Inhibition of adipose tissue lipolysis increases intramuscular lipid and glycogen use in vivo in humans

The impact of forearm immobilization and acipimox administration on muscle amino acid metabolism and insulin sensitivity in healthy, young volunteers

Although the mechanisms underpinning short-term muscle disuse atrophy and associated insulin resistance remain to be elucidated, perturbed lipid metabolism might be involved. Our aim was to determine the impact of acipimox administration [i.e., pharmacologically lowering circulating nonesterified fatty acid (NEFA) availability] on muscle amino acid metabolism and insulin sensitivity during short-term disuse. Eighteen healthy individuals (age: 22 ± 1 years; body mass index: 24.0 ± 0.6 kg·m-2) underwent 2 days forearm immobilization with placebo (PLA; n = 9) or acipimox (ACI; 250 mg Olbetam; n = 9) ingestion four times daily. Before and after immobilization, whole body glucose disposal rate (GDR), forearm glucose uptake (FGU; i.e., muscle insulin sensitivity), and amino acid kinetics were measured under fasting and hyperinsulinemic-hyperaminoacidemic-euglycemic clamp conditions using forearm balance and l-[ring-2H5]-phenylalanine infusions. Immobilization did not affect GDR but decreased insulin-stimulated FGU in both groups, more so in ACI (from 53 ± 8 to 12 ± 5 µmol·min-1) than PLA (from 52 ± 8 to 38 ± 13 µmol·min-1; P < 0.05). In ACI only, and in contrast to our hypothesis, fasting arterialized NEFA concentrations were elevated to 1.3 ± 0.1 mmol·L-1 postimmobilization (P < 0.05), and fasting forearm NEFA balance increased approximately fourfold (P = 0.10). Forearm phenylalanine net balance decreased following immobilization (P < 0.10), driven by an increased rate of appearance [from 32 ± 5 (fasting) and 21 ± 4 (clamp) preimmobilization to 53 ± 8 and 31 ± 4 postimmobilization; P < 0.05] while the rate of disappearance was unaffected by disuse or acipimox. Disuse-induced insulin resistance is accompanied by early signs of negative net muscle amino acid balance, which is driven by accelerated muscle amino acid efflux. Acutely elevated NEFA availability worsened muscle insulin resistance without affecting amino acid kinetics, suggesting increased muscle NEFA uptake may contribute to inactivity-induced insulin resistance but does not cause anabolic resistance.NEW & NOTEWORTHY We demonstrate that 2 days of forearm cast immobilization in healthy young volunteers leads to the rapid development of insulin resistance, which is accompanied by accelerated muscle amino acid efflux in the absence of impaired muscle amino acid uptake. Acutely elevated fasting nonesterified fatty acid (NEFA) availability as a result of acipimox supplementation worsened muscle insulin resistance without affecting amino acid kinetics, suggesting increased muscle NEFA uptake may contribute to inactivity-induced insulin resistance but does not cause anabolic resistance.

The impact of short-term forearm immobilization and acipimox administration on muscle amino acid metabolism and insulin sensitivity in healthy, young volunteers

The mechanisms underpinning short-term muscle disuse atrophy remain to be elucidated, but perturbations in lipid metabolism may be involved. Specifically, positive muscle non-esterified fatty acid (NEFA) balance has been implicated in the development of disuse-induced insulin and anabolic resistance. Our aim was to determine the impact of acipimox administration (i.e. pharmacologically lowering circulating NEFA availability) on muscle amino acid metabolism and insulin sensitivity during short-term disuse. Eighteen healthy individuals (age 22±1 years, BMI 24.0±0.6 kg·m-2) underwent 2 days of forearm cast immobilization with placebo (PLA; n=9, 5M/4F) or acipimox (ACI; 250 mg Olbetam; n=9, 4M/5F) ingestion four times daily. Before and after immobilization, whole-body glucose disposal rate (GDR), forearm glucose uptake (FGU, i.e. muscle insulin sensitivity), and amino acid kinetics were measured under fasting and hyperinsulinaemic-hyperaminoacidaemic-euglycaemic clamp conditions using arteriovenous forearm balance and intravenous L-[ring-2H5]phenylalanine infusions. Immobilization did not affect GDR but decreased insulin-stimulated FGU in both groups, but to a greater degree in ACI (from 53±8 to 12±5 μmol·min-1) than in PLA (from 52±8 to 38±13 μmol·min-1; P<0.05). In ACI only, fasting arterialised NEFA concentrations were elevated to 1.3±0.1 mmol·L-1 post-immobilization (P<0.05), and fasting forearm NEFA balance increased ~4-fold (P=0.10). Forearm phenylalanine net balance tended to decrease following immobilization (P<0.10), driven by increases in phenylalanine rates of appearance (from 32±5 (fasting) and 21±4 (clamp) pre-immobilization to 53±8 and 31±4 post-immobilization; P<0.05) while rates of disappearance were unaffected and no effects of acipimox observed. Altogether, we show disuse-induced insulin resistance is accompanied by early signs of negative net muscle amino acid balance, which is driven by accelerated muscle amino acid efflux. Acutely elevated NEFA availability worsened muscle insulin resistance without affecting muscle amino acid kinetics, suggesting that disuse-associated increased muscle NEFA uptake may contribute to inactivity-induced insulin resistance but does not represent an early mechanism causing anabolic resistance.

Carbohydrate intake before and during high intensity exercise with reduced muscle glycogen availability affects the speed of muscle reoxygenation and performance

Muscle glycogen state and carbohydrate (CHO) supplementation before and during exercise may impact responses to high-intensity interval training (HIIT). This study determined cardiorespiratory, substrate metabolism, muscle oxygenation, and performance when completing HIIT with or without CHO supplementation in a muscle glycogen depleted state. On two occasions, in a cross-over design, eight male cyclists performed a glycogen depletion protocol prior to HIIT during which either a 6% CHO drink (60 g.hr^-1) or placebo (%CHO, PLA) was consumed. HIIT consisted of 5 × 2 min at 80% peak power output (PPO), 3 × 10-min bouts of steady-state (SS) cycling (50, 55, 60% PPO), and a time-to-exhaustion (TTE) test. There was no difference in SS [Formula: see text], HR, substrate oxidation and gross efficiency (GE %) between CHO and PLA conditions. A faster rate of muscle reoxygenation (%. s^-1) existed in PLA after the 1st (Δ - 0.23 ± 0.22, d = 0.58, P < 0.05) and 3rd HIIT intervals (Δ - 0.34 ± 0.25, d = 1.02, P < 0.05). TTE was greater in CHO (7.1 ± 5.4 min) than PLA (2.5 ± 2.3 min, d = 0.98, P < 0.05). CHO consumption before and during exercise under reduced muscle glycogen conditions did not suppress fat oxidation, suggesting a strong regulatory role of muscle glycogen on substrate metabolism. However, CHO ingestion provided a performance benefit under intense exercise conditions commenced with reduced muscle glycogen. More research is needed to understand the significance of altered muscle oxygenation patterns during exercise.

Intramuscular lipid utilization during exercise: a systematic review, meta-analysis, and meta-regression

Intramuscular lipid (IMCL) utilization during exercise was controversial as numerous studies did not observe a decline in IMCL content post-exercise when assessed in muscle biopsies using biochemical techniques. Contemporary techniques including immunofluorescence microscopy and ¹H-magnetic resonance spectroscopy (¹H-MRS) offer advantages over biochemical techniques. The primary aim of this systematic review, meta-analysis, and meta-regression was to examine the net degradation of IMCL in response to an acute bout of cycling exercise in humans, as assessed with different analytical approaches. A secondary aim was to explore the factors influencing IMCL degradation including feeding status, exercise variables, and participant characteristics. A total of 44 studies met the inclusion criteria using biochemical, immunofluorescence, and ¹H-MRS techniques. A meta-analysis was completed using a random effects model and percentage change in IMCL content calculated from the standardized mean difference. Cycling exercise resulted in a net degradation of IMCL regardless of technique (total effect -23.7%, 95% CI = -28.7 to -18.7%) and there was no difference when comparing fasted versus fed-state exercise (P > 0.05). IMCL degradation using immunofluorescence techniques detected larger effects in type I fibers compared with whole muscle using biochemical techniques (P = 0.003) and in type I fibers compared with type II fibers (P < 0.001). Although IMCL degradation was associated with exercise duration, V̇o_2max, and BMI, none of these factors independently related to the change in IMCL content. These findings provide strong evidence that the analytical approach can influence the assessment of IMCL degradation in human skeletal muscle in response to exercise.

Comparison of Metabolic Characteristics of Physically Active Individuals with Different Training Habits during Incremental Treadmill Test

The number of people engaging in self-conducted regular physical activity is increasing, but the effects of home fitness and individually planned workouts on health and metabolism are unknown. We aimed to examine the effects of regular training conducted without the supervision of professionals on exercise metabolism in our cross-sectional observational study. Forty-five physically active volunteers, classified into three groups, based on the type and frequency of their training (group 1 frequent long-term endurance, group 2 three times per week aerobic training, and group 3 two times per week short aerobic and resistance training), fulfilled a vita maxima incremental treadmill test. Aerobic capacity (VO2max), MET (metabolic equivalent of task), and metabolic responses were examined. The results were evaluated by ANOVA and Bonferroni and Scheffe multiple comparison analysis using Microsoft Excel and SPSS 23 programs. (p < 0.05). Significant differences were found between group 1 and 3 in VO2max (p = 0.46) and MET (p = 0.46) between group 1 and 2, in FatmaxHR (heart rate on maximum fat oxidation) (p= 0.04). We concluded self-conducted regular physical activity has positive effects on metabolism and health. Aerobic training performed four times per week showed the most beneficial effects on metabolism and health maintenance. In addition, based on our findings, strength training performed two times per week is recommended.

High intramuscular triglyceride turnover rates and the link to insulin sensitivity: influence of obesity, type 2 diabetes and physical activity

Large intramuscular triglyceride (IMTG) stores in sedentary, obese individuals have been linked to insulin resistance, yet well-trained athletes exhibit high IMTG levels whilst maintaining insulin sensitivity. Contrary to previous assumptions, it is now known that IMTG content per se does not result in insulin resistance. Rather, insulin resistance is caused, at least in part, by the presence of high concentrations of harmful lipid metabolites, such as diacylglycerols and ceramides in muscle. Several mechanistic differences between obese sedentary individuals and their highly trained counterparts have been identified, which determine the differential capacity for IMTG synthesis and breakdown in these populations. In this review, we first describe the most up-to-date mechanisms by which a low IMTG turnover rate (both breakdown and synthesis) leads to the accumulation of lipid metabolites and results in skeletal muscle insulin resistance. We then explore current and potential exercise and nutritional strategies that target IMTG turnover in sedentary obese individuals, to improve insulin sensitivity. Overall, improving IMTG turnover should be an important component of successful interventions that aim to prevent the development of insulin resistance in the ever-expanding sedentary, overweight and obese populations. Novelty: A description of the most up-to-date mechanisms regulating turnover of the IMTG pool. An exploration of current and potential exercise/nutritional strategies to target and enhance IMTG turnover in obese individuals. Overall, highlights the importance of improving IMTG turnover to prevent the development of insulin resistance.

Contribution of skeletal muscle and serum lipids to muscle contraction induced by neuromuscular electrical stimulation in older individuals

Intramyocellular lipids (IMCL) stored in droplets in muscle cells and free fatty acids (FFA) from fat cells in the blood are the main substrates of adenosine triphosphate during continuous muscle contractions of relatively lower intensity. Although it is known that the lipid oxidative capacity decreases with aging, the effect of IMCL and FFA on muscle contraction in older individuals remains unclear. The purpose of this study was to investigate the contribution of skeletal muscle lipids and blood lipids as energy sources for muscle contraction in older individuals. Eighteen older individuals (mean age: 70.4 ± 3.5 years) underwent muscle contraction intervention induced by intermittent neuromuscular electrical stimulation (NMES) to the vastus lateralis for 30 min. Fasting blood samples were obtained and proton magnetic resonance spectroscopy (1 H-MRS) was performed before and after NMES, and the parameters (including IMCL and extramyocellular lipid [EMCL]) from 1 H-MRS, along with FFA and adiponectin levels, were analyzed using the blood samples of all participants. Levels of IMCL and EMCL did not change (p > 0.05); however, FFA and adiponectin levels decreased from 1.1 ± 0.5 mEq/L to 0.8 ± 0.2 mEq/L and 12.0 ± 5.3 μg/ml to 11.4 ± 5.0 μg/ml, after NMES (p < 0.05), respectively. These findings indicate that serum lipids, but not skeletal muscle lipids, are the energy substrate utilized during involuntary muscle contraction in older individuals.

Exogenous Ketone Supplements in Athletic Contexts: Past, Present, and Future

The ketone bodies acetoacetate (AcAc) and β-hydroxybutyrate (βHB) have pleiotropic effects in multiple organs including brain, heart, and skeletal muscle by serving as an alternative substrate for energy provision, and by modulating inflammation, oxidative stress, catabolic processes, and gene expression. Of particular relevance to athletes are the metabolic actions of ketone bodies to alter substrate utilisation through attenuating glucose utilisation in peripheral tissues, anti-lipolytic effects on adipose tissue, and attenuation of proteolysis in skeletal muscle. There has been long-standing interest in the development of ingestible forms of ketone bodies that has recently resulted in the commercial availability of exogenous ketone supplements (EKS). These supplements in the form of ketone salts and ketone esters, in addition to ketogenic compounds such as 1,3-butanediol and medium chain triglycerides, facilitate an acute transient increase in circulating AcAc and βHB concentrations, which has been termed 'acute nutritional ketosis' or 'intermittent exogenous ketosis'. Some studies have suggested beneficial effects of EKS to endurance performance, recovery, and overreaching, although many studies have failed to observe benefits of acute nutritional ketosis on performance or recovery. The present review explores the rationale and historical development of EKS, the mechanistic basis for their proposed effects, both positive and negative, and evidence to date for their effects on exercise performance and recovery outcomes before concluding with a discussion of methodological considerations and future directions in this field.

Factors Influencing AMPK Activation During Cycling Exercise: A Pooled Analysis and Meta-Regression

BACKGROUND

The 5' adenosine monophosphate (AMP)-activated protein kinase (AMPK) is a cellular energy sensor that is activated by increases in the cellular AMP/adenosine diphosphate:adenosine triphosphate (ADP:ATP) ratios and plays a key role in metabolic adaptations to endurance training. The degree of AMPK activation during exercise can be influenced by many factors that impact on cellular energetics, including exercise intensity, exercise duration, muscle glycogen, fitness level, and nutrient availability. However, the relative importance of these factors for inducing AMPK activation remains unclear, and robust relationships between exercise-related variables and indices of AMPK activation have not been established.

OBJECTIVES

The purpose of this analysis was to (1) investigate correlations between factors influencing AMPK activation and the magnitude of change in AMPK activity during cycling exercise, (2) investigate correlations between commonly reported measures of AMPK activation (AMPK-α2 activity, phosphorylated (p)-AMPK, and p-acetyl coenzyme A carboxylase (p-ACC), and (3) formulate linear regression models to determine the most important factors for AMPK activation during exercise.

METHODS

Data were pooled from 89 studies, including 982 participants (93.8% male, maximal oxygen consumption [[Formula: see text]] 51.9 ± 7.8 mL kg^-1 min^-1). Pearson's correlation analysis was performed to determine relationships between effect sizes for each of the primary outcome markers (AMPK-α2 activity, p-AMPK, p-ACC) and factors purported to influence AMPK signaling (muscle glycogen, carbohydrate ingestion, exercise duration and intensity, fitness level, and muscle metabolites). General linear mixed-effect models were used to examine which factors influenced AMPK activation.

RESULTS

Significant correlations (r = 0.19-0.55, p < .05) with AMPK activity were found between end-exercise muscle glycogen, exercise intensity, and muscle metabolites phosphocreatine, creatine, and free ADP. All markers of AMPK activation were significantly correlated, with the strongest relationship between AMPK-α2 activity and p-AMPK (r = 0.56, p < 0.001). The most important predictors of AMPK activation were the muscle metabolites and exercise intensity.

CONCLUSION

Muscle glycogen, fitness level, exercise intensity, and exercise duration each influence AMPK activity during exercise when all other factors are held constant. However, disrupting cellular energy charge is the most influential factor for AMPK activation during endurance exercise.

Interactions between insulin and exercise

The interaction between insulin and exercise is an example of balancing and modifying the effects of two opposing metabolic regulatory forces under varying conditions. While insulin is secreted after food intake and is the primary hormone increasing glucose storage as glycogen and fatty acid storage as triglycerides, exercise is a condition where fuel stores need to be mobilized and oxidized. Thus, during physical activity the fuel storage effects of insulin need to be suppressed. This is done primarily by inhibiting insulin secretion during exercise as well as activating local and systemic fuel mobilizing processes. In contrast, following exercise there is a need for refilling the fuel depots mobilized during exercise, particularly the glycogen stores in muscle. This process is facilitated by an increase in insulin sensitivity of the muscles previously engaged in physical activity which directs glucose to glycogen resynthesis. In physically trained individuals, insulin sensitivity is also higher than in untrained individuals due to adaptations in the vasculature, skeletal muscle and adipose tissue. In this paper, we review the interactions between insulin and exercise during and after exercise, as well as the effects of regular exercise training on insulin action.

The Anabolic Response to Plant-Based Protein Ingestion

There is a global trend of an increased interest in plant-based diets. This includes an increase in the consumption of plant-based proteins at the expense of animal-based proteins. Plant-derived proteins are now also frequently applied in sports nutrition. So far, we have learned that the ingestion of plant-derived proteins, such as soy and wheat protein, result in lower post-prandial muscle protein synthesis responses when compared with the ingestion of an equivalent amount of animal-based protein. The lesser anabolic properties of plant-based versus animal-derived proteins may be attributed to differences in their protein digestion and amino acid absorption kinetics, as well as to differences in amino acid composition between these protein sources. Most plant-based proteins have a low essential amino acid content and are often deficient in one or more specific amino acids, such as lysine and methionine. However, there are large differences in amino acid composition between various plant-derived proteins or plant-based protein sources. So far, only a few studies have directly compared the muscle protein synthetic response following the ingestion of a plant-derived protein versus a high(er) quality animal-derived protein. The proposed lower anabolic properties of plant- versus animal-derived proteins may be compensated for by (i) consuming a greater amount of the plant-derived protein or plant-based protein source to compensate for the lesser quality; (ii) using specific blends of plant-based proteins to create a more balanced amino acid profile; (iii) fortifying the plant-based protein (source) with the specific free amino acid(s) that is (are) deficient. Clinical studies are warranted to assess the anabolic properties of the various plant-derived proteins and their protein sources in vivo in humans and to identify the factors that may or may not compromise the capacity to stimulate post-prandial muscle protein synthesis rates. Such work is needed to determine whether the transition towards a more plant-based diet is accompanied by a transition towards greater dietary protein intake requirements.

Carbohydrate improves exercise capacity but does not affect subcellular lipid droplet morphology, AMPK and p53 signalling in human skeletal muscle

KEY POINTS

Muscle glycogen and intramuscular triglycerides (IMTG, stored in lipid droplets) are important energy substrates during prolonged exercise. Exercise-induced changes in lipid droplet (LD) morphology (i.e. LD size and number) have not yet been studied under nutritional conditions typically adopted by elite endurance athletes, that is, after carbohydrate (CHO) loading and CHO feeding during exercise. We report for the first time that exercise reduces IMTG content in both central and peripheral regions of type I and IIa fibres, reflective of decreased LD number in both fibre types whereas reductions in LD size were exclusive to type I fibres. Additionally, CHO feeding does not alter subcellular IMTG utilisation, LD morphology or muscle glycogen utilisation in type I or IIa/II fibres. In the absence of alterations to muscle fuel selection, CHO feeding does not attenuate cell signalling pathways with regulatory roles in mitochondrial biogenesis.

ABSTRACT

We examined the effects of carbohydrate (CHO) feeding on lipid droplet (LD) morphology, muscle glycogen utilisation and exercise-induced skeletal muscle cell signalling. After a 36 h CHO loading protocol and pre-exercise meal (12 and 2 g kg^-1 , respectively), eight trained males ingested 0, 45 or 90 g CHO h^-1 during 180 min cycling at lactate threshold followed by an exercise capacity test (150% lactate threshold). Muscle biopsies were obtained pre- and post-completion of submaximal exercise. Exercise decreased (P < 0.01) glycogen concentration to comparable levels (∼700 to 250 mmol kg^-1 DW), though utilisation was greater in type I (∼40%) versus type II fibres (∼10%) (P < 0.01). LD content decreased in type I (∼50%) and type IIa fibres (∼30%) (P < 0.01), with greater utilisation in type I fibres (P < 0.01). CHO feeding did not affect glycogen or IMTG utilisation in type I or II fibres (all P > 0.05). Exercise decreased LD number within central and peripheral regions of both type I and IIa fibres, though reduced LD size was exclusive to type I fibres. Exercise induced (all P < 0.05) comparable AMPK^Thr172 (∼4-fold), p53^Ser15 (∼2-fold) and CaMKII^Thr268 phosphorylation (∼2-fold) with no effects of CHO feeding (all P > 0.05). CHO increased exercise capacity where 90 g h^-1 (233 ± 133 s) > 45 g h^-1 (156 ± 66 s; P = 0.06) > 0 g h^-1 (108 ± 54 s; P = 0.03). In conditions of high pre-exercise CHO availability, we conclude CHO feeding does not influence exercise-induced changes in LD morphology, glycogen utilisation or cell signalling pathways with regulatory roles in mitochondrial biogenesis.

The causal pathway effects of a physical activity intervention on adiposity in children: The KISS Study cluster randomized clinical trial

BACKGROUND

Very little information on the potential mechanisms of the physical activity interventions effects on adiposity is available. We evaluated the possible mediating factors of a physical activity school-based intervention on the sum of skinfolds in children.

METHODS

This is a cluster randomized trial, secondary analysis of the KISS study. Children (n = 499) from the first and fifth grades were randomly assigned to intervention or control group. Adiposity was estimated by four skinfolds, aerobic fitness assessed by the shuttle run test, and insulin, triglycerides, total cholesterol, high-density lipoprotein (HDL), and glucose collected via fasting blood samples.

RESULTS

The intervention affected aerobic fitness (0.140 SD, 95% CI 0.011 to 0.270), triglycerides (0.217 SD, 95% CI -0.409 to -0.025), cholesterol/HDL ratio (-0.191 SD, 95% CI -0.334 to -0.047), glucose (-0.330 SD, 95% CI -0.538 to -0.121), and skinfolds (-0.122 SD, 95% CI -0.189 to -0.056). No intervention effect on insulin was found. We observed that changes in aerobic fitness impacted children's triglycerides and cholesterol/HDL ratio and consecutively the glucose levels mediating 30% of the intervention effect on skinfolds.

CONCLUSIONS

Our findings provided evidence of the positive metabolic distress caused by a physical activity intervention on adiposity levels in children.

Skeletal muscle energy metabolism during exercise

The continual supply of ATP to the fundamental cellular processes that underpin skeletal muscle contraction during exercise is essential for sports performance in events lasting seconds to several hours. Because the muscle stores of ATP are small, metabolic pathways must be activated to maintain the required rates of ATP resynthesis. These pathways include phosphocreatine and muscle glycogen breakdown, thus enabling substrate-level phosphorylation ('anaerobic') and oxidative phosphorylation by using reducing equivalents from carbohydrate and fat metabolism ('aerobic'). The relative contribution of these metabolic pathways is primarily determined by the intensity and duration of exercise. For most events at the Olympics, carbohydrate is the primary fuel for anaerobic and aerobic metabolism. Here, we provide an overview of exercise metabolism and the key regulatory mechanisms ensuring that ATP resynthesis is closely matched to the ATP demand of exercise. We also summarize various interventions that target muscle metabolism for ergogenic benefit in athletic events.

Exercise and Muscle Lipid Content, Composition, and Localization: Influence on Muscle Insulin Sensitivity

Accumulation of lipid in skeletal muscle is thought to be related to the development of insulin resistance and type 2 diabetes. Initial work in this area focused on accumulation of intramuscular triglyceride; however, bioactive lipids such as diacylglycerols and sphingolipids are now thought to play an important role. Specific species of these lipids appear to be more negative toward insulin sensitivity than others. Adding another layer of complexity, localization of lipids within the cell appears to influence the relationship between these lipids and insulin sensitivity. This article summarizes how accumulation of total lipids, specific lipid species, and localization of lipids influence insulin sensitivity in humans. We then focus on how these aspects of muscle lipids are impacted by acute and chronic aerobic and resistance exercise training. By understanding how exercise alters specific species and localization of lipids, it may be possible to uncover specific lipids that most heavily impact insulin sensitivity.

High Protein Diet and Metabolic Plasticity in Non-Alcoholic Fatty Liver Disease: Myths and Truths

Non-alcoholic fatty liver disease (NAFLD) is characterized by lipid accumulation within the liver affecting 1 in 4 people worldwide. As the new silent killer of the twenty-first century, NAFLD impacts on both the request and the availability of new liver donors. The liver is the first line of defense against endogenous and exogenous metabolites and toxins. It also retains the ability to switch between different metabolic pathways according to food type and availability. This ability becomes a disadvantage in obesogenic societies where most people choose a diet based on fats and carbohydrates while ignoring vitamins and fiber. The chronic exposure to fats and carbohydrates induces dramatic changes in the liver zonation and triggers the development of insulin resistance. Common believes on NAFLD and different diets are based either on epidemiological studies, or meta-analysis, which are not controlled evidences; in most of the cases, they are biased on test-subject type and their lifestyles. The highest success in reverting NAFLD can be attributed to diets based on high protein instead of carbohydrates. In this review, we discuss the impact of NAFLD on body metabolic plasticity. We also present a detailed analysis of the most recent studies that evaluate high-protein diets in NAFLD with a special focus on the liver and the skeletal muscle protein metabolisms.

Concordance between 13 C:12 C ratio technique respect to indirect calorimetry to estimate carbohydrate and Fat oxidation rates by means stoichiometric equations during exercise. A reliability and agreement study

Indirect calorimetry is a tool used routinely by sport/exercise physiologist to assess the metabolic response to training and to nutritional interventions. There are different stoichiometric equations to estimate fat (FatOxR ) and carbohydrates (CHOOxR ) oxidation rates, however there are not enough information in literature about what are the most accurate equations. The purpose of this study was to determine the concordance between indirect calorimetry and a method of reference for stoichiometric equations used to estimate FatOxR and CHOOxR . Concordance between indirect calorimetry and the method of reference (13 C to 12 C ratio (13 C:12 C ratio) technique) for key stoichiometric equations was assessed in well-trained triathletes. Subjects carried out a carbohydrate depletion-repletion protocol, labeling the glycogen stores with 13 C, and a laboratory test to assess the 13 C metabolic response during a wide range of aerobic intensities during exercise. All the equations showed a narrow agreement interval (Δ) (CHOOxR nPC (protein component negligible): -0.308, 0.308, CHOOxR PC (protein component): -0.268, 0.268, FatOxR nPC and PC: -0.032, 0.032 (g·min-1 )). FatOxR showed a similar concordance (28-32%) with CHOOxR nPC ranging from 55% to 75%, and for CHOOxR PC between 51% to 71%. None of the stoichiometric equations met a perfect agreement with the method of reference. The Jeukendrup and Wallis equation showed the best concordance for CHOOxR nPC whilst the Frayn and Ferrannini (Glu) equations had the best agreement for CHOOxR PC. All FatOxR equations showed similar concordances and they are able to be used indistinctly.

Intramuscular triglyceride synthesis: importance in muscle lipid partitioning in humans

Intramuscular triglyceride (IMTG) concentration is elevated in insulin-resistant individuals and was once thought to promote insulin resistance. However, endurance-trained athletes have equivalent concentration of IMTG compared with individuals with type 2 diabetes, and have very low risk of diabetes, termed the "athlete's paradox." We now know that IMTG synthesis is positively related to insulin sensitivity, but the exact mechanisms for this are unclear. To understand the relationship between IMTG synthesis and insulin sensitivity, we measured IMTG synthesis in obese control subjects, endurance-trained athletes, and individuals with type 2 diabetes during rest, exercise, and recovery. IMTG synthesis rates were positively related to insulin sensitivity, cytosolic accumulation of DAG, and decreased accumulation of C18:0 ceramide and glucosylceramide. Greater rates of IMTG synthesis in athletes were not explained by alterations in FFA concentration, DGAT1 mRNA expression, or protein content. IMTG synthesis during exercise in Ob and T2D indicate utilization as a fuel despite unchanged content, whereas IMTG concentration decreased during exercise in athletes. mRNA expression for genes involved in lipid desaturation and IMTG synthesis were increased after exercise and recovery. Further, in a subset of individuals, exercise decreased cytosolic and membrane di-saturated DAG content, which may help explain insulin sensitization after acute exercise. These data suggest IMTG synthesis rates may influence insulin sensitivity by altering intracellular lipid localization, and decreasing specific ceramide species that promote insulin resistance.

Metabolic Responses to Carbohydrate Ingestion during Exercise: Associations between Carbohydrate Dose and Endurance Performance

Carbohydrate (CHO) ingestion during exercise lasting less than three hours improves endurance exercise performance but there is still debate about the optimal dose. We utilised stable isotopes and blood metabolite profiles to further examine metabolic responses to CHO (glucose only) ingestion in the 20–64 g·h⁻¹ range, and to determine the association with performance outcome. In a double-blind, randomized cross-over design, male cyclists (n = 20, mean ± SD, age 34 ± 10 years, mass 75.8 ± 9 kg, peak power output 394 ± 36 W, VO_2max 62 ± 9 mL·kg⁻¹·min⁻¹) completed four main experimental trials. Each trial involved a two-hour constant load ride (185 ± 25 W) followed by a time trial, where one of three CHO beverages, or a control (water), were administered every 15 min, providing 0, 20, 39 or 64 g CHO·h⁻¹. Dual glucose tracer techniques, indirect calorimetry and blood analyses were used to determine glucose kinetics, exogenous CHO oxidation (EXO), endogenous CHO and fat oxidation; and metabolite responses. Regression analysis revealed that total exogenous CHO oxidised in the second hour of exercise, and suppression of serum NEFA concentration provided the best prediction model of performance outcome. However, the model could only explain ~19% of the variance in performance outcome. The present data demonstrate that consuming ~40 g·h⁻¹ of CHO appears to be the minimum ingestion rate required to induce metabolic effects that are sufficient to impact upon performance outcome. These data highlight a lack of performance benefit and few changes in metabolic outcomes beyond an ingestion rate of 39 g·h⁻¹. Further work is required to explore dose-response effects of CHO feeding and associations between multiple metabolic parameters and subsequent performance outcome.

Molecular Regulation of Fatty Acid Oxidation in Skeletal Muscle during Aerobic Exercise

This review summarizes how fatty acid (FA) oxidation is regulated in skeletal muscle during exercise. From the available evidence it seems that acetyl-CoA availability in the mitochondrial matrix adjusts FA oxidation to exercise intensity and duration. This is executed at the step of mitochondrial fatty acyl import, as the extent of acetyl group sequestration by carnitine determines the availability of carnitine for the carnitine palmitoyltransferase 1 (CPT1) reaction. The rate of glycolysis seems therefore to be central to the amount of β-oxidation-derived acetyl-CoA that is oxidized in the tricarboxylic acid (TCA) cycle. FA oxidation during exercise is also determined by FA availability to mitochondria, dependent on trans-sarcolemmal FA uptake via cluster of differentiation 36/SR-B2 (CD36) and FAs mobilized from myocellular lipid droplets.

Pronounced limb and fibre type differences in subcellular lipid droplet content and distribution in elite skiers before and after exhaustive exercise

KEY POINTS

Although lipid droplets in skeletal muscle are an important energy source during endurance exercise, our understanding of lipid metabolism in this context remains incomplete. Using transmission electron microscopy, two distinct subcellular pools of lipid droplets can be observed in skeletal muscle - one beneath the sarcolemma and the other between myofibrils. At rest, well-trained leg muscles of cross-country skiers contain 4- to 6-fold more lipid droplets than equally well-trained arm muscles, with a 3-fold higher content in type 1 than in type 2 fibres. During exhaustive exercise, lipid droplets between the myofibrils but not those beneath the sarcolemma are utilised by both type 1 and 2 fibres. These findings provide insight into compartmentalisation of lipid metabolism within skeletal muscle fibres.

ABSTRACT

Although the intramyocellular lipid pool is an important energy store during prolonged exercise, our knowledge concerning its metabolism is still incomplete. Here, quantitative electron microscopy was used to examine subcellular distribution of lipid droplets in type 1 and 2 fibres of the arm and leg muscles before and after 1 h of exhaustive exercise. Intermyofibrillar lipid droplets accounted for 85-97% of the total volume fraction, while the subsarcolemmal pool made up 3-15%. Before exercise, the volume fractions of intermyofibrillar and subsarcolemmal lipid droplets were 4- to 6-fold higher in leg than in arm muscles (P < 0.001). Furthermore, the volume fraction of intermyofibrillar lipid droplets was 3-fold higher in type 1 than in type 2 fibres (P < 0.001), with no fibre type difference in the subsarcolemmal pool. Following exercise, intermyofibrillar lipid droplet volume fraction was 53% lower (P = 0.0082) in both fibre types in arm, but not leg muscles. This reduction was positively associated with the corresponding volume fraction prior to exercise (R²  = 0.84, P < 0.0001). No exercise-induced change in the subsarcolemmal pool could be detected. These findings indicate clear differences in the subcellular distribution of lipid droplets in the type 1 and 2 fibres of well-trained arm and leg muscles, as well as preferential utilisation of the intermyofibrillar pool during prolonged exhaustive exercise. Apparently, the metabolism of lipid droplets within a muscle fibre is compartmentalised.

Triglyceride metabolism in exercising muscle

Triglycerides are stored within lipid droplets in skeletal muscle and can be hydrolyzed to produce fatty acids for energy production through β-oxidation and oxidative phosphorylation. While there was some controversy regarding the quantitative importance of intramyocellular triglyceride (IMTG) as a metabolic substrate, recent advances in proton magnetic resonance spectroscopy and confocal microscopy support earlier tracer and biopsy studies demonstrating a substantial contribution of IMTG to energy production, particularly during moderate-intensity endurance exercise. This review provides an update on the understanding of IMTG utilization during exercise, with a focus on describing the key regulatory proteins that control IMTG breakdown and how these proteins respond to acute exercise and in the adaptation to exercise training. This article is part of a Special Issue entitled: Recent Advances in Lipid Droplet Biology edited by Rosalind Coleman and Matthijs Hesselink.

Metabolism of ketone bodies during exercise and training: physiological basis for exogenous supplementation

Optimising training and performance through nutrition strategies is central to supporting elite sportspeople, much of which has focused on manipulating the relative intake of carbohydrate and fat and their contributions as fuels for energy provision. The ketone bodies, namely acetoacetate, acetone and β-hydroxybutyrate (βHB), are produced in the liver during conditions of reduced carbohydrate availability and serve as an alternative fuel source for peripheral tissues including brain, heart and skeletal muscle. Ketone bodies are oxidised as a fuel source during exercise, are markedly elevated during the post-exercise recovery period, and the ability to utilise ketone bodies is higher in exercise-trained skeletal muscle. The metabolic actions of ketone bodies can alter fuel selection through attenuating glucose utilisation in peripheral tissues, anti-lipolytic effects on adipose tissue, and attenuation of proteolysis in skeletal muscle. Moreover, ketone bodies can act as signalling metabolites, with βHB acting as an inhibitor of histone deacetylases, an important regulator of the adaptive response to exercise in skeletal muscle. Recent development of ketone esters facilitates acute ingestion of βHB that results in nutritional ketosis without necessitating restrictive dietary practices. Initial reports suggest this strategy alters the metabolic response to exercise and improves exercise performance, while other lines of evidence suggest roles in recovery from exercise. The present review focuses on the physiology of ketone bodies during and after exercise and in response to training, with specific interest in exploring the physiological basis for exogenous ketone supplementation and potential benefits for performance and recovery in athletes.

Liver glycogen metabolism during and after prolonged endurance-type exercise

Carbohydrate and fat are the main substrates utilized during prolonged endurance-type exercise. The relative contribution of each is determined primarily by the intensity and duration of exercise, along with individual training and nutritional status. During moderate- to high-intensity exercise, carbohydrate represents the main substrate source. Because endogenous carbohydrate stores (primarily in liver and muscle) are relatively small, endurance-type exercise performance/capacity is often limited by endogenous carbohydrate availability. Much exercise metabolism research to date has focused on muscle glycogen utilization, with little attention paid to the contribution of liver glycogen. (13)C magnetic resonance spectroscopy permits direct, noninvasive measurements of liver glycogen content and has increased understanding of the relevance of liver glycogen during exercise. In contrast to muscle, endurance-trained athletes do not exhibit elevated basal liver glycogen concentrations. However, there is evidence that liver glycogenolysis may be lower in endurance-trained athletes compared with untrained controls during moderate- to high-intensity exercise. Therefore, liver glycogen sparing in an endurance-trained state may account partly for training-induced performance/capacity adaptations during prolonged (>90 min) exercise. Ingestion of carbohydrate at a relatively high rate (>1.5 g/min) can prevent liver glycogen depletion during moderate-intensity exercise independent of the type of carbohydrate (e.g., glucose vs. sucrose) ingested. To minimize gastrointestinal discomfort, it is recommended to ingest specific combinations or types of carbohydrates (glucose plus fructose and/or sucrose). By coingesting glucose with either galactose or fructose, postexercise liver glycogen repletion rates can be doubled. There are currently no guidelines for carbohydrate ingestion to maximize liver glycogen repletion.

The Physiological Regulation of Skeletal Muscle Fatty Acid Supply and Oxidation During Moderate-Intensity Exercise

Energy substrates that are important to the working muscle at moderate intensities are the non-esterified fatty acids (NEFAs) taken up from the circulation and NEFAs originating from lipolysis of the intramuscular triacylglycerol (IMTAG). Moreover, NEFA from lipolysis via lipoprotein lipase (LPL) in the muscle of the very-low-density lipoproteins and in the (semi) post-prandial state chylomicrons may also contribute. In this review, the NEFA fluxes and oxidation by skeletal muscle during prolonged moderate-intensity exercise are described in terms of the integration of physiological systems. Steps involved in the regulation of the active muscle NEFA uptake include (1) increased energy demand; (2) delivery of NEFA to the muscle; (3) transport of NEFA into the muscle by NEFA transporters; and (4) activation of the NEFAs and either oxidation or re-esterification into IMTAG. The increased metabolic demand of the exercising muscle is the main driving force for all physiological regulatory processes. It elicits functional hyperemia, increasing the recruitment of capillaries and muscle blood flow resulting in increased NEFA delivery and accessibility to NEFA transporters and LPL. It also releases epinephrine that augments adipose tissue NEFA release and thereby NEFA delivery to the active muscle. Moreover, NEFA transporters translocate to the plasma membrane, further increasing the NEFA uptake. The majority of the NEFAs taken up by the active muscle is oxidized and a minor portion is re-esterified to IMTAG. Net IMTAG lipolysis occurs; however, the IMTAG contribution to total fat oxidation is rather limited compared to plasma-derived NEFA oxidation, suggesting a complex role and regulation of IMTAG utilization.

The effect of AQP3 deficiency on fuel selection during a single bout of exhausting exercise

Aquaporin-3 (AQP3) is an integral membrane protein that facilitates the transport of water and glycerol across cell membranes. However, the precise localization and function of AQP3 in skeletal muscles is currently unknown. In this study, we investigated the capacity of AQP3 knockout mice to perform a single bout of exhausting exercise and analyzed the parameters related to skeletal muscle energy metabolism during exhausting exercise. Mice were exposed to a single bout of treadmill running at a speed of 12 m/min with 10° inclination until exhaustion, and sacrificed immediately, 24 h and 48 h after exercise. Both immunohistochemistry and double immunofluorescence staining revealed that AQP3 is expressed at the cell surface with no evidence of colocalization with either AQP1 or AQP4 in hamstring skeletal muscles. When exposed to a single bout of exhaustive exercise, AQP3 knockout mice fatigued more easily with the average time to exhaustion shorter than the wild-type mice. After exhausting exercise, plasma glucose, muscle glycogen, muscle triglyceride, and muscle free fatty acid levels decreased compared with the values before exercise in both AQP3 knockout and wild-type mice. However, muscle glycerol concentration after exercise decreased in the wild-type mice, but rather increased in AQP3 knockout mice. These findings suggest that decreased glycerol efflux from the skeletal muscles in AQP3 knockout mice may result in low exercise capacity, presumably due to the limitations in the constant energy supply through hepatic gluconeogenesis from glycerol during the prolonged endurance exercise.

Effects of acute exercise on lipid content and dietary lipid uptake in liver and skeletal muscle of lean and diabetic rats

Insulin resistance is associated with ectopic lipid accumulation. Physical activity improves insulin sensitivity, but the impact of exercise on lipid handling in insulin-resistant tissues remains to be elucidated. The present study characterizes the effects of acute exercise on lipid content and dietary lipid partitioning in liver and skeletal muscle of lean and diabetic rats by use of magnetic resonance spectroscopy (MRS). After baseline measurements, rats were randomized to exercise or no-exercise groups. A subset of animals was subjected to MRS directly after 1 h of treadmill running for measurement of total intrahepatocellular lipid (IHCL) and intramyocellular lipid (IMCL) content (n=7 lean and diabetic rats). The other animals were administered 13C-labeled lipids orally after treadmill visit (with or without exercise) followed by MRS measurements after 4 and 24 h to determine the 13C enrichment of IHCL and IMCL (n=8 per group). Total IHCL and IMCL content were fivefold higher in diabetic vs. lean rats (P<0.001). Exercise did not significantly affect IHCL content but reduced IMCL by 25±7 and 33±4% in lean and diabetic rats (P<0.05), respectively. Uptake of dietary lipids in liver and muscle was 2.3-fold greater in diabetic vs. lean rats (P<0.05). Prior exercise did not significantly modulate dietary lipid uptake into muscle, but in liver of both lean and diabetic rats, lipid uptake was 44% reduced after acute exercise (P<0.05). In conclusion, IMCL but not IHCL represents a viable substrate source during exercise in both lean and diabetic rats, and exercise differentially affects dietary lipid uptake in muscle and liver.

Integrative biology of exercise

Exercise represents a major challenge to whole-body homeostasis provoking widespread perturbations in numerous cells, tissues, and organs that are caused by or are a response to the increased metabolic activity of contracting skeletal muscles. To meet this challenge, multiple integrated and often redundant responses operate to blunt the homeostatic threats generated by exercise-induced increases in muscle energy and oxygen demand. The application of molecular techniques to exercise biology has provided greater understanding of the multiplicity and complexity of cellular networks involved in exercise responses, and recent discoveries offer perspectives on the mechanisms by which muscle "communicates" with other organs and mediates the beneficial effects of exercise on health and performance.

Kinetics and utilization of lipid sources during acute exercise and acipimox

Overweight is associated with abnormalities of lipid metabolism, many of which are reversed by exercise. We investigated the impact of experimental antilipolysis and acute exercise on lipid kinetics and oxidation from VLDL-TG, plasma FFA, and "residual lipids" in overweight men (n = 8) using VLDL-TG and palmitate tracers in combination with muscle biopsies in a randomized, placebo-controlled design. Participants received placebo or acipimox on each study day (4 h of rest, 90 min of exercise at 50% V(O(2 max))). Exercise suppressed VLDL-TG secretion significantly during placebo but not acipimox (placebo-rest: 64.2 ± 9.4; placebo-exercise: 48.3 ± 8.0; acipimox-rest: 55.2 ± 13.4; acipimox-exercise: 52.0 ± 10.9). Resting oxidation of VLDL-TG FA and FFA was significantly reduced during acipimox compared with placebo, whereas "residual lipid oxidation" increased significantly [VLDL-TG oxidation (placebo: 18 ± 3 kcal/h; acipimox: 11 ± 2 kcal/h), FFA oxidation (placebo: 14 ± 2 kcal/h; acipimox: 4 ± 0.5 kcal/h), and residual lipid oxidation (placebo: 3 ± 5 kcal/h; acipimox: 14 ± 5 kcal/h)]. Additionally, during exercise on both placebo and acipimox, oxidation of VLDL-TG and FFA increased, but the relative contribution to total lipid oxidation diminished, except for FFA, which remained unchanged during acipimox. Residual lipid oxidation increased significantly during exercise in both absolute and relative terms. Changes in selected cellular enzymes and proteins provided no explanations for kinetic changes. In conclusion, suppressed FFA availability blunts the effect of exercise on VLDL-TG secretion and modifies the contribution of lipid sources for oxidation.

Influence of lipolysis and fatty acid availability on fuel selection during exercise

The aim of the present study was to investigate the influence of substrate availability on fuel selection during exercise. Eight endurance-trained male cyclists performed 90-min exercise at 70% of their maximal oxygen uptake in a cross-over design, either in rested condition (CON) or the day after 2-h exercise practised at 70% of maximal oxygen uptake (EX). Subjects were given a sucrose load (0.75 g kg(-1) body weight) 45 min after the beginning of the 90-min exercise test. Lipolysis was measured in subcutaneous abdominal adipose tissue (SCAT) by microdialysis and substrate oxidation by indirect calorimetry. Lipid oxidation increased during exercise and tended to decrease during sucrose ingestion in both conditions. Lipid oxidation was higher during the whole experimental period in the EX group (p = 0.004). Interestingly, fuel selection, assessed by the change in respiratory exchange ratio (RER), was increased in the EX session (p = 0.002). This was paralleled by a higher rate of SCAT lipolysis reflected by dialysate glycerol, plasma glycerol, and fatty acids (FA) levels (p < 0.001). Of note, we observed a significant relationship between whole-body fat oxidation and dialysate glycerol in both sessions (r (2) = 0.33, p = 0.02). In conclusion, this study highlights the limiting role of lipolysis and plasma FA availability to whole-body fat oxidation during exercise in endurance-trained subjects. This study shows that adipose tissue lipolysis is a determinant of fuel selection during exercise in healthy subjects.

Fuel selection during short-term submaximal treadmill exercise in the cold is not affected by pre-exercise low-intensity shivering

Exercise and shivering rely on different metabolic pathways and consequently, fuel selection. The present study examined the effects of a pre-exercise low-intensity shivering protocol on fuel selection during submaximal exercise in a cold environment. Nine male subjects exercised 4 times for 60 min at 50% (LOW) or 70% (MOD) of their peak oxygen consumption on a motorized treadmill in a climatic chamber set at 0 °C with (SHIV) and without (CON) a pre-exercise cooling protocol, inducing low-intensity shivering. Thermal, cardiorespiratory and metabolic responses were measured every 15 min whereas blood samples were collected every 30 min to assess serum nonesterified fatty acids (NEFA), glycerol, glucose, β-hydroxybutyrate (BHB) and plasma catecholamine concentrations. Rectal and skin temperatures were lower in the SHIV condition, within LOW and MOD conditions, during the first 45 min of exercise. Norepinephrine (NE) concentration was greater in SHIV vs. CON within LOW (1.39 ± 0.17 vs. 0.98 ± 0.17 ng·mL(-1)) and MOD (1.50 ± 0.20 vs. 1.01 ± 0.09 ng·mL(-1)), whereas NEFA, glycerol and BHB were greater in SHIV vs. CON (1060 ± 49 vs. 898 ± 78 μmol·L(-1); 0.27 ± 0.02 vs. 0.22 ± 0.03 mmol·L(-1); 0.39 ± 0.06 vs. 0.27 ± 0.04 mmol·L(-1), respectively) within MOD only. No changes were observed in fat or carbohydrate oxidation between SHIV and CON during exercise. Despite increases in NE, NEFA, glycerol and BHB from pre-exercise low-intensity shivering, fuel selection during short-term submaximal exercise in the cold was unaltered.

Effect of prolonged exercise and pre-exercise dietary manipulation on hepatic triglycerides in trained men

The purpose of this study was to examine the effect of exercise and pre-exercise dietary manipulation on hepatic triglyceride concentration (HTGC). HTGC was measured by proton magnetic resonance spectroscopy ((1)H-MRS) before and after 90 min of moderate intensity cycling in six endurance trained males following 67 h of mixed diet (M) and an isocaloric minimal carbohydrate (2%) high fat (83%) diet (HF). Diets were administered by balanced crossover design. Whole-body fat oxidation, plasma-free fatty acid (FFA), glycerol and triglyceride concentrations were significantly elevated during exercise in HF versus M (P < 0.05 for all). There was no significant treatment × time interaction for HTGC (P = 0.368). However, there was a significant net increase in HTGC (time effect) during the combined 6 h exercise and post-exercise period (P = 0.037). In conclusion, we observed no measurable net change in the hepatic triglyceride pool across a period involving a prolonged exercise bout. Furthermore, manipulation of pre-exercise dietary intake did not influence the interaction between the hepatic triglyceride concentration and exercise in lean trained men. This supports the contention that hepatic triglycerides do not meaningfully contribute to the high rate of fat oxidation observed during acute exercise, or the enhancement of this with regular exercise training and/or pre-exercise dietary manipulation.

Intramyocellular lipid content and insulin sensitivity are increased following a short-term low-glycemic index diet and exercise intervention

The relationship between intramyocellular (IMCL) and extramyocellular lipid (EMCL) accumulation and skeletal muscle insulin resistance is complex and dynamic. We examined the effect of a short-term (7-day) low-glycemic index (LGI) diet and aerobic exercise training intervention (EX) on IMCL and insulin sensitivity in older, insulin-resistant humans. Participants (66 ± 1 yr, BMI 33 ± 1 kg/m(2)) were randomly assigned to a parallel, controlled feeding trial [either an LGI (LGI/EX, n = 7) or high GI (HGI/EX, n = 8) eucaloric diet] combined with supervised exercise (60 min/day, 85% HR(max)). Insulin sensitivity was determined via 40 mU·m(-2)·min(-1) hyperinsulinemic euglycemic clamp and soleus IMCL and EMCL content was assessed by (1)H-MR spectroscopy with correction for fiber orientation. BMI decreased (kg/m(2) -0.6 ± 0.2, LGI/EX; -0.7 ± 0.2, HGI/EX P < 0.0004) after both interventions with no interaction effect of diet composition. Clamp-derived insulin sensitivity increased by 0.91 ± 0.21 (LGI/EX) and 0.17 ± 0.55 mg·kg(-1)·min(-1) (HGI/EX), P = 0.04 (effect of time). HOMA-IR was reduced by -1.1 ± 0.4 (LGI/EX) and -0.1 ± 0.2 (HGI/EX), P = 0.007 (effect of time), P = 0.02 (time × trial). Although both interventions increased IMCL content, (Δ: 2.3 ± 1.3, LGI/EX; 1.4 ± 0.9, HGI/EX, P = 0.03), diet composition did not significantly effect the increase. However, the LGI/EX group showed a robust increase in the [IMCL]/[EMCL] ratio compared with the HGI/EX group (Δ: 0.5 ± 0.2 LGI/EX vs. 0.07 ± 0.1, P = 0.03). The LGI/EX group also demonstrated greater reductions in [EMCL] than the HGI/EX group (Δ: -5.8 ± 3.4, LGI/EX; 2.3 ± 1.1, HGI/EX, P = 0.03). Changes in muscle lipids and insulin sensitivity were not correlated; however, the change in [IMCL]/[EMCL] was negatively associated with the change in FPI (r = -0.78, P = 0.002) and HOMA-IR (r = -0.61, P = 0.03). These data suggest that increases in the IMCL pool following a low glycemic diet and exercise intervention may represent lipid repartitioning from EMCL. The lower systemic glucose levels that prevail while eating a low glycemic diet may promote redistribution of lipid stores in the muscle.

The effect of exercise and nutrition on intramuscular fat metabolism and insulin sensitivity

Intramuscular triacylglycerol (IMTG) is both a dynamic fat-storage depot that can expand during periods of elevated lipid availability and a fatty acid source that can be utilized during periods of increased energy expenditure in active individuals. Although many studies have investigated the lifestyle determinants of IMTG content, the results are far from consistent, and studies attempting to unravel the mechanisms behind IMTG metabolism are in their infancy. The limited evidence available suggests that the enzymes responsible for skeletal muscle lipolysis and IMTG synthesis play an important role in determining the fate of fatty acids and therefore the concentration of lipid metabolites and insulin sensitivity of skeletal muscle. This review provides a summary of current knowledge on the effects of acute and chronic exercise as well as energy intake and macronutrient composition of the diet upon the metabolism of IMTG and the implications for metabolic health.

Low expression of long-chain acyl-CoA dehydrogenase in human skeletal muscle

PURPOSE

Long-chain acyl-CoA dehydrogenase (LCAD) is a mitochondrial flavoenzyme thought to be one of the major enzymes responsible for the first step of long-chain fatty acid (LCFA) beta-oxidation. Surprisingly, recent studies have shown LCAD is hardly detectable in human tissues such as liver and heart. Skeletal muscle is the largest organ in the body in terms of mass, and accounts for the majority of LCFA oxidation, especially during exercise. The purpose of this study was to investigate the expression levels of LCAD in human skeletal muscle.

METHODS

Muscle biopsies were obtained from the vastus lateralis of healthy athletic men and women, and examined for mRNA abundance, protein content, and enzyme activity of LCAD. We compared LCAD content with that of very-long chain acyl-CoA dehydrogenase (VLCAD) and medium chain acyl-CoA dehydrogenase (MCAD); two mitochondrial beta-oxidation enzymes that have overlapping chain-length specificity to that of LCAD. LCAD protein content and enzyme activity were also examined in enriched mitochondrial protein fractions. As controls, LCAD presence in skeletal muscle was compared to human heart, liver, and mouse skeletal muscle.

RESULTS

The mRNA presence of LCAD in human skeletal muscle is significantly less than VLCAD and MCAD (0.08+/-0.01 vs 7.3+/-0.5 vs 2.4+/-0.2 respectively, P<or=0.0001). LCAD protein was undetectable in human muscle homogenates, and coordinately LCAD enzyme activity was undetectable in enriched mitochondrial samples.

CONCLUSION

LCAD is minimally expressed in human skeletal muscle and likely does not play a significant role in LCFA oxidation.

Exercise therapy in type 2 diabetes

Structured exercise is considered an important cornerstone to achieve good glycemic control and improve cardiovascular risk profile in Type 2 diabetes. Current clinical guidelines acknowledge the therapeutic strength of exercise intervention. This paper reviews the wide pathophysiological problems associated with Type 2 diabetes and discusses the benefits of exercise therapy on phenotype characteristics, glycemic control and cardiovascular risk profile in Type 2 diabetes patients. Based on the currently available literature, it is concluded that Type 2 diabetes patients should be stimulated to participate in specifically designed exercise intervention programs. More attention should be paid to cardiovascular and musculoskeletal deconditioning as well as motivational factors to improve long-term treatment adherence and clinical efficacy. More clinical research is warranted to establish the efficacy of exercise intervention in a more differentiated approach for Type 2 diabetes subpopulations within different stages of the disease and various levels of co-morbidity.

Continuous low- to moderate-intensity exercise training is as effective as moderate- to high-intensity exercise training at lowering blood HbA(1c) in obese type 2 diabetes patients

AIMS/HYPOTHESIS

Exercise represents an effective interventional strategy to improve glycaemic control in type 2 diabetes patients. However, the impact of exercise intensity on the benefits of exercise training remains to be established. In the present study, we compared the clinical benefits of 6 months of continuous low- to moderate-intensity exercise training with those of continuous moderate- to high-intensity exercise training, matched for energy expenditure, in obese type 2 diabetes patients.

METHODS

Fifty male obese type 2 diabetes patients (age 59 +/- 8 years, BMI 32 +/- 4 kg/m(2)) participated in a 6 month continuous endurance-type exercise training programme. All participants performed three supervised exercise sessions per week, either 55 min at 50% of whole body peak oxygen uptake (VO(2)peak (low to moderate intensity) or 40 min at 75% of VO(2)peak (moderate to high intensity). Oral glucose tolerance, blood glycated haemoglobin, lipid profile, body composition, maximal workload capacity, whole body and skeletal muscle oxidative capacity and skeletal muscle fibre type composition were assessed before and after 2 and 6 months of intervention.

RESULTS

The entire 6 month intervention programme was completed by 37 participants. Continuous endurance-type exercise training reduced blood glycated haemoglobin levels, LDL-cholesterol concentrations, body weight and leg fat mass, and increased VO(2)peak, lean muscle mass and skeletal muscle cytochrome c oxidase and citrate synthase activity (p < 0.05). No differences were observed between the groups training at low to moderate or moderate to high intensity.

CONCLUSIONS/INTERPRETATION

When matched for energy cost, prolonged continuous low- to moderate-intensity endurance-type exercise training is equally effective as continuous moderate- to high-intensity training in lowering blood glycated haemoglobin and increasing whole body and skeletal muscle oxidative capacity in obese type 2 diabetes patients.

TRIAL REGISTRATION

ISRCTN32206301

FUNDING

None.

Adipose triacylglycerol lipase deletion alters whole body energy metabolism and impairs exercise performance in mice

Adipose triacylglycerol lipase (ATGL) and hormone-sensitive lipase (HSL) are essential for efficient lipolysis in adipose tissue and skeletal muscle. Herein, we utilized whole body knockout mice to address the importance of ATGL and HSL for metabolic function and exercise performance. ATGL deletion severely disrupts whole-body substrate partitioning at rest; reducing plasma free fatty acid (FFA) availability (WT: 0.49 +/- 0.06 vs. ATGL(-/-) 0.34 +/- 0.03 mM), which in turn enhances carbohydrate oxidation during fasting (mean RER, WT: 0.86 +/- 0.02, ATGL(-/-) 0.90 +/- 0.01) and is associated with depleted muscle and liver glycogen stores. While plasma FFA was modestly reduced (23%) and whole body carbohydrate metabolism increased in HSL(-/-) mice, resting glycogen storage was not compromised. Studies in isolated muscles revealed that the capacity of ATGL and HSL(-/-) muscle to transport exogenous fatty acids is not compromised and the capacity to oxidize fatty acids is actually increased (3.7- and 1.3-fold above WT for ATGL and HSL). The exercise-induced increase in plasma FFA and glycerol was blunted with ATGL or HSL deletion, demonstrating an impaired capacity for exercise-induced lipolysis in these mice. Carbohydrate oxidation was increased concomitantly during exercise in ATGL(-/-) and HSL(-/-) mice, resulting in more muscle and liver glycogen depletion. Maximal running velocity and endurance capacity were reduced by 42% and 46% in ATGL(-/-) mice, but not in HSL(-/-) mice. The reduction in performance in ATGL(-/-) mice was not due to defective muscle contractile performance. These results demonstrate an essential role for both ATGL and HSL in maintaining adequate FFA supply to sustain normal substrate metabolism at rest and during exercise.

Dietary glycemic index influences lipid oxidation but not muscle or liver glycogen oxidation during exercise

The glycemic index (GI) of dietary carbohydrates influences glycogen storage in skeletal muscle and circulating nonesterified fatty acid (NEFA) concentrations. We hypothesized that diets differing only in GI would alter intramuscular lipid oxidation and glycogen usage in skeletal muscle and liver during subsequent exercise. Endurance-trained individuals (n = 9) cycled for 90 min at 70% Vo(2peak) and then consumed either high- or low-GI meals over the following 12 h. The following day after an overnight fast, the 90-min cycle was repeated. (1)H and (13)C magnetic resonance spectroscopy was used before and after exercise to assess intramuscular lipid and glycogen content of the vastus muscle group and liver. Blood and expired air samples were collected at 15-min intervals throughout exercise. NEFA availability was reduced during exercise in the high- compared with the low-GI trial (area under curve 44.5 +/- 6.0 vs. 38.4 +/- 7.30 mM/h, P < 0.05). Exercise elicited an approximately 55% greater reduction in intramyocellular triglyceride (IMCL) in the high- vs. low-GI trial (1.6 +/- 0.2 vs. 1.0 +/- 0.3 mmol/kg wet wt, P < 0.05). There was no difference in the exercise-induced reduction of the glycogen pool in skeletal muscle (76 +/- 8 vs. 68 +/- 5 mM) or in liver (65 +/- 8 vs. 71 +/- 4 mM) between the low- and high-GI trials, respectively. High-GI recovery diets reduce NEFA availability and increase reliance on IMCL during moderate-intensity exercise. Skeletal muscle and liver glycogen storage or usage are not affected by the GI of an acute recovery diet.

Acute exercise-induced changes in basal VLDL-triglyceride kinetics leading to hypotriglyceridemia manifest more readily after resistance than endurance exercise

Resistance training is considered less effective than endurance training in lowering plasma triglyceride (TG) concentrations. Acutely, however, a single bout of strenuous exercise, whether endurance or resistance, increases the efficiency of very low-density lipoprotein (VLDL)-TG removal from the circulation and leads to hypotriglyceridemia. The comparative effects of these two types of exercise on VLDL-TG metabolism are not known. We therefore examined basal VLDL-TG kinetics by using stable isotope-labeled tracers in seven healthy, nonobese, untrained young men in the postabsorptive state, the morning after a single 90-min bout of either low-intensity endurance exercise (approximately 30% of peak oxygen consumption) or high-intensity resistance exercise (3 sets of 10 repetitions for 12 exercises at 80% of peak torque production), matched for total energy expenditure (approximately 400 kcal), or an equivalent period of rest on the preceding afternoon. Compared with rest, resistance exercise lowered fasting plasma VLDL-TG concentration by -28 +/- 10% (P = 0.034), increased VLDL-TG plasma clearance rate by 30 +/- 8% (P = 0.003), and shortened the mean residence time (MRT) of VLDL-TG in the circulation by -36 +/- 11 min (P = 0.016), whereas endurance exercise had no effect (all P > 0.05). Basal VLDL-TG plasma clearance rate was greater (P = 0.003) and VLDL-TG MRT was shorter (P = 0.012) the morning after resistance than endurance exercise. We conclude that, for the same total energy expenditure, resistance exercise is more potent than endurance exercise in eliciting changes in VLDL-TG metabolism that have been linked with hypotriglyceridemia, and it should thus be considered as an alternative to or in addition to endurance exercise for the control of plasma TG concentrations.

Hormone-sensitive lipase is necessary for normal mobilization of lipids during submaximal exercise

For the working muscle there are a number of fuels available for oxidative metabolism, including glycogen, glucose, and nonesterified fatty acids. Nonesterified fatty acids originate from lipolysis in white adipose tissue, hydrolysis of VLDL triglycerides, or hydrolysis of intramyocellular triglyceride stores. A key enzyme in the mobilization of fatty acids from intracellular lipid stores is hormone-sensitive lipase (HSL). The aim of the present study was to investigate the metabolic response of HSL-null mice challenged with exercise or fasting and to examine whether other lipases are able to fully compensate for the lack of HSL. The results showed that HSL-null mice have reduced capacity to perform aerobic exercise. The liver glycogen stores were more rapidly depleted in HSL-null mice during treadmill exercise, and HSL-null mice had reduced plasma concentrations of both glycerol and nonesterified fatty acids after exercise and fasting, respectively. The data support the hypothesis that in the absence of HSL, mice are not able to respond to an exercise challenge with increased mobilization of the lipid stores. Consequently, the impact of the lipid-sparing effect on liver glycogen is reduced in the HSL-null mice, resulting in faster depletion of this energy source, contributing to the decreased endurance during submaximal exercise.

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

PURPOSE

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

METHODS

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

RESULTS

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

CONCLUSIONS

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