Mitochondrial Respiration is Associated with Lower Energy Expenditure and Lower Aerobic Capacity in African American Women

Racial and Gender Differences in Cardiorespiratory Fitness and Atrial Fibrillation

The expanding field of cardiorespiratory fitness (CRF) in individuals with and without atrial fibrillation (AF) presents a complex landscape, demanding careful interpretation of the existing research. AF, characterized by significant mortality and morbidity, prompts the exploration of strategies to mitigate its impact. Increasing physical activity (PA) levels emerges as a promising avenue to address AF risk factors, such as obesity, hypertension, and diabetes mellitus, through mechanisms of reduced vasoconstriction, endothelin-1 modulation, and improved insulin sensitivity. However, caution is warranted, as recent investigations suggest a heightened incidence of AF, particularly in athletes engaged in high-intensity exercise, due to the formation of ectopic foci and changes in cardiac anatomy. Accordingly, patients should adhere to guideline-recommended amounts of low-to-moderate PA to balance benefits and minimize adverse effects. When looking closer at the current evidence, gender-specific differences have been observed and challenged conventional understanding, with women demonstrating decreased AF risk even at extreme exercise levels. This phenomenon may be rooted in divergent hemodynamic and structural responses to exercise between men and women. Existing research is predominantly observational and limited to racially homogenous populations, which underscores the need for comprehensive studies encompassing diverse, non-White ethnic groups in athlete and non-athlete populations. These individuals exhibit a disproportionately high burden of AF risk factors that could be addressed through improved CRF. Despite the limitations, randomized control trials offer promising evidence for the efficacy of CRF interventions in patients with preexisting AF, showcasing improvements in clinically significant AF outcomes and patient quality of life. The potential of CRF as a countermeasure to the consequences of AF remains an area of great promise, urging future research to delve deeper to explore its role within specific racial and gender contexts. This comprehensive understanding will contribute to the development of tailored strategies for optimizing cardiovascular health and AF prevention in all those who are affected.

Bioactive peptides PDBSN improve mitochondrial function and suppression the oxidative stress in human adiposity cells

Introduction: Mitochondria are essential for generating cellular energy and are significant in the pathogenesis of obesity. Peptide PDBSN has been demonstrated to inhibit the adipogenic differentiation of adipocytes in vitro and improves metabolic homoeostasis in vivo. Therefore, in this study, we further investigated the effects of PDBSN on the morphology, synthesis, and function of adipocyte mitochondria. Methods: Human visceral and subcutaneous primary preadipocytes (HPA-v and HPA-s) were cultured into mature adipocytes. Intracellular triglyceride content was assessed using oil-red O staining and tissue triglyceride determination. Gene and protein levels associated with mitochondrial synthesis were detected using real-time quantitative polymerase chain reaction and western blotting. Mitochondrial membrane potentials and ROS were detected using fluorescent indicators. Morphological changes were observed by electron microscopy. Results: PDBSN significantly increased mitochondrial membrane potential (MMP), while decreasing intracellular triglyceride (TG) and intracellular reactive oxygen species (ROS) levels. On the other hand, the transcription and protein levels of genetic marker genes PGC1-α and MTFA were significantly up-regulated after PDBSN administration. Further studies showed that transcriptional and protein levels of mitochondrial fusion and fission genetic markers MFN1, MFN2, NRF1, and DRP1 increased. Conclusion: PDBSN significantly reduces intracellular TG and ROS levels and increases MMP. The maximum respiratory capacity in adults significantly increases after PDBSN administration, and ROS levels are significantly reduced. This suggests that PDBSN improves mitochondrial function to some extent, which not only provides an essential basis for the pathophysiology of obesity but also provides insights for the development of new drugs to treat obesity and metabolic diseases.

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.

Resting and total energy expenditure of patients with long-chain fatty acid oxidation disorders (LC-FAODs)

The basis of medical nutrition therapy for patients with LC-FAODs is to provide adequate energy to maintain anabolism and prevent catabolism. In practice, energy needs are estimated based on formulas derived from normal populations but it is unknown if energy expenditure among patients with LC-FAODs is similar to the normal population. We measured resting energy expenditure (REE), total energy expenditure (TEE) and body composition in 31 subjects with LC-FAODs ranging in age from 7 to 64 years. Measured REE was lower than estimated REE by various prediction equations and measured TEE was lower than estimated TEE. It is possible that the lower energy expenditure based on prediction formulas from the normal population is due to differences in body composition; we compared body composition to normal data from the 2017-18 National Health and Nutrition Examination Survey (NHANES). Fat free mass and fat mass was similar between subjects with an LC-FAOD and NHANES normal data suggesting no difference in body composition. We then compared measured REE and TEE to normal published data from the Dietary Reference Intakes (DRI). Measured REE and TEE were significantly lower among subjects with LC-FAODs compared to normal published energy expenditure data. Our results suggests patients with a LC-FAOD exhibit a lower REE and therefore actually have a slightly lower TEE than estimated. Current prediction equations may overestimate energy expenditure of patients with a LC-FAOD.

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.

Sex and race contribute to variation in mitochondrial function and insulin sensitivity

OBJECTIVE

Insulin sensitivity is lower in African American (AA) versus Caucasian American (CA). We tested the hypothesis that lower insulin sensitivity in AA could be explained by mitochondrial respiratory rates, coupling efficiency, myofiber composition, or H2 O2 emission. A secondary aim was to determine whether sex affected the results.

METHODS

AA and CA men and women, 19-45 years, BMI 17-43 kg m2 , were assessed for insulin sensitivity (SIClamp ) using a euglycemic clamp at 120 mU/m2 /min, muscle mitochondrial function using high-resolution respirometry, H2 O2 emission using amplex red, and % myofiber composition.

RESULTS

SIClamp was greater in CA (p < 0.01) and women (p < 0.01). Proportion of type I myofibers was lower in AA (p < 0.01). Mitochondrial respiratory rates, coupling efficiency, and H2 O2 production did not differ with race. Mitochondrial function was positively associated with insulin sensitivity in women but not men. Statistical adjustment for mitochondrial function, H2 O2 production, or fiber composition did not eliminate the race difference in SIClamp .

CONCLUSION

Neither mitochondrial respiratory rates, coupling efficiency, myofiber composition, nor mitochondrial reactive oxygen species production explained lower SIClamp in AA compared to CA. The source of lower insulin sensitivity in AA may be due to other aspects of skeletal muscle that have yet to be identified.

Associations Between Maternal Lifetime Stress and Placental Mitochondrial DNA Mutations in an Urban Multiethnic Cohort

BACKGROUND

Disrupted placental functioning due to stress can have lifelong implications. Cumulative stress and trauma are likely to have lasting impacts on maternal physiological functioning and offspring development, resulting in increased risk for later-life complex disorders for which racial disparities exist.

METHODS

This study examined the association between maternal lifetime stress and placental mitochondrial DNA mutational load in an urban multiethnic cohort. Maternal lifetime exposure to stressful events was assessed using the validated Life Stressor Checklist-Revised. Whole mitochondrial DNA sequencing was performed and mutations were determined for 365 placenta samples with complete exposure and covariate data. Multivariable regression was used to model maternal lifetime stress in relation to placental mitochondrial DNA mutational load. Racial/ethnic differences were examined by cross-product terms and contrast statements. Gene-wise analyses were conducted.

RESULTS

We identified 13,189 heteroplasmies (Phred score > 10,000, minor allele frequency < 0.5, number of mutant reads > 1). Women experiencing increased psychosocial stress over their lifetime exhibited a higher number of total placental mitochondrial mutations (β = .23, 95% confidence interval = .03 to .42) and heteroplasmic mutations (β = .18, 95% confidence interval = .05 to .31) but not homoplasmic mutations (β = -.008, 95% confidence interval = -.03 to .01); the strongest associations were observed among Black women and genes coding for NADH dehydrogenase and cytochrome c oxidase subunits.

CONCLUSIONS

Cumulative maternal lifetime stress is associated with a greater mitochondrial mutational load, particularly among Black women. The impact of racial/ethnic differences in mutational load on placental function directly affecting offspring development and/or leading to chronic disease disparities warrants further investigation.

Greater Skeletal Muscle Oxidative Capacity Is Associated With Higher Resting Metabolic Rate: Results From the Baltimore Longitudinal Study of Aging

Resting metabolic rate (RMR) tends to decline with aging. The age-trajectory of decline in RMR is similar to changes that occur in muscle mass, muscle strength, and fitness, but while the decline in these phenotypes has been related to changes of mitochondrial function and oxidative capacity, whether lower RMR is associated with poorer mitochondrial oxidative capacity is unknown. In 619 participants of the Baltimore Longitudinal Study of Aging, we analyzed the cross-sectional association between RMR (kcal/day), assessed by indirect calorimetry, and skeletal muscle maximal oxidative phosphorylation capacity, assessed as postexercise phosphocreatine recovery time constant (τ PCr), by phosphorous magnetic resonance spectroscopy. Linear regression models were used to evaluate the relationship between τ PCr and RMR, adjusting for potential confounders. Independent of age, sex, lean body mass, muscle density, and fat mass, higher RMR was significantly associated with shorter τ PCr, indicating greater mitochondrial oxidative capacity. Higher RMR is associated with a higher mitochondrial oxidative capacity in skeletal muscle. This association may reflect a relationship between better muscle quality and greater mitochondrial health.

Impaired glucose partitioning in primary myotubes from severely obese women with type 2 diabetes

The purpose of this study was to determine whether intramyocellular glucose partitioning was altered in primary human myotubes derived from severely obese women with type 2 diabetes. Human skeletal muscle cells were obtained from lean nondiabetic and severely obese Caucasian females with type 2 diabetes [body mass index (BMI): 23.6 ± 2.6 vs. 48.8 ± 1.9 kg/m², fasting glucose: 86.9 ± 1.6 vs. 135.6 ± 12.0 mg/dL, n = 9/group]. 1-[¹⁴C]-Glucose metabolism (glycogen synthesis, glucose oxidation, and nonoxidized glycolysis) and 1- and 2-[¹⁴C]-pyruvate oxidation were examined in fully differentiated myotubes under basal and insulin-stimulated conditions. Tricarboxylic acid cycle intermediates were determined via targeted metabolomics. Myotubes derived from severely obese individuals with type 2 diabetes exhibited impaired insulin-mediated glucose partitioning with reduced rates of glycogen synthesis and glucose oxidation and increased rates of nonoxidized glycolytic products, when compared with myotubes derived from the nondiabetic individuals (P < 0.05). Both 1- and 2-[¹⁴C]-pyruvate oxidation rates were significantly blunted in myotubes from severely obese women with type 2 diabetes compared with myotubes from the nondiabetic controls. Lastly, concentrations of tricarboxylic acid cycle intermediates, namely, citrate (P < 0.05), cis-aconitic acid (P = 0.07), and α-ketoglutarate (P < 0.05), were lower in myotubes from severely obese women with type 2 diabetes. These data suggest that intramyocellular insulin-mediated glucose partitioning is intrinsically altered in the skeletal muscle of severely obese women with type 2 diabetes in a manner that favors the production of glycolytic end products. Defects in pyruvate dehydrogenase and tricarboxylic acid cycle may be responsible for this metabolic derangement associated with type 2 diabetes.

Calorie Restriction and Aging in Humans

Calorie restriction (CR), the reduction of dietary intake below energy requirements while maintaining optimal nutrition, is the only known nutritional intervention with the potential to attenuate aging. Evidence from observational, preclinical, and clinical trials suggests the ability to increase life span by 1-5 years with an improvement in health span and quality of life. CR moderates intrinsic processes of aging through cellular and metabolic adaptations and reducing risk for the development of many cardiometabolic diseases. Yet, implementation of CR may require unique considerations for the elderly and other specific populations. The objectives of this review are to summarize the evidence for CR to modify primary and secondary aging; present caveats for implementation in special populations; describe newer, alternative approaches that have comparative effectiveness and fewer deleterious effects; and provide thoughts on the future of this important field of study.

Decreased Mitochondrial Dynamics Is Associated with Insulin Resistance, Metabolic Rate, and Fitness in African Americans

CONTEXT

African American women (AAW) have a higher incidence of insulin resistance and are at a greater risk for the development of obesity and type 2 diabetes than Caucasian women (CW). Although several factors have been proposed to mediate these racial disparities, the mechanisms remain poorly defined. We previously demonstrated that sedentary lean AAW have lower peripheral insulin sensitivity, reduced maximal aerobic fitness (VO2max), and lower resting metabolic rate (RMR) than CW. We have also demonstrated that skeletal muscle mitochondrial respiration is lower in AAW and appears to play a role in these racial differences.

OBJECTIVE

The goal of this study was to assess mitochondrial pathways and dynamics to examine the potential mechanisms of lower insulin sensitivity, RMR, VO2max, and mitochondrial capacity in AAW.

DESIGN

To achieve this goal, we assessed several mitochondrial pathways in skeletal muscle using gene array technology and semiquantitative protein analysis.

RESULTS

We report alterations in mitochondrial pathways associated with inner membrane small molecule transport genes, fusion-fission, and autophagy in lean AAW. These differences were associated with lower insulin sensitivity, RMR, and VO2max.

CONCLUSIONS

Together these data suggest that the metabolic racial disparity of insulin resistance, RMR, VO2max, and mitochondrial capacity may be mediated by perturbations in mitochondrial pathways associated with membrane transport, fission-fusion, and autophagy. The mechanisms contributing to these differences remain unknown.

VO2max is associated with measures of energy expenditure in sedentary condition but does not predict weight change

BACKGROUND/OBJECTIVES

Energy expenditure measured under sedentary conditions predicts weight change but evidence that directly measured VO₂max is associated with weight change is lacking. The aim of this study was to determine the associations of VO₂max with measures of predominantly sedentary 24-h thermogenesis, and subsequent weight change.

SUBJECTS/METHODS

Three hundred fifty-seven individuals (162 females; 27 Blacks, 72 Caucasians, and 258 American Indians) had measures of body composition, resting metabolic rate (RMR), and intermittent treadmill run test for assessment of VO₂max. On a separate day, 24-h energy expenditure (EE), diet-induced thermogenesis (DIT) expressed as "awake and fed" thermogenesis (AFT), sleeping metabolic rate (SMR), and spontaneous physical activity (SPA) were measured in a whole-room indirect calorimeter. Follow-up weight for 217 individuals was available (median follow-up time, 9.5 y; mean weight change, 12.4 ± 14.9 kg).

RESULTS

After adjustment for fat free mass, fat mass, age, sex, and race, a higher VO₂max was associated with a higher RMR (β = 68.2 kcal/day per L/min, P < 0.01) and 24-h EE (β = 62.2 kcal/day per L/min, P < 0.05) and including additional adjustment for energy intake higher AFT (β = 66.1 kcal/day per L/min, P = 0.01). Neither SMR (P > 0.2) nor SPA (P > 0.8) were associated with VO₂max. VO₂max at baseline did not predict follow-up weight after adjustment for baseline weight, follow-up time, sex, and race (P > 0.4).

CONCLUSION

VO₂max is associated with measures of EE including 24-h EE, RMR and DIT implying a common mechanism regulating the energetics of skeletal muscle during exercise and thermogenesis. However, this did not translate to VO₂max as a predictor of weight change.