TG peak at lower ASP levels suggesting increased ASP sensitivity. This may explain why AA women may have fewer metabolic complications, such as diabetes and CVD, when compared to their Caucasian counterparts at the same level of obesity.

Fat oxidation in black and white youth: a metabolic phenotype potentially predisposing black girls to obesity

INTRODUCTION

The prevalence of obesity is higher in Blacks with racial divergence in adiposity in girls starting during adolescence. Our hypothesis is that in Black children, puberty associated increase in fat oxidation is diminished and could play a role in predisposing to fat accretion triggered during puberty. Thus, we examined the relationships between race, pubertal development, and postabsorptive fat oxidation in youth.

SUBJECTS AND METHODS

This was a cross-sectional design of healthy Black (n=50) and white (n=51) youth. Resting metabolic rate (RMR) and substrate oxidation rate were measured after an overnight fast with indirect calorimetry. Body composition was measured by dual-energy x-ray absorptiometry.

RESULTS AND DISCUSSION

Within each race, RMR (kcal/kg fat free mass.min) was lower (puberty effect; P<0.05) in the pubertal vs. prepubertal group independent of gender. In girls, RMR was lower (race effect; P<0.05) in Blacks vs. whites. In girls but not boys, Blacks had lower (race effect; P=0.033) fat oxidation (micromol/kg fat free mass.min) compared with whites independent of pubertal status. Furthermore, the difference in fat oxidation between the prepubertal vs. pubertal groups tended to be greater (puberty x race interaction; P=0.089) in white girls (3.7+/-0.5 vs. 6.5+/-0.5) than in Black girls (3.4+/-0.6 vs. 4.5+/-0.5). These data suggest that the lower fat oxidation and RMR during puberty in Black girls could be a risk factor predisposing to obesity. This metabolic phenotype could potentially explain the divergence in adiposity in Black girls during adolescence against the backdrop of an obesogenic environment.

Cessation of daily wheel running differentially alters fat oxidation capacity in liver, muscle, and adipose tissue

Physical inactivity is associated with the increased risk of developing chronic metabolic diseases. To understand early alterations caused by physical inactivity, we utilize an animal model in which rats are transitioned from daily voluntary wheel running to a sedentary condition. In the hours and days following this transition, adipose tissue mass rapidly increases, due in part to increased lipogenesis. However, whether a concurrent decrease in fatty acid oxidative capacity (FAO) in skeletal muscle, liver, and adipose tissue occurs during this period is unknown. Following 6 wk of access to voluntary running wheels (average distance of approximately 6 km a night), rats were rapidly transitioned to a sedentary state by locking the wheels for 5 h (WL5) or 173 h (WL173). Complete ([(14)C]palmitate oxidation to (14)CO(2)) and incomplete ([(14)C]palmitate oxidation to (14)C-labeled acid soluble metabolites) was determined in isolated mitochondrial and whole homogenate preparations from skeletal muscle and liver and in isolated adipocytes. Strikingly, the elevated complete FAO in the red gastrocnemius at WL5 fell to that of rats that never ran (SED) by WL173. In contrast, hepatic FAO was elevated at WL173 above both WL5 and SED groups, while in isolated adipocytes, FAO remained higher in both running groups (WL5 and WL173) compared with the SED group. The alterations in muscle and liver fat oxidation were associated with changes in carnitine palmitoyl transferase-1 activity and inhibition, but not significant changes in other mitochondrial enzyme activities. In addition, peroxisome proliferator-activated receptor coactivator-1alpha mRNA levels that were higher in both skeletal muscle and liver at WL5 fell to SED levels at WL173. This study is the first to demonstrate that the transition from high to low daily physical activity causes rapid, tissue-specific changes in FAO.

Fat infiltration in muscle: new evidence for familial clustering and associations with diabetes

OBJECTIVE

Increased fat infiltration in skeletal muscle has been associated with diabetes. Quantitative computed tomography (QCT) can be used to measure muscle density, which reflects the lipid content of skeletal muscle such that greater fat infiltration in skeletal muscle is associated with lower muscle density. The relative contribution of genetic and environmental factors to fat infiltration in skeletal muscle has not been assessed. Therefore, our aim is to determine genetic and environmental contributions to measures of skeletal muscle composition, and describe their associations with type 2 diabetes in multigenerational families of African ancestry.

METHODS AND PROCEDURES

Peripheral QCT (pQCT) measures of skeletal muscle density were obtained for the calf in 471 individuals (60% women; mean 43 years) belonging to eight large, multigenerational Afro-Caribbean families (mean family size 51 individuals; 3,535 relative pairs).

RESULTS

The proportion of variance in muscle density due to additive genetic effects (residual heritability) was 35.0% (P < 0.001) and significant covariates (age, gender, BMI, and parity) explained 55.0% of the total phenotypic variation in muscle density. Muscle density was lower (P < 0.001) in 62 diabetics (69.5 mg/cm(3)) than in 339 nondiabetics (74.3 mg/cm(3)) and remained significant after adjusting for age, gender, and BMI (P = 0.005) or age, gender, and waist circumference (P = 0.01).

DISCUSSION

Our results provide new evidence that ectopic lipid deposition in skeletal muscle is a heritable trait and is associated with diabetes, independent of overall and central obesity in families of African heritage. Genome-wide screens and candidate gene studies are warranted to identify the genetic factors contributing to ectopic deposition of skeletal muscle fat.

Adipose tissue infiltration in skeletal muscle: age patterns and association with diabetes among men of African ancestry

BACKGROUND

Although obesity is strongly associated with diabetes, the greater prevalence of diabetes in persons of African ancestry than in those of other ancestries cannot be explained simply by differences in total or central adiposity.

OBJECTIVE

We examined whether skeletal muscle composition is associated with diabetes in 1249 men of African ancestry aged >or=40 y.

DESIGN

Anthropometry and fasting serum glucose were measured, and lower-leg skeletal muscle composition was assessed with peripheral quantitative computerized tomography (pQCT).

RESULTS

The prevalence of diabetes in this population was high (21%). We observed an age-associated adipose tissue remodeling in skeletal muscle and greater intermuscular (IMAT) and lesser subcutaneous (SAT) adipose tissue area with advancing age (P < 0.0001). Multivariate stepwise logistic regression identified more IMAT and less SAT to be significantly associated with a greater prevalence of diabetes. Even among normal-weight men [body mass index (BMI; in kg/m(2)) < 25], diabetic men had significantly (P = 0.01) more IMAT than did those without diabetes. Greater IMAT was also associated with a greater prevalence of hyperglycemia in men with a family history of diabetes than in those without such history (P for interaction = 0.02).

CONCLUSIONS

These findings underscore the independent associations of subcutaneous and intermuscular fat among men of African ancestry, an effect that may be modified by a family history of diabetes. Further studies are needed to identify the genetic and physiologic mechanisms that influence the distribution and remodeling of adipose tissue in skeletal muscle with aging.'

Peptide-YY levels after a fat load in black and white women

OBJECTIVE

To determine whether basal plasma peptide-YY (PYY) levels in overweight, middle-aged black women are different from those of white women of similar BMI and age and ascertain whether there is a difference between the two groups in plasma PYY levels in response to a liquid high fat load.

RESEARCH METHODS AND PROCEDURES

Using a commercial radioimmunoassay kit, the concentration of PYY was measured at baseline and at 2, 4, 6, and 8 hours after ingesting a fatty liquid meal (86.5% of the calories from fat) in 12 black and 12 white women who were matched for age and BMI.

RESULTS

PYY levels (picograms per milliliter) at baseline and at every other time-point of the test meal were significantly lower in the black than in the white group. In addition, the change in PYY concentration from baseline was lower in the black than in the white group only at 8 hours after the meal.

DISCUSSION

The lower baseline level and the blunted PYY response of the black women to the fat load indicated that this signal for appetite suppression was reduced, which, in turn, might contribute to the enhanced obesity of the black women.

Metabolic inflexibility in substrate use is present in African-American but not Caucasian healthy, premenopausal, nondiabetic women

CONTEXT

There is an increased prevalence of obesity and insulin resistance in African-American compared with Caucasian females. Metabolic inflexibility (MI) is the inability to switch the use of lipids and carbohydrates in the peripheral tissue (i.e. muscle) based upon substrate availability.

OBJECTIVE

We examined whether MI exists in African-American females. MAIN OUTCOME MEASURES AND DESIGN: We measured substrate use differences during eucaloric, macronutrient-manipulated diets [high fat (50% fat, 35% carbohydrate, 15% protein) vs. low fat (30% fat, 55% carbohydrate, 15% protein)] between Caucasian and African-American women. We also compared differences in substrate use in response to insulin infusion during two-step pancreatic-euglycemic clamps and epinephrine infusion during lipolysis studies. In each study, similar groups of Caucasian and African-American women were compared.

RESULTS

Caucasians had significantly higher fat oxidation (FO) (P = 0.01) and lower carbohydrate oxidation (P < 0.01) during the high-fat vs. low-fat diet, whereas no significant differences were observed in African-Americans. The African-American women also failed to significantly suppress FO during the second step of the pancreatic-euglycemic clamp despite a doubling of their fasting plasma insulin and failed to increase their FO or decrease their carbohydrate oxidation in response to epinephrine infusion as much as Caucasian women did. The response of free fatty acid turnover rates to insulin and epinephrine stimulation was similar between races.

CONCLUSION

The impaired substrate use observed in African-American women during these three studies demonstrates the existence of MI and may contribute to their greater prevalence of obesity and insulin resistance.

Skeletal muscle fat oxidation is increased in African-American and white women after 10 days of endurance exercise training

OBJECTIVE

Obesity is associated with lower rates of skeletal muscle fatty acid oxidation (FAO), which is linked to insulin resistance. FAO is reduced further in obese African-American (AAW) vs. white women (CW) and may also be lower in lean AAW vs. CW. In lean CW, endurance exercise training (EET) elevates the oxidative capacity of skeletal muscle. Therefore, we determined whether EET would elevate skeletal muscle FAO similarly in AAW and CW with a lower lipid oxidative capacity.

RESEARCH METHODS AND PROCEDURES

In vitro rates of FAO were assessed in rectus abdominus muscle strips using [1- 14C] palmitate (Pal) from lean AAW [BMI = 24.2 +/- 0.9 (standard error) kg/m2] and CW (23.6 +/- 0.8 kg/m2) undergoing voluntary abdominal surgery. Lean AAW (22 +/- 0.9 kg/m(2)) and CW (24 +/- 0.8 kg/m2) and obese AAW (36 +/- 1.2 kg/m2) and CW (40 +/- 1.3 kg/m2) underwent 10 consecutive days of EET on a cycle ergometer (60 min/d, 75% peak oxygen uptake). FAO was measured in vastus lateralis homogenates as captured 14CO2 using [1- 14C] Pal, palmitoyl-CoA (Pal-CoA), and palmityl-carnitine (Pal-Car).

RESULTS

Muscle strip experiments showed suppressed rates of FAO (p = 0.03) in lean AAW vs. CW. EET increased the rates of skeletal muscle Pal oxidation (p = 0.05) in both lean AAW and CW. In obese subjects, Pre-EET Pal (but not Pal-CoA or Pal-Car) oxidation was lower (p = 0.05) in AAW vs. CW. EET increased Pal oxidation 100% in obese AAW (p < 0.05) and 59% (p < 0.05) in obese CW. Similar increases (p < 0.05) in post-EET FAO were observed for Pal-CoA and Pal-Car in both groups.

DISCUSSION

Both lean and obese AAW possess a lower capacity for skeletal muscle FAO, but EET increases FAO similarly in both AAW and CW. These data suggest the use of EET for treatment against obesity and diabetes for both AAW and CW.

Differences in transport of fatty acids and expression of fatty acid transporting proteins in adipose tissue of obese black and white women

We have reported that the rate of de novo triglyceride (TG) synthesis by omental, but not subcutaneous, adipose tissue was higher in African-American women (AAW) than in Caucasian women (CAW). The purpose of this study was to explore the potential mechanisms underlying this increase. Toward that end, we determined the activities of key enzymes in the pathway of TG synthesis, the rates of uptake of fatty acids by adipocytes, mRNA and protein levels of the fatty acid-transporting proteins FAT/CD36 and FATP, and mRNA and protein levels of PPARgamma in omental fat of AAW and CAW. The results showed 1) no difference in the activity of phosphofructokinase, glycerol-3-phosphate dehydrogenase, or diacylglycerol acyltransferase; 2) a higher rate of fatty acid uptake by adipocytes of the AAW; 3) an increase in the mRNA and protein levels of CD36 and FATP4 in the fat of the AAW; and 4) an increase in the mRNA and protein levels of PPARgamma, which can stimulate the expression of CD36 and FATP. These results suggest that the increase in the transport of fatty acid, which is mediated by the overexpression of the transport proteins in the omental adipose tissue of the AAW, might contribute to the higher prevalence of obesity in AAW.

Fatty acid oxidation by skeletal muscle homogenates from morbidly obese black and white American women

The purpose of this study was to determine if there were differences in the capacity of skeletal muscle from morbidly obese Black and White American women to oxidize fatty acids. The oxidation rates of (14)C-palmitate, (14)C-palmitoyl-CoA, and (14)C-palmitoyl-carnitine were measured in whole homogenates of rectus abdominus from Black and White women who were similar in age and body mass index (BMI). The activities of muscle citrate synthase (CS), beta-hydroxy acyl-CoA dehydrogenase (beta-HAD), and mitochondrial and microsomal acyl-CoA synthetase (ACS) were measured in the 2 groups. The results showed that the rate of (14)C-palmitate oxidation by muscle of Black women was 25% that of Whites (8.7 +/- 1.5 v 34.4 +/- 6.8 nmol (14)CO(2) produced/gram tissue wet weight/ hour; P <.05), but the rates of (14)C-palmitoyl-CoA and (14)C-palmitoyl-carnitine oxidation were not different in the 2 groups. No differences were found in the activities of CS or beta-HAD. However, the activities of both mitochondrial and microsomal ACS were lower in the Black women than the Whites (mitochondrial ACS 25.1 +/- 3.9 v 36.4 +/- 5.0 nmol/mg protein/min; P <.05; microsomal ACS 6.2 +/- 0.5 v 8.5 +/- 0.5; nmol/mg protein/min; P <.005). The lower rate of palmitate oxidation, and the lack of differences in the rates of palmitoyl-CoA and palmitoyl-carnitine oxidation indicate that there is a defect in the activation of the fatty acid in the muscle of the Black women. This was confirmed by the decrease in mitochondrial ACS activity in the Black women. The decreased fatty acid oxidation by skeletal muscle of obese Black women could result in shunting these fuels from muscle to adipose tissue for storage, which may contribute to the maintenance of obesity in the Black women.

Muscle fiber type is associated with obesity and weight loss

The purpose of this study was to test the hypothesis that muscle fiber type is related to obesity. Fiber type was compared 1) in lean and obese women, 2) in Caucasian (C) and African-American (AA) women, and 3) in obese individuals who lost weight after gastric bypass surgery. When lean (body mass index 24.0 +/- 0.9 kg/m(2), n = 28) and obese (34.8 +/- 0.9 kg/m(2), n = 25) women were compared, there were significant (P < 0.05) differences in muscle fiber type. The obese women possessed fewer type I (41.5 +/- 1.8 vs. 54.6 +/- 1.8%) and more type IIb (25.1 +/- 1.5 vs. 14.4 +/- 1.5%) fibers than the lean women. When ethnicity was accounted for, the percentage of type IIb fibers in obese AA was significantly higher than in obese C (31.0 +/- 2.4% vs. 19.2 +/- 1.9%); fewer type I fibers were also found in obese AA (34.5 +/- 2.8% vs. 48.6 +/- 2.2%). These data are consistent with the higher incidence of obesity and greater weight gain reported in AA women. With weight loss intervention, there was a positive relationship (r = 0.72, P < 0.005) between the percentage of excess weight loss and the percentage of type I fibers in morbidly obese patients. These findings indicate that there is a relationship between muscle fiber type and obesity.