Effect of exercise training on metabolic flexibility in response to a high-fat diet in obese individuals

Assessing metabolic flexibility response to a multifibre diet: a randomised-controlled trial

INTRODUCTION

Metabolic flexibility (MetF), defined as the ability to switch between fat and glucose oxidation, is increasingly recognised as a critical marker for assessing responses to dietary interventions. Previously, we showed that the consumption of multifibre bread improved insulin sensitivity and reduced low-density lipoprotein cholesterol (LDLc) levels in overweight and obese individuals. As a secondary objective, we aimed to explore whether our intervention could also improve MetF.

METHODS

In this study, 39 subjects at cardiometabolic risk participated in a double-blind, randomised, crossover trial lasting 8 weeks, repeated twice. During each phase, participants consumed either 150 g of standard bread daily or bread enriched with a mixture of seven dietary fibres. MetF response was assessed using a mixed-meal tolerance test (MMTT), analysing changes in respiratory quotient (∆RQ) measured using indirect calorimetry.

RESULTS

Although there were no significant differences in ∆RQ changes induced by dietary fibre between the two diets, these changes were positively correlated with postprandial triglyceride excursion (∆TG) at baseline. Subgroup analysis of baseline fasting and postprandial plasma metabolites was conducted to characterise MetF responders. These responders exhibited higher baseline fasting LDLc levels and greater post-MMTT ∆TG.

CONCLUSION

In conclusion, although dietary fibres did not directly impact MetF in this study, our findings highlight potential determinants of MetF response, warranting further investigation in dedicated future interventions.

Lipidomics of infant mesenchymal stem cells associate with the maternal milieu and child adiposity

Our objective was to interrogate mesenchymal stem cell (MSC) lipid metabolism and gestational exposures beyond maternal body mass that may contribute to child obesity risk. MSCs were cultured from term infants of mothers with obesity (n = 16) or normal weight (n = 15). In MSCs undergoing myogenesis in vitro, we used lipidomics to distinguish phenotypes by unbiased cluster analysis and lipid challenge (24-hour excess fatty acid [24hFA]). We measured MSC AMP-activated protein kinase (AMPK) activity and fatty acid oxidation (FAO), and a composite index of maternal glucose, insulin, triglycerides, free fatty acids, TNF-α, and high-density lipoprotein and total cholesterol in fasting blood from mid and late gestation (~17 and ~27 weeks, respectively). We measured child adiposity at birth (n = 29), 4-6 months (n = 29), and 4-6 years (n = 13). Three MSC clusters were distinguished by triacylglycerol (TAG) stores, with greatest TAGs in Cluster 2. All clusters increased acylcarnitines and TAGs with 24hFA, although Cluster 2 was more pronounced and corresponded to AMPK activation and FAO. Maternal metabolic markers predicted MSC clusters and child adiposity at 4-6 years (both highest in Cluster 3). Our data support the notion that MSC phenotypes are predicted by comprehensive maternal metabolic milieu exposures, independent of maternal BMI, and suggest utility as an at-birth predictor for child adiposity, although validation with larger longitudinal samples is warranted.

Nutritional Mechanisms of Cancer Cachexia

Cancer cachexia is a complex systemic wasting syndrome. Nutritional mechanisms that span energy intake, nutrient metabolism, body composition, and energy balance may be impacted by, and may contribute to, the development of cachexia. To date, clinical management of cachexia remains elusive. Leaning on discoveries and novel methodologies from other fields of research may bolster new breakthroughs that improve nutritional management and clinical outcomes. Characteristics that compare and contrast cachexia and obesity may reveal opportunities for cachexia research to adopt methodology from the well-established field of obesity research. This review outlines the known nutritional mechanisms and gaps in the knowledge surrounding cancer cachexia. In parallel, we present how obesity may be a different side of the same coin and how obesity research has tackled similar research questions. We present insights into how cachexia research may utilize nutritional methodology to expand our understanding of cachexia to improve definitions and clinical care in future directions for the field.

Associations between regional adipose tissue distribution and skeletal muscle bioenergetics in older men and women

OBJECTIVE

The aim of this study was to examine associations of ectopic adipose tissue (AT) with skeletal muscle (SM) mitochondrial bioenergetics in older adults.

METHODS

Cross-sectional data from 829 adults ≥70 years of age were used. Abdominal, subcutaneous, and visceral AT and thigh muscle fat infiltration (MFI) were quantified by magnetic resonance imaging. SM mitochondrial energetics were characterized in vivo (31P-magnetic resonance spectroscopy; ATPmax) and ex vivo (high-resolution respirometry maximal oxidative phosphorylation [OXPHOS]). ActivPal was used to measure physical activity ([PA]; step count). Linear regression adjusted for covariates was applied, with sequential adjustment for BMI and PA.

RESULTS

Independent of BMI, total abdominal AT (standardized [Std.] β = -0.21; R2 = 0.09) and visceral AT (Std. β = -0.16; R2 = 0.09) were associated with ATPmax (p < 0.01; n = 770) but not following adjustment for PA (p ≥ 0.05; n = 658). Visceral AT (Std. β = -0.16; R2 = 0.25) and thigh MFI (Std. β = -0.11; R2 = 0.24) were associated with carbohydrate-supported maximal OXPHOS independent of BMI and PA (p < 0.05; n = 609). Total abdominal AT (Std. β = -0.19; R2 = 0.24) and visceral AT (Std. β = -0.17; R2 = 0.24) were associated with fatty acid-supported maximal OXPHOS independent of BMI and PA (p < 0.05; n = 447).

CONCLUSIONS

Skeletal MFI and abdominal visceral, but not subcutaneous, AT are inversely associated with SM mitochondrial bioenergetics in older adults independent of BMI. Associations between ectopic AT and in vivo mitochondrial bioenergetics are attenuated by PA.

Fasting and Glucose Metabolism Differentially Impact Peripheral Inflammation in Human Type 2 Diabetes

Cytokines produced by peripheral T-helper 1/17 cells disproportionately contribute to the inflammation (i.e., metaflammation) that fuels type 2 diabetes (T2D) pathogenesis. Shifts in the nutrient milieu could influence inflammation through changes in T-cell metabolism. We aimed to determine whether changes in glucose utilization alter cytokine profiles in T2D. Peripheral blood mononuclear cells (PBMCs), CD4+ T-cells, and CD4+CD25- T-effector (Teff) cells were isolated from age-matched humans classified by glycemic control and BMI. Cytokines secreted by CD3/CD28-stimulated PBMCs and Teff were measured in supernatants with multiplex cytokine assays and a FLEXMAP-3D. Metabolic activity of stimulated CD4+ T-cells was measured by a Seahorse XFe96 analyzer. In this study, we demonstrated that T-cell stimulated PBMCs from non-fasted people with T2D produced higher amounts of cytokines compared to fasting. Although dysglycemia characterizes T2D, cytokine production by PBMCs or CD4+ T-cells in T2D was unaltered by hyperglycemic media. Moreover, pharmacological suppression of mitochondrial glucose oxidation did not change T-cell metabolism in T2D, yet enhanced cytokine competency. In conclusion, fasting and glucose metabolism differentially impact peripheral inflammation in human T2D, suggesting that glucose, along with fatty acid metabolites per our previous work, partner to regulate metaflammation. These data expose a major disconnect in the use of glycemic control drugs to target T2D-associated metaflammation.

Impact of aerobic fitness status, menstrual cycle phase, and oral contraceptive use on exercise substrate oxidation and metabolic flexibility in females

The influence of menstrual cycle phase and fitness status on metabolism during high-intensity interval exercise (HIIE) was assessed. Twenty-five females (24.4 (3.6) years) were categorized by normal menstrual cycle (n = 14) vs. oral contraceptive (OC) use (n = 11) and by aerobic fitness, high-fitness females (HFF; n = 13) vs. low-fitness females (LFF; n = 12). HIIE was four sets of four repetitions with a 3 min rest between intervals on a cycle ergometer at a power output halfway between the ventilatory threshold and V̇O2peak and performed during follicular (FOL: days 2-7 or inactive pills) and luteal phases (LUT: day ∼21 or 3rd week of active pills). Substrate oxidation was assessed via indirect calorimetry, blood lactate via finger stick, and recovery of skeletal muscle oxidative metabolism (mV̇O2) via continuous-wave near-infrared spectroscopy. HFF oxidized more fat (g·kg-1) during the full session (FOL: p = 0.050, LUT: p = 0.001), high intervals (FOL: p = 0.048, LUT: p = 0.001), low intervals (FOL: p = 0.032, LUT: p = 0.024), and LUT recovery (p = 0.033). Carbohydrate oxidation area under the curve was greater in HFF during FOL (FOL: p = 0.049, LUT: p = 0.124). Blood lactate was lower in LFF in FOL (p ≤ 0.05) but not in LUT. Metabolic flexibility (Δ fat oxidation g·kg-1·min-1) was greater in HFF than LFF during intervals 2-3 in FOL and 1-4 in LUT (p ≤ 0.05). Fitness status more positively influences exercise metabolic flexibility during HIIE than cycle phase or OC use.

The link between obesity and aging - insights into cardiac energy metabolism

Obesity and aging are well-established risk factors for a range of diseases, including cardiovascular diseases and type 2 diabetes. Given the escalating prevalence of obesity, the aging population, and the subsequent increase in cardiovascular diseases, it is crucial to investigate the underlying mechanisms involved. Both aging and obesity have profound effects on the energy metabolism through various mechanisms, including metabolic inflexibility, altered substrate utilization for energy production, deregulated nutrient sensing, and mitochondrial dysfunction. In this review, we aim to present and discuss the hypothesis that obesity, due to its similarity in changes observed in the aging heart, may accelerate the process of cardiac aging and exacerbate the clinical outcomes of elderly individuals with obesity.

MCUb is an inducible regulator of calcium-dependent mitochondrial metabolism and substrate utilization in muscle

Mitochondria use the electron transport chain to generate high-energy phosphate from oxidative phosphorylation, a process also regulated by the mitochondrial Ca2+ uniporter (MCU) and Ca2+ levels. Here, we show that MCUb, an inhibitor of MCU-mediated Ca2+ influx, is induced by caloric restriction, where it increases mitochondrial fatty acid utilization. To mimic the fasted state with reduced mitochondrial Ca2+ influx, we generated genetically altered mice with skeletal muscle-specific MCUb expression that showed greater fatty acid usage, less fat accumulation, and lower body weight. In contrast, mice lacking Mcub in skeletal muscle showed increased pyruvate dehydrogenase activity, increased muscle malonyl coenzyme A (CoA), reduced fatty acid utilization, glucose intolerance, and increased adiposity. Mechanistically, pyruvate dehydrogenase kinase 4 (PDK4) overexpression in muscle of Mcub-deleted mice abolished altered substrate preference. Thus, MCUb is an inducible control point in regulating skeletal muscle mitochondrial Ca2+ levels and substrate utilization that impacts total metabolic balance.

Associations between regional adipose tissue distribution and skeletal muscle bioenergetics in older men and women

Objective

Examine the association of ectopic adipose tissue (AT) with skeletal muscle (SM) mitochondrial bioenergetics in older adults.

Methods

Cross-sectional data from 829 older adults ≥70 years was used. Total abdominal, subcutaneous, and visceral AT; and thigh muscle fat infiltration (MFI) was quantified by MRI. SM mitochondrial energetics were characterized using in vivo 31 P-MRS (ATP max ) and ex vivo high-resolution respirometry (maximal oxidative phosphorylation (OXPHOS)). ActivPal was used to measure PA (step count). Linear regression models adjusted for covariates were applied, with sequential adjustment for BMI and PA.

Results

Independent of BMI, total abdominal (standardized (Std.) β=-0.21; R 2 =0.09) and visceral AT (Std. β=-0.16; R 2 =0.09) were associated with ATP max ( p <0.01), but not after further adjustment for PA (p≥0.05). Visceral AT (Std. β=-0.16; R 2 =0.25) and thigh MFI (Std. β=-0.11; R 2 =0.24) were negatively associated with carbohydrate-supported maximal OXPHOS independent of BMI and PA ( p <0.05). Total abdominal AT (Std. β=-0.19; R 2 =0.24) and visceral AT (Std. β=-0.17; R 2 =0.24) were associated with fatty acid-supported maximal OXPHOS independent of BMI and PA (p<0.05).

Conclusions

Skeletal MFI and abdominal visceral, but not subcutaneous AT, are inversely associated with SM mitochondrial bioenergetics in older adults independent of BMI. Associations between ectopic AT and in vivo mitochondrial bioenergetics are attenuated by PA.

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.

The impact of sprint interval training with or without weight loss on substrate oxidation in adults: A secondary analysis of the i-FLEX study

Endurance exercise training and weight loss (WL) have been associated with changes in fat oxidation. However, there is limited evidence investigating the impact of sprint interval training (SIT)-induced WL on fat oxidation in adults. To investigate the impact of SIT with or without WL on fat oxidation, 34 adults aged 19-60 years (males, n = 15) took part in 4-week SIT. SIT consisted of 30-s Wingates starting with two intervals and working up to four interspersed with 4 min of active recovery. Fat oxidation was estimated via indirect calorimetry using a metabolic cart during submaximal cycling. Following the intervention, participants were classified into a WL group (weight change >0 kg) or a non-WL group (weight change ≤0 kg). No difference in resting fat oxidation (p = 0.642) and respiratory exchange ratio (RER) (p = 0.646) were observed between the groups. There was a significant interaction for the WL group with increased submaximal fat oxidation usage (p = 0.005) and decreased submaximal RER over the duration of the study (p = 0.017). When adjusted for baseline weight and sex, submaximal fat oxidation usage remained significant (p < 0.05), while RER did not (p = 0.081). The WL group had higher work volume, relative peak power, and mean power than the non-WL group (p < 0.05). Short-term SIT elicited significant improvements in submaximal RER and fat oxidation (FOx) in adults that lost weight, which may be explained by an increase in work volume throughout SIT training.

FAT/CD36 Participation in Human Skeletal Muscle Lipid Metabolism: A Systematic Review

Fatty acid translocase/cluster of differentiation 36 (FAT/CD36) is a multifunctional membrane protein activated by a high-fat diet, physical exercise, fatty acids (FAs), leptin, and insulin. The principal function of FAT/CD36 is to facilitate the transport of long-chain fatty acids through cell membranes such as myocytes, adipocytes, heart, and liver. Under high-energy expenditure, the different isoforms of FAT/CD36 in the plasma membrane and mitochondria bind to the mobilization and oxidation of FAs. Furthermore, FAT/CD36 is released in its soluble form and becomes a marker of metabolic dysfunction. Studies with healthy animals and humans show that physical exercise and a high-lipid diet increase FAT/CD36 expression and caloric expenditure. However, several aspects such as obesity, diabetes, Single Nucleotide polymorphisms (SNPs), and oxidative stress affect the normal FAs metabolism and function of FAT/CD36, inducing metabolic disease. Through a comprehensive systematic review of primary studies, this work aimed to document molecular mechanisms related to FAT/CD36 in FAs oxidation and trafficking in skeletal muscle under basal conditions, physical exercise, and diet in healthy individuals.

Metabolic Flexibility and Its Impact on Health Outcomes

A metabolically flexible state exists when there is a rapid switch between glucose and fatty acids during the transition between the fed and fasting state. This flexibility in fuel choice serves to prevent hyperglycemia following a meal and simultaneously ensures an adequate amount of blood glucose is available for delivery to the brain and exclusively glycolytic tissues during fasting. The modern era is characterized by chronic overnutrition in which a mixture of fuels is delivered to the mitochondria in an unabated manner thereby uncoupling the feast and famine situation. The continuous influx of fuel leads to accumulation of reducing equivalents in the mitochondria and an increase in the mitochondrial membrane potential. These changes create a microenvironment fostering the generation of reactive oxygen species and other metabolites leading to deleterious protein modification, cell injury, and ultimately clinical disease. Insulin resistance may also play a primary role in this deleterious effect. The imbalance between mitochondrial energy delivery and use is made worse with a sedentary lifestyle. Maneuvers that restore energy balance across the mitochondria activate pathways that remove or repair damaged molecules and restore the plasticity characteristic of normal energy metabolism. Readily available strategies to maintain energy balance across the mitochondria include exercise, various forms of caloric restriction, administration of sodium-glucose cotransporter-2 inhibitors, cold exposure, and hypobaric hypoxia.

Molecular Mechanisms of Exercise and Healthspan

Healthspan is the period of our life without major debilitating diseases. In the modern world where unhealthy lifestyle choices and chronic diseases taper the healthspan, which lead to an enormous economic burden, finding ways to promote healthspan becomes a pressing goal of the scientific community. Exercise, one of humanity's most ancient and effective lifestyle interventions, appears to be at the center of the solution since it can both treat and prevent the occurrence of many chronic diseases. Here, we will review the current evidence and opinions about regular exercise promoting healthspan through enhancing the functionality of our organ systems and preventing diseases.

Inflammation and Fibrogenesis in MAFLD: Role of the Hepatic Immune System

Metabolic (dysfunction)-associated fatty liver disease (MAFLD) is the definition recently proposed to better circumscribe the spectrum of conditions long known as non-alcoholic fatty liver disease (NAFLD) that range from simple steatosis without inflammation to more advanced liver diseases. The progression of MAFLD, as well as other chronic liver diseases, toward cirrhosis, is driven by hepatic inflammation and fibrogenesis. The latter, result of a "chronic wound healing reaction," is a dynamic process, and the understanding of its underlying pathophysiological events has increased in recent years. Fibrosis progresses in a microenvironment where it takes part an interplay between fibrogenic cells and many other elements, including some cells of the immune system with an underexplored or still unclear role in liver diseases. Some therapeutic approaches, also acting on the immune system, have been probed over time to evaluate their ability to improve inflammation and fibrosis in NAFLD, but to date no drug has been approved to treat this condition. In this review, we will focus on the contribution of the liver immune system in the progression of NAFLD, and on therapies under study that aim to counter the immune substrate of the disease.

Twenty-four hour assessments of substrate oxidation reveal differences in metabolic flexibility in type 2 diabetes that are improved with aerobic training

AIMS/HYPOTHESIS

The aim of this study was to assess metabolic flexibility (MetFlex) in participants with type 2 diabetes within the physiologically relevant conditions of sleeping, the post-absorptive (fasting) state and during meals using 24 h whole-room indirect calorimetry (WRIC) and to determine the impact of aerobic training on these novel features of MetFlex.

METHODS

Normal-weight, active healthy individuals (active; n = 9), obese individuals without type 2 diabetes (ND; n = 9) and obese individuals with type 2 diabetes (n = 23) completed baseline metabolic assessments. The type 2 diabetes group underwent a 10 week supervised aerobic training intervention and repeated the metabolic assessments. MetFlex was assessed by indirect calorimetry in response to insulin infusion and during a 24 h period in a whole-room indirect calorimeter. Indices of MetFlex evaluated by WRIC included mean RQ and RQ kinetic responses after ingesting a standard high-carbohydrate breakfast (RQ_BF) and sleep RQ (RQ_sleep). Muscle mitochondrial energetics were assessed in the vastus lateralis muscle in vivo and ex vivo using ³¹P-magnetic resonance spectroscopy and high-resolution respirometry, respectively.

RESULTS

The three groups had significantly different RQ_sleep values (active 0.823 ± 0.04, ND 0.860 ± 0.01, type 2 diabetes 0.842 ± 0.03; p < 0.05). The active group had significantly faster RQ_BF and more stable RQ_sleep responses than the ND and type 2 diabetes groups, as demonstrated by steeper and flatter slopes, respectively. Following the training intervention, the type 2 diabetes group displayed significantly increased RQ_BF slope. Several indices of RQ kinetics had significant associations with in vivo and ex vivo muscle mitochondrial capacities.

CONCLUSIONS/INTERPRETATION

Twenty-four hour WRIC revealed that physiological RQ responses exemplify differences in MetFlex across a spectrum of metabolic health and correlated with skeletal muscle mitochondrial energetics. Defects in certain features of MetFlex were improved with aerobic training, emphasising the need to assess multiple aspects of MetFlex and disentangle insulin resistance from MetFlex in type 2 diabetes.

TRIAL REGISTRATION

ClinicalTrials.gov NCT01911104.

FUNDING

This study was funded by the ADA (grant no. 7-13-JF-53).

Effects of sprint interval training on substrate oxidation in adults living with and without obesity: The i-FLEX study

Metabolic flexibility is the ability to adapt substrate oxidation according to metabolic demand. Exercise increases fat oxidation responses in individuals living with obesity; however, limited research exists on the relationship between substrate oxidation and insulin sensitivity after sprint interval training (SIT). The primary objective was to investigate changes in substrate oxidation at rest and during submaximal exercise, and in insulin sensitivity after 4 weeks of SIT in individuals living with or without obesity. The secondary objective was to investigate correlations between changes in substrate oxidation and insulin sensitivity following SIT. Adults living with obesity (n = 16, body mass index (BMI) = 34.1 kg/m2  ± 3.8) and without obesity (n = 18, BMI = 22.9 kg/m2  ± 1.6) took part in a 4-week SIT intervention. Participants completed three sessions of SIT per week, consisting of repeated sets of a 30-s Wingate separated by 4 m of active recovery. Substrate oxidation at rest and during submaximal exercise was measured using VCO2 /VO2 . Insulin sensitivity was calculated using the Matsuda index. No difference in substrate oxidation at rest was observed for either group (p > 0.05), while a significant increase in fat oxidation was observed in individuals living with obesity [F(1,31) = 14.55, p = 0.001] during the submaximal exercise test. A significant decrease in insulin sensitivity was observed among individuals without obesity [F(1,31) = 5.010, p = 0.033]. No correlations were observed between changes in substrate oxidation and insulin sensitivity (p > 0.05). Following SIT, individuals living with obesity increased submaximal fat oxidation compared to individuals without obesity. No correlations were observed between substrate oxidation and insulin sensitivity. Thus, SIT impacts fat oxidation during exercise in individuals living with obesity while having no such influence on insulin sensitivity.

Influence of Exercise on the Human Gut Microbiota of Healthy Adults: A Systematic Review

To summarize the literature on the influence of exercise on the gut microbiota of healthy adults.

Effects of Feeding Time on Markers of Muscle Metabolic Flexibility Following Acute Aerobic Exercise in Trained Mice Undergoing Time Restricted Feeding

Time-restricted feeding (TRF) is becoming a popular way of eating in physically active populations, despite a lack of research on metabolic and performance outcomes as they relate to the timing of food consumption in relation to the time of exercise. The purpose of this study was to determine if the timing of feeding/fasting after exercise training differently affects muscle metabolic flexibility and response to an acute bout of exercise. Male C57BL/6 mice were randomized to one of three groups for 8 weeks. The control had ad libitum access to food before and after exercise training. TRF-immediate had immediate access to food for 6 h following exercise training and the TRF-delayed group had access to food 5-h post exercise for 6 h. The timing of fasting did not impact performance in a run to fatigue despite TRF groups having lower hindlimb muscle mass. TRF-delayed had lower levels of muscle HSL mRNA expression and lower levels of PGC-1α expression but displayed no changes in electron transport chain enzymes. These results suggest that in young populations consuming a healthy diet and exercising, the timing of fasting may not substantially impact metabolic flexibility and running performance.

Altered Metabolic Flexibility in Inherited Metabolic Diseases of Mitochondrial Fatty Acid Metabolism

In general, metabolic flexibility refers to an organism's capacity to adapt to metabolic changes due to differing energy demands. The aim of this work is to summarize and discuss recent findings regarding variables that modulate energy regulation in two different pathways of mitochondrial fatty metabolism: β-oxidation and fatty acid biosynthesis. We focus specifically on two diseases: very long-chain acyl-CoA dehydrogenase deficiency (VLCADD) and malonyl-CoA synthetase deficiency (acyl-CoA synthetase family member 3 (ACSF3)) deficiency, which are both characterized by alterations in metabolic flexibility. On the one hand, in a mouse model of VLCAD-deficient (VLCAD^-/-) mice, the white skeletal muscle undergoes metabolic and morphologic transdifferentiation towards glycolytic muscle fiber types via the up-regulation of mitochondrial fatty acid biosynthesis (mtFAS). On the other hand, in ACSF3-deficient patients, fibroblasts show impaired mitochondrial respiration, reduced lipoylation, and reduced glycolytic flux, which are compensated for by an increased β-oxidation rate and the use of anaplerotic amino acids to address the energy needs. Here, we discuss a possible co-regulation by mtFAS and β-oxidation in the maintenance of energy homeostasis.

Metabolic flexibility is unimpaired during exercise in the cold following acute glucose ingestion in young healthy adults

PURPOSE

Metabolic flexibility is compromised in individuals suffering from metabolic diseases, lipo- and glucotoxicity, and mitochondrial dysfunctions. Exercise studies performed in cold environments have demonstrated an increase in lipid utilization, which could lead to a compromised substrate competition, glycotoxic-lipotoxic state, or metabolic inflexibility. Whether metabolic flexibility is altered during incremental maximal exercise to volitional fatigue in a cold environment remains unclear.

METHODS

Ten young healthy participants performed four maximal incremental treadmill tests to volitional fatigue, in a fasted state, in a cold (0 °C) or a thermoneutral (22.0 °C) environment, with and without a pre-exercise ingestion of a 75-g glucose solution. Metabolic flexibility was assessed via indirect calorimetry using the change in respiratory exchange ratio (ΔRER), maximal fat oxidation (ΔMFO), and where MFO occurred along the exercise intensity spectrum (ΔFat_max), while circulating lactate and glucose levels were measured pre and post exercise.

RESULTS

Multiple linear mixed-effects regressions revealed an increase in glucose oxidation from glucose ingestion and an increase in lipid oxidation from the cold during exercise (p < 0.001). No differences were observed in metabolic flexibility as assessed via ΔRER (0.05 ± 0.03 vs. 0.05 ± 0.03; p = 0.734), ΔMFO (0.21 ± 0.18 vs. 0.16 ± 0.13 g min^-1; p = 0.133) and ΔFat_max (13.3 ± 19.0 vs. 0.6 ± 21.3 %V̇O_2peak; p = 0.266) in cold and thermoneutral, respectively.

CONCLUSIONS

Following glucose loading, metabolic flexibility was unaffected during exercise to volitional fatigue in a cold environment, inducing an increase in lipid oxidation. These results suggest that competing pathways responsible for the regulation of fuel selection during exercise and cold exposure may potentially be mechanistically independent. Whether long-term metabolic influences of high-fat diets and acute lipid overload in cold and warm environments would impact metabolic flexibility remain unclear.

Metabolism and Metabolic Disorders and the Microbiome: The Intestinal Microbiota Associated With Obesity, Lipid Metabolism, and Metabolic Health-Pathophysiology and Therapeutic Strategies

Changes in the intestinal microbiome have been associated with obesity and type 2 diabetes, in epidemiological studies and studies of the effects of fecal transfer in germ-free mice. We review the mechanisms by which alterations in the intestinal microbiome contribute to development of metabolic diseases, and recent advances, such as the effects of the microbiome on lipid metabolism. Strategies have been developed to modify the intestinal microbiome and reverse metabolic alterations, which might be used as therapies. We discuss approaches that have shown effects in mouse models of obesity and metabolic disorders, and how these might be translated to humans to improve metabolic health.

Tuning fatty acid oxidation in skeletal muscle with dietary fat and exercise

Both the consumption of a diet rich in fatty acids and exercise training result in similar adaptations in several skeletal muscle proteins. These adaptations are involved in fatty acid uptake and activation within the myocyte, the mitochondrial import of fatty acids and further metabolism of fatty acids by β-oxidation. Fatty acid availability is repeatedly increased postprandially during the day, particularly during high dietary fat intake and also increases during, and after, aerobic exercise. As such, fatty acids are possible signalling candidates that regulate transcription of target genes encoding proteins involved in muscle lipid metabolism. The mechanism of signalling might be direct or indirect targeting of peroxisome proliferator-activated receptors by fatty acid ligands, by fatty acid-induced NAD⁺-stimulated activation of sirtuin 1 and/or fatty acid-mediated activation of AMP-activated protein kinase. Lactate might also have a role in lipid metabolic adaptations. Obesity is characterized by impairments in fatty acid oxidation capacity, and individuals with obesity show some rigidity in increasing fatty acid oxidation in response to high fat intake. However, individuals with obesity retain improvements in fatty acid oxidation capacity in response to exercise training, thereby highlighting exercise training as a potential method to improve lipid metabolic flexibility in obesity.

Plasma Lactate as a Marker for Metabolic Health

Blood lactate concentrations traditionally have been used as an index of exercise intensity or clinical hyperlactatemia. However, more recent data suggest that fasting plasma lactate can also be indicative of the risk for subsequent metabolic disease. The hypothesis presented is that fasting blood lactate accumulation reflects impaired mitochondrial substrate use, which in turn influences metabolic disease risk.

Difference in Housing Temperature-Induced Energy Expenditure Elicits Sex-Specific Diet-Induced Metabolic Adaptations in Mice

OBJECTIVE

The aim of this study was to test whether increased energy expenditure (EE), independent of physical activity, reduces acute diet-induced weight gain through tighter coupling of energy intake to energy demand and enhanced metabolic adaptations.

METHODS

Indirect calorimetry and quantitative magnetic resonance imaging were used to assess energy metabolism and body composition during 7-day high-fat/high-sucrose (HFHS) feeding in male and female mice housed at divergent temperatures (20°C vs. 30°C).

RESULTS

As previously observed, 30°C housing resulted in lower total EE and energy intake compared with 20°C mice regardless of sex. Interestingly, housing temperature did not impact HFHS-induced weight gain in females, whereas 30°C male mice gained more weight than 20°C males. Energy intake coupling to EE during HFHS feeding was greater in 20°C versus 30°C housing, with females greater at both temperatures. Fat mass gain was greater in 30°C mice compared with 20°C mice, whereas females gained less fat mass than males. Strikingly, female 20°C mice gained considerably more fat-free mass than 30°C mice. Reduced fat mass gain was associated with greater metabolic flexibility to HFHS, whereas fat-free mass gain was associated with diet-induced adaptive thermogenesis.

CONCLUSIONS

These data reveal that EE and sex interact to impact energy homeostasis and metabolic adaptation to acute HFHS feeding, altering weight gain and body composition change.

Contribution of macronutrients to obesity: implications for precision nutrition

The specific metabolic contribution of consuming different energy-yielding macronutrients (namely, carbohydrates, protein and lipids) to obesity is a matter of active debate. In this Review, we summarize the current research concerning associations between the intake of different macronutrients and weight gain and adiposity. We discuss insights into possible differential mechanistic pathways where macronutrients might act on either appetite or adipogenesis to cause weight gain. We also explore the role of dietary macronutrient distribution on thermogenesis or energy expenditure for weight loss and maintenance. On the basis of the data discussed, we describe a novel way to manage excessive body weight; namely, prescribing personalized diets with different macronutrient compositions according to the individual's genotype and/or enterotype. In this context, the interplay of macronutrient consumption with obesity incidence involves mechanisms that affect appetite, thermogenesis and metabolism, and the outcomes of these mechanisms are altered by an individual's genotype and microbiota. Indeed, the interactions of the genetic make-up and/or microbiota features of a person with specific macronutrient intakes or dietary pattern consumption help to explain individualized responses to macronutrients and food patterns, which might represent key factors for comprehensive precision nutrition recommendations and personalized obesity management.

Postprandial skeletal muscle metabolism following a high-fat diet in sedentary and endurance-trained males

Our objective was to determine the influence of a high-fat diet (HFD) on fasting and postprandial skeletal muscle substrate metabolism in endurance-trained (ET) compared with sedentary (SED) humans. SED (n = 17) and ET (n = 7) males were control-fed a 10-day moderate-fat diet followed by a 5-day isocaloric HFD (55% fat, 30% carbohydrate). Skeletal muscle biopsies were taken in the fasted condition and 4 h after a high-fat meal (820 kcals; 63% fat and 25% carbohydrate). Palmitate-induced suppression of pyruvate oxidation, an indication of substrate preference, and oxidation of fat and glucose were measured in homogenized skeletal muscle in fasted and fed states. Postprandial responses were calculated as percent changes from fasting to fed states. Postprandial suppression of pyruvate oxidation was maintained after the HFD in ET, but not SED skeletal muscle, suggesting greater adaptability to dietary intake changes in the former. Fasting total fat oxidation increased due to the HFD in ET skeletal muscle (P = 0.006), which was driven by incomplete fat oxidation (P = 0.008). Fasting fat oxidation remained unchanged in skeletal muscle of SED individuals. Yet, postprandial fat oxidation was similar between groups. Fasting glucose oxidation was elevated after the HFD in ET (P = 0.036), but not SED, skeletal muscle. Postprandial glucose oxidation was reduced due to the HFD in SED (P = 0.002), but not ET, skeletal muscle. These findings provide insight into differing substrate metabolism responses between SED and ET individuals and highlight the role that the prevailing diet may play in modulating fasting and postprandial metabolic responses in skeletal muscle.NEW & NOTEWORTHY The relationship between high dietary fat intake and physical activity level and their combined effect on skeletal muscle substrate metabolism remains unclear. We assessed the influence of the prevailing diet in modulating substrate oxidation in skeletal muscle of endurance-trained compared with sedentary humans during a high-fat challenge meal. Collectively, our findings demonstrate the adaptability of skeletal muscle in endurance-trained individuals to high dietary fat intake.

Metabolic flexibility is impaired in women who are pregnant and overweight/obese and related to insulin resistance and inflammation

CONTEXT

Maternal obesity is a significant public health concern that contributes to unfavorable outcomes such as inflammation and insulin resistance. Women with obesity may have impaired metabolic flexibility (i.e. an inability to adjust substrate metabolism according to fuel availability). Impaired metabolic flexibility during pregnancy may mediate poor pregnancy outcomes in women with obesity.

PURPOSE

The purposes of this study were to: 1) compare metabolic flexibility between overweight/obese and lean women; and 2) determine the relationships between metabolic flexibility, inflammation following a high-fat meal, and maternal metabolic health outcomes (i.e. gestational weight gain and insulin resistance).

PROCEDURES

This interventional physiology study assessed lipid oxidation rates via indirect calorimetry before and after consumption of a high-fat meal. The percent change in lipid metabolism was calculated to determine 'metabolic flexibility.' Maternal inflammatory profiles (CRP, IL-6, IL-8, IL-10, IL-12, TNF-α) and insulin resistance (HOMA-IR) were determined via plasma analyses.

MAIN FINDINGS

64 women who were pregnant (lean = 35, overweight/obese = 29) participated between 32 and 38 weeks gestation. Lean women had significantly higher metabolic flexibility compared to overweight/obese women (lean 48.0 ± 34.1% vs overweight/obese 29.3 ± 34.3%, p = .035). Even when controlling for pre-pregnancy BMI, there was a negative relationship between metabolic flexibility and percent change in CRP among the overweight/obese group (r = -0.526, p = .017). Metabolic flexibility (per kg fat free mass) was negatively correlated with postprandial HOMA-IR (2 h: r = -0.325, p = .016; 4 h: r = -0.319, p = .019).

CONCLUSIONS

Overweight and obese women who are pregnant are less 'metabolically flexible' than lean women, and this is related to postprandial inflammation and insulin resistance.

Palmitoylethanolamide counteracts hepatic metabolic inflexibility modulating mitochondrial function and efficiency in diet-induced obese mice

Peroxisome proliferator-activated receptor (PPAR)-α activation controls hepatic lipid homeostasis, stimulating fatty acid oxidation, and adapting the metabolic response to lipid overload and storage. Here, we investigate the effect of palmitoylethanolamide (PEA), an endogenous PPAR-α ligand, in counteracting hepatic metabolic inflexibility and mitochondrial dysfunction induced by high-fat diet (HFD) in mice. Long-term PEA administration (30 mg/kg/die per os) in HFD mice limited hepatic lipid accumulation, increased energy expenditure, and markedly reduced insulin resistance. In isolated liver mitochondria, we have demonstrated PEA capability to modulate mitochondrial oxidative capacity and energy efficiency, leading to the reduction of intracellular lipid accumulation and oxidative stress. Moreover, we have evaluated the effect of PEA on mitochondrial bioenergetics of palmitate-challenged HepG2 cells, using Seahorse analyzer. In vitro data showed that PEA recovered mitochondrial dysfunction and reduced lipid accumulation in insulin-resistant HepG2 cells, increasing fatty acid oxidation. Mechanistic studies showed that PEA effect on lipid metabolism was limited by AMP-activated protein kinase (AMPK) inhibition, providing evidence for a pivotal role of AMPK in PEA-induced adaptive metabolic setting. All these findings identify PEA as a modulator of hepatic lipid and glucose homeostasis, limiting metabolic inflexibility induced by nutrient overload.

High-intensity exercise to promote accelerated improvements in cardiorespiratory fitness (HI-PACE): study protocol for a randomized controlled trial

Background

African Americans have a disproportionate prevalence and incidence of type 2 diabetes compared with Caucasians. Recent evidence indicates that low cardiorespiratory fitness (CRF) level, an independent risk factor for type 2 diabetes, is also more prevalent in African Americans than Caucasians. Numerous studies in Caucasian populations suggest that vigorous exercise intensity may promote greater improvements in CRF and other type 2 diabetes risk factors (e.g., reduction of glucose/insulin levels, pulse wave velocity, and body fat) than moderate intensity. However, current evidence comparing health benefits of different aerobic exercise intensities on type 2 diabetes risk factors in African Americans is negligible. This is clinically important as African Americans have a greater risk for type 2 diabetes and are less likely to meet public health recommendations for physical activity than Caucasians. The purpose of the HI-PACE (High-Intensity exercise to Promote Accelerated improvements in CardiorEspiratory fitness) study is to evaluate whether high-intensity aerobic exercise elicits greater improvements in CRF, insulin action, and arterial stiffness than moderate-intensity exercise in African Americans.

Methods/Design

A randomized controlled trial will be performed on overweight and obese (body mass index of 25–45 kg/m²) African Americans (35–65 years) (n = 60). Participants will be randomly assigned to moderate-intensity (MOD-INT) or high-intensity (HIGH-INT) aerobic exercise training or a non-exercise control group (CON) for 24 weeks. Supervised exercise will be performed at a heart rate associated with 45–55% and 70–80% of VO₂ max in the MOD-INT and HIGH-INT groups, respectively, for an exercise dose of 600 metabolic equivalents of task (MET)-minutes per week (consistent with public health recommendations). The primary outcome is change in CRF. Secondary outcomes include change in insulin sensitivity (measured via an intravenous glucose tolerance test), skeletal muscle mitochondrial oxidative capacity (via near-infrared spectroscopy), skeletal muscle measurements (i.e., citrate synthase, COX IV, GLUT-4, CPT-1, and PGC1-α), arterial stiffness (via carotid-femoral pulse wave velocity), body fat, C-reactive protein, and psychological outcomes (quality of life/exercise enjoyment).

Discussion

The anticipated results of the HI-PACE study will provide vital information on the health effects of high-intensity exercise in African Americans. This study will advance health disparity research and has the potential to influence future public health guidelines for physical activity.

Trial registration

ClinicalTrials.gov identifier: NCT02892331. Registered on September 8, 2016.

Electronic supplementary material

The online version of this article (10.1186/s13063-019-3611-1) contains supplementary material, which is available to authorized users.

Mechanisms Preserving Insulin Action during High Dietary Fat Intake

Prolonged intervention studies investigating molecular metabolism are necessary for a deeper understanding of dietary effects on health. Here we provide mechanistic information about metabolic adaptation to fat-rich diets. Healthy, slightly overweight men ingested saturated or polyunsaturated fat-rich diets for 6 weeks during weight maintenance. Hyperinsulinemic clamps combined with leg balance technique revealed unchanged peripheral insulin sensitivity, independent of fatty acid type. Both diets increased fat oxidation potential in muscle. Hepatic insulin clearance increased, while glucose production, de novo lipogenesis, and plasma triacylglycerol decreased. High fat intake changed the plasma proteome in the immune-supporting direction and the gut microbiome displayed changes at taxonomical and functional level with polyunsaturated fatty acid (PUFA). In mice, eucaloric feeding of human PUFA and saturated fatty acid diets lowered hepatic triacylglycerol content compared with low-fat-fed control mice, and induced adaptations in the liver supportive of decreased gluconeogenesis and lipogenesis. Intake of fat-rich diets thus induces extensive metabolic adaptations enabling disposition of dietary fat without metabolic complications.

Sex-specific effects of maternal and postweaning high-fat diet on skeletal muscle mitochondrial respiration

Exposure to maternal over-nutrition in utero is linked with developmental programming of obesity, metabolic syndrome and cardiovascular disease in offspring, which may be exacerbated by postnatal high-fat (HF) diet. Skeletal muscle mitochondrial function contributes to substrate metabolism and is impaired in metabolic disease. We examined muscle mitochondrial respiration in male and female mice exposed to maternal HF diet in utero, followed by postweaning HF diet until middle age. After in utero exposure to maternal control (Con) or HF diet (45% kcal fat; 39.4% lard, 5.5% soybean oil), offspring were weaned to Con or HF, creating four groups: Con/Con (male/female (m/f), n=8/8), Con/HF (m/f, n=7/4), HF/Con (m/f, n=9/6) and HF/HF (m/f, n=4/4). Oxidative phosphorylation (OXPHOS) and electron transfer system (ETS) capacity were measured in permeabilized gastrocnemius bundles. Maternal HF diet increased fasting glucose and lean body mass in males and body fat percentage in both sexes (P⩽0.05). Maximal adenosine diphosphate-stimulated respiration (complex I OXPHOS) was decreased by maternal HF diet in female offspring (-21%, P=0.053), but not in male (-0%, P>0.05). Sexually divergent responses were exacerbated in offspring weaned to HF diet. In females, OXPHOS capacity was lower (-28%, P=0.041) when weaned to high-fat (HF/HF) v. control diet (HF/Con). In males, OXPHOS (+33%, P=0.009) and ETS (+42%, P=0.016) capacity increased. Our data suggest that maternal lard-based HF diet, rich in saturated fat, affects offspring skeletal muscle respiration in a sex-dependent manner, and these differences are exacerbated by HF diet in adulthood.

Metabolic Flexibility as an Adaptation to Energy Resources and Requirements in Health and Disease

The ability to efficiently adapt metabolism by substrate sensing, trafficking, storage, and utilization, dependent on availability and requirement, is known as metabolic flexibility. In this review, we discuss the breadth and depth of metabolic flexibility and its impact on health and disease. Metabolic flexibility is essential to maintain energy homeostasis in times of either caloric excess or caloric restriction, and in times of either low or high energy demand, such as during exercise. The liver, adipose tissue, and muscle govern systemic metabolic flexibility and manage nutrient sensing, uptake, transport, storage, and expenditure by communication via endocrine cues. At a molecular level, metabolic flexibility relies on the configuration of metabolic pathways, which are regulated by key metabolic enzymes and transcription factors, many of which interact closely with the mitochondria. Disrupted metabolic flexibility, or metabolic inflexibility, however, is associated with many pathological conditions including metabolic syndrome, type 2 diabetes mellitus, and cancer. Multiple factors such as dietary composition and feeding frequency, exercise training, and use of pharmacological compounds, influence metabolic flexibility and will be discussed here. Last, we outline important advances in metabolic flexibility research and discuss medical horizons and translational aspects.

Sedentary behaviour is a key determinant of metabolic inflexibility

Metabolic flexibility is defined as the ability to adapt substrate oxidation rates in response to changes in fuel availability. The inability to switch between the oxidation of lipid and carbohydrate appears to be an important feature of chronic disorders such as obesity and type 2 diabetes. Laboratory assessment of metabolic flexibility has traditionally involved measurement of the respiratory quotient (RQ) by indirect calorimetry during the fasted to fed transition (e.g. mixed meal challenge) or during a hyperinsulinaemic-euglycaemic clamp. Under these controlled experimental conditions, 'metabolic inflexibility' is characterized by lower fasting fat oxidation (higher fasting RQ) and/or an impaired ability to oxidize carbohydrate during feeding or insulin-stimulated conditions (lower postprandial or clamp RQ). This experimental paradigm has provided fundamental information regarding the role of substrate oxidation in the development of obesity and insulin resistance. However, the key determinants of metabolic flexibility among relevant clinical populations remain unclear. Herein, we propose that habitual physical activity levels are a primary determinant of metabolic flexibility. We present evidence demonstrating that high levels of physical activity predict metabolic flexibility, while physical inactivity and sedentary behaviours trigger a state of metabolic 'inflexibility', even among individuals who meet physical activity recommendations. Furthermore, we describe alternative experimental approaches to studying the concept of metabolic flexibility across a range of activity and inactivity. Finally, we address the promising use of strategies that aim to reduce sedentary behaviours as therapy to improve metabolic flexibility and reduce weight gain risk.

Exercise in vivo marks human myotubes in vitro: Training-induced increase in lipid metabolism

BACKGROUND AND AIMS

Physical activity has preventive as well as therapeutic benefits for overweight subjects. In this study we aimed to examine effects of in vivo exercise on in vitro metabolic adaptations by studying energy metabolism in cultured myotubes isolated from biopsies taken before and after 12 weeks of extensive endurance and strength training, from healthy sedentary normal weight and overweight men.

METHODS

Healthy sedentary men, aged 40-62 years, with normal weight (body mass index (BMI) < 25 kg/m2) or overweight (BMI ≥ 25 kg/m2) were included. Fatty acid and glucose metabolism were studied in myotubes using [14C]oleic acid and [14C]glucose, respectively. Gene and protein expressions, as well as DNA methylation were measured for selected genes.

RESULTS

The 12-week training intervention improved endurance, strength and insulin sensitivity in vivo, and reduced the participants' body weight. Biopsy-derived cultured human myotubes after exercise showed increased total cellular oleic acid uptake (30%), oxidation (46%) and lipid accumulation (34%), as well as increased fractional glucose oxidation (14%) compared to cultures established prior to exercise. Most of these exercise-induced increases were significant in the overweight group, whereas the normal weight group showed no change in oleic acid or glucose metabolism.

CONCLUSIONS

12 weeks of combined endurance and strength training promoted increased lipid and glucose metabolism in biopsy-derived cultured human myotubes, showing that training in vivo are able to induce changes in human myotubes that are discernible in vitro.

Metabolic Inflexibility with Obesity and the Effects of Fenofibrate on Skeletal Muscle Fatty Acid Oxidation

This study was designed to investigate mechanisms of lipid metabolic inflexibility in human obesity and the ability of fenofibrate (FENO) to increase skeletal muscle fatty acid oxidation (FAO) in primary human skeletal muscle cell cultures (HSkMC) exhibiting metabolic inflexibility. HSkMC from 10 lean and 10 obese, insulin resistant subjects were treated with excess fatty acid for 24 h (24hFA) to gauge lipid-related metabolic flexibility. Metabolically inflexible HSkMC from obese individuals were then treated with 24hFA in combination with FENO to determine effectiveness for increasing FAO. Mitochondrial enzyme activity and FAO were measured in skeletal muscle from subjects with prediabetes (n=11) before and after 10 weeks of fenofibrate in vivo. 24hFA increased FAO to a greater extent in HSkMC from lean versus obese subjects (+49% vs. +9%, for lean vs. obese, respectively; p<0.05) indicating metabolic inflexibility with obesity. Metabolic inflexibility was not observed for measures of cellular respiration in permeabilized cells using carbohydrate substrate. Fenofibrate co-incubation with 24hFA, increased FAO in a subset of HSkMC from metabolically inflexible, obese subjects (p<0.05), which was eliminated by PPARα antagonist. In vivo, fenofibrate treatment increased skeletal muscle FAO in a subset of subjects with prediabetes but did not affect gene transcription or mitochondrial enzyme activity. Lipid metabolic inflexibility observed in HSkMC from obese subjects is not due to differences in electron transport flux, but rather upstream decrements in lipid metabolism. Fenofibrate increases the capacity for FAO in human skeletal muscle cells, though its role in skeletal muscle metabolism in vivo remains unclear.

Contribution of Maladaptive Adipose Tissue Expansion to Development of Cardiovascular Disease

The overweight and obesity epidemic has led to an increase in the metabolic syndrome and associated cardiovascular disease (CVD). These abnormalities include insulin resistance, type 2 diabetes mellitus, vascular stiffness, hypertension, stroke, and coronary heart disease. Visceral white adipocyte tissue (WAT) expansion and associated fibrosis/stiffness of WAT promote insulin resistance and CVD through increases in proinflammatory adipokines, oxidative stress, activation of renin-angiotensin-aldosterone system, dysregulation of adipocyte apoptosis and autophagy, dysfunctional immune modulation, and adverse changes in the gut microbiome. The expansion of WAT is partly determined by activation of peroxisome proliferator-activated receptor gamma and mammalian target of rapamycin/ribosomal S6 kinase signaling pathways. Further, the chronic activation of these signaling pathways may not only induce adipocyte hypertrophy and fibrosis, but also contribute to systemic inflammation, and impairment of insulin metabolic signaling in fat, liver, and skeletal muscle tissue. Therefore, the interplay of adipocyte dysfunction, maladaptive immune and inflammatory responses, and associated metabolic disorders often coexist leading to systemic low-grade inflammation and insulin resistance that are associated with increased CVD in obese individuals. © 2017 American Physiological Society. Compr Physiol 7:253-262, 2017.

Measuring the short-term substrate utilization response to high-carbohydrate and high-fat meals in the whole-body indirect calorimeter

The paper demonstrates that minute‐to‐minute metabolic response to meals with different macronutrient content can be measured and discerned in the whole‐body indirect calorimeter. The ability to discriminate between high‐carbohydrate and high‐fat meals is achieved by applying a modified regularization technique with additional constraints imposed on oxygen consumption rate. These additional constraints reduce the differences in accuracy between the oxygen and carbon dioxide analyzers. The modified technique was applied to 63 calorimeter sessions that were each 24 h long. The data were collected from 16 healthy volunteers (eight males, eight females, aged 22–35 years). Each volunteer performed four 24‐h long calorimeter sessions. At each session, they received one of four treatment combinations involving exercise (high or low intensity) and diet (a high‐fat or high‐carbohydrate shake for lunch). One volunteer did not complete all four assignments, which brought the total number of sessions to 63 instead of 64. During the 24‐h stay in the calorimeter, subjects wore a continuous glucose monitoring system, which was used as a benchmark for subject's postprandial glycemic response. The minute‐by‐minute respiratory exchange ratio (RER) data showed excellent agreement with concurrent subcutaneous glucose concentrations in postprandial state. The averaged minute‐to‐minute RER response to the high‐carbohydrate shake was significantly different from the response to high‐fat shake. Also, postprandial RER slopes were significantly different for two dietary treatments. The results show that whole‐body respiration calorimeters can be utilized as tools to study short‐term kinetics of substrate oxidation in humans.

Diacylglycerol kinase-δ regulates AMPK signaling, lipid metabolism, and skeletal muscle energetics

Decrease of AMPK-related signal transduction and insufficient lipid oxidation contributes to the pathogenesis of obesity and type 2 diabetes. Previously, we identified that diacylglycerol kinase-δ (DGKδ), an enzyme involved in triglyceride biosynthesis, is reduced in skeletal muscle from type 2 diabetic patients. Here, we tested the hypothesis that DGKδ plays a role in maintaining appropriate AMPK action in skeletal muscle and energetic aspects of contraction. Voluntary running activity was reduced in DGKδ(+/-) mice, but glycogen content and mitochondrial markers were unaltered, suggesting that DGKδ deficiency affects skeletal muscle energetics but not mitochondrial protein abundance. We next determined the role of DGKδ in AMPK-related signal transduction and lipid metabolism in isolated skeletal muscle. AMPK activation and signaling were reduced in DGKδ(+/-) mice, concomitant with impaired lipid oxidation and elevated incorporation of free fatty acids into triglycerides. Strikingly, DGKδ deficiency impaired work performance, as evident by altered force production and relaxation dynamics in response to repeated contractions. In conclusion, DGKδ deficiency impairs AMPK signaling and lipid metabolism, thereby highlighting the deleterious role of excessive lipid metabolites in the development of peripheral insulin resistance and type 2 diabetes pathogenesis. DGKδ deficiency also influences skeletal muscle energetics, which may lead to low physical activity levels in type 2 diabetes.

A 3-day high-fat/low-carbohydrate diet does not alter exercise-induced growth hormone response in healthy males

OBJECTIVE

The purpose of the present study was to examine the effects of 3 days isoenergetic high-fat/low-carbohydrate diet (HF-LC) relative to low-fat/high-carbohydrate diet (LF-HC) on the exercise-induced growth hormone (GH) response in healthy male subjects.

DESIGN

Ten healthy young males participated in this study. Each subject consumed the HF-LC (18±1% protein, 61±2% fat, 21±1% carbohydrate, 2720 kcal per day) for 3 consecutive days after consuming the LF-HC (18±1% protein, 20±1% fat, 62±1% carbohydrate, 2755 kcal per day) for 3 consecutive days. After each dietary intervention period, the hormonal and metabolic responses to an acute exercise (30 min of continuous pedaling at 60% of V˙O2max) were compared. The intramyocellular lipid (IMCL) contents in the vastus lateralis, soleus, and tibialis anterior were evaluated by proton magnetic resonance spectroscopy.

RESULTS

Serum GH concentrations increased significantly during the exercise after both the HF-LC and LF-HC periods (P<0.05). However, the exercise-induced GH response was not significantly different between the two periods. Fat utilization and lipolytic responses during the exercise were enhanced significantly after the HF-LC period compared with the LF-HC period. IMCL content did not differ significantly in any portion of muscle after the dietary interventions.

CONCLUSIONS

We could not show that short-term HF-LC consumption changed significantly exercise-induced GH response or IMCL content in healthy young males.

Differential epigenetic and transcriptional response of the skeletal muscle carnitine palmitoyltransferase 1B (CPT1B) gene to lipid exposure with obesity

The ability to increase fatty acid oxidation (FAO) in response to dietary lipid is impaired in the skeletal muscle of obese individuals, which is associated with a failure to coordinately upregulate genes involved with FAO. While the molecular mechanisms contributing to this metabolic inflexibility are not evident, a possible candidate is carnitine palmitoyltransferase-1B (CPT1B), which is a rate-limiting step in FAO. The present study was undertaken to determine if the differential response of skeletal muscle CPT1B gene transcription to lipid between lean and severely obese subjects is linked to epigenetic modifications (DNA methylation and histone acetylation) that impact transcriptional activation. In primary human skeletal muscle cultures the expression of CPT1B was blunted in severely obese women compared with their lean counterparts in response to lipid, which was accompanied by changes in CpG methylation, H3/H4 histone acetylation, and peroxisome proliferator-activated receptor-δ and hepatocyte nuclear factor 4α transcription factor occupancy at the CPT1B promoter. Methylation of specific CpG sites in the CPT1B promoter that correlated with CPT1B transcript level blocked the binding of the transcription factor upstream stimulatory factor, suggesting a potential causal mechanism. These findings indicate that epigenetic modifications may play important roles in the regulation of CPT1B in response to a physiologically relevant lipid mixture in human skeletal muscle, a major site of fatty acid catabolism, and that differential DNA methylation may underlie the depressed expression of CPT1B in response to lipid, contributing to the metabolic inflexibility associated with severe obesity.

Defects in mitochondrial efficiency and H2O2 emissions in obese women are restored to a lean phenotype with aerobic exercise training

The notion that mitochondria contribute to obesity-induced insulin resistance is highly debated. Therefore, we determined whether obese (BMI 33 kg/m(2)), insulin-resistant women with polycystic ovary syndrome had aberrant skeletal muscle mitochondrial physiology compared with lean, insulin-sensitive women (BMI 23 kg/m(2)). Maximal whole-body and mitochondrial oxygen consumption were not different between obese and lean women. However, obese women exhibited lower mitochondrial coupling and phosphorylation efficiency and elevated mitochondrial H2O2 (mtH2O2) emissions compared with lean women. We further evaluated the impact of 12 weeks of aerobic exercise on obesity-related impairments in insulin sensitivity and mitochondrial energetics in the fasted state and after a high-fat mixed meal. Exercise training reversed obesity-related mitochondrial derangements as evidenced by enhanced mitochondrial bioenergetics efficiency and decreased mtH2O2 production. A concomitant increase in catalase antioxidant activity and decreased DNA oxidative damage indicate improved cellular redox status and a potential mechanism contributing to improved insulin sensitivity. mtH2O2 emissions were refractory to a high-fat meal at baseline, but after exercise, mtH2O2 emissions increased after the meal, which resembles previous findings in lean individuals. We demonstrate that obese women exhibit impaired mitochondrial bioenergetics in the form of decreased efficiency and impaired mtH2O2 emissions, while exercise effectively restores mitochondrial physiology toward that of lean, insulin-sensitive individuals.

Metabolomic analysis reveals altered skeletal muscle amino acid and fatty acid handling in obese humans

OBJECTIVE

Investigate the effects of obesity and high-fat diet (HFD) exposure on fatty acid oxidation and TCA cycle intermediates and amino acids in skeletal muscle to better characterize energy metabolism.

METHODS

Plasma and skeletal muscle metabolomic profiles were measured from lean and obese males before and after a 5-day HFD in the 4 h postprandial condition.

RESULTS

At both time points, plasma short-chain acylcarnitine species (SCAC) were higher in the obese subjects, while the amino acids glycine, histidine, methionine, and citrulline were lower in skeletal muscle of obese subjects. Skeletal muscle medium-chain acylcarnitines (MCAC) C6, C8, C10:2, C10:1, C10, and C12:1 increased in obese subjects, but decreased in lean subjects, from pre- to post-HFD. Plasma content of C10:1 was also decreased in the lean but increased in the obese subjects from pre- to post-HFD. CD36 increased from pre- to post-HFD in obese but not lean subjects.

CONCLUSIONS

Lower skeletal muscle amino acid content and accumulation of plasma SCAC in obese subjects could reflect increased anaplerosis for TCA cycle intermediates, while accumulation of MCAC suggests limitations in β-oxidation. These measures may be important markers of or contributors to dysregulated metabolism observed in skeletal muscle of obese humans.

Maximal oxidative capacity during exercise is associated with skeletal muscle fuel selection and dynamic changes in mitochondrial protein acetylation

Maximal exercise-associated oxidative capacity is strongly correlated with health and longevity in humans. Rats selectively bred for high running capacity (HCR) have improved metabolic health and are longer-lived than their low-capacity counterparts (LCR). Using metabolomic and proteomic profiling, we show that HCR efficiently oxidize fatty acids (FAs) and branched-chain amino acids (BCAAs), sparing glycogen and reducing accumulation of short- and medium-chain acylcarnitines. HCR mitochondria have reduced acetylation of mitochondrial proteins within oxidative pathways at rest, and there is rapid protein deacetylation with exercise, which is greater in HCR than LCR. Fluxomic analysis of valine degradation with exercise demonstrates a functional role of differential protein acetylation in HCR and LCR. Our data suggest that efficient FA and BCAA utilization contribute to high intrinsic exercise capacity and the health and longevity benefits associated with enhanced fitness.

The impact of obesity and exercise on cognitive aging

Obesity is a major concern in the aging population and degrades health, motor functions and cognition in older adults. The effects of obesity are pervasive and challenging to health-care systems, making this a widespread and critically important public health dilemma. In this review, we examine the relationship between obesity, cognitive aging, and related dysfunctions. Potential neural mechanisms underlying such relationship are described. We propose that cost-effective exercises can be employed to cope with obesity and cognitive declines in older adults. Finally, we discuss implications and future research directions.

Mitochondrial function in metabolic health: a genetic and environmental tug of war

BACKGROUND

The increased prevalence of obesity and its co-morbidities and their strong association with inactivity have produced an 'exercise-deficient phenotype' in which individuals with a particular combination of disease-susceptible genes collide with environmental influences to cross a biological 'threshold' that ultimately manifests as overt clinical conditions (i.e., risk-factors for disease states). These risk-factors have been linked to impairments in skeletal muscle mitochondrial function.

SCOPE OF REVIEW

The question of whether 'inborn' mitochondrial deficiencies and/or defective mitochondrial metabolism contribute to metabolic disease, or if environmental factors are the major determinant, will be examined.

MAJOR CONCLUSIONS

We contend that impaired whole-body insulin resistance along with impaired skeletal muscle handling of carbohydrate and lipid fuels (i.e., metabolic inflexibility) is associated with a reduced skeletal muscle mitochondrial content which, in large part, is a maladaptive response to an 'inactivity cycle' which predisposes to a reduced level of habitual physical activity. While genetic components play a role in the pathogenesis of metabolic disease, exercise is a powerful environmental stimulus capable of restoring the metabolic flexibility of fuel selection and reduces risk-factors for metabolic disease in genetically-susceptible individuals.

GENERAL SIGNIFICANCE

Given the apathy towards voluntary physical activity in most Western societies, it is clear that there is an urgent need for innovative, clinically-effective exercise strategies, coupled with changes in current attitudes and methods of delivering exercise prescription and dietary advice, in order to improve metabolic health and reduce metabolic disease risk at the population level. This article is part of a Special Issue entitled Frontiers of Mitochondrial Research.

Exercise and obesity-induced insulin resistance in skeletal muscle

The skeletal muscle in our body is a major site for bioenergetics and metabolism during exercise. Carbohydrates and fats are the primary nutrients that provide the necessary energy required to maintain cellular activities during exercise. The metabolic responses to exercise in glucose and lipid regulation depend on the intensity and duration of exercise. Because of the increasing prevalence of obesity, recent studies have focused on the cellular and molecular mechanisms of obesity-induced insulin resistance in skeletal muscle. Accumulation of intramyocellular lipid may lead to insulin resistance in skeletal muscle. In addition, lipid intermediates (e.g., fatty acyl-coenzyme A, diacylglycerol, and ceramide) impair insulin signaling in skeletal muscle. Recently, emerging evidence linking obesity-induced insulin resistance to excessive lipid oxidation, mitochondrial overload, and mitochondrial oxidative stress have been provided with mitochondrial function. This review will provide a brief comprehensive summary on exercise and skeletal muscle metabolism, and discuss the potential mechanisms of obesity-induced insulin resistance in skeletal muscle.

Short-term aerobic exercise training increases postprandial pancreatic polypeptide but not peptide YY concentrations in obese individuals

Objective

Short-term exercise training improves glycemic control, but the effect of short-term training on postprandial satiety peptide responses or perceived satiety remains unknown. We tested the hypothesis that short-term aerobic exercise training (15 days) would alter postprandial pancreatic and gut peptide [pancreatic polypeptide (PP) and peptide YY (PYY)] responses and perceived appetite and satiety in obese individuals.

Subjects

Thirteen healthy obese men and women (age: 42±2 y; BMI: 30-45 kg/m²)

Measurements

Subjects were studied before and after 15 days of training (walking 1 h at 70-75% VO_2peak). On the study day, subjects consumed 1500 kcal as 6 meals (250 kcal: 9 g protein, 40 g CHO, 6 g fat) while blood samples and satiety measurements were taken at baseline and every 20 min for 12 h. Blood was analyzed for pancreatic polypeptide (PP), peptide YY (PYY), glucose, and insulin levels. Appetite and satiety was assessed with a visual analog scale throughout the day.

Results

Incremental area under the curve (iAUC) for PP increased significantly with training (pre 2788±753; post 3845±830 pg/ml·min for 12-h, p<0.001), but there was no difference in the PP response to each meal. The initial PP response to the first meal increased (ΔPP_{min 20-0}: pre 86±25; post 140±36 pg/ml, p<0.05) with training. PYY iAUC showed no significant changes with training but showed a significant main effect of time across meals, with the largest response being to the first meal (P<0.005). There were no changes in satiety, glucose, or insulin levels with training.

Conclusion

Short-term exercise training increases postprandial PP concentrations in obese individuals; however, PYY levels and glycemic control remain unaffected. Both PP and PYY show meal-induced increases at all meals but PYY has a greater response at the first meal with reduced responses at subsequent meals.