Fuel selection, muscle fibre

Changes Induced by Aging and Long-Term Exercise and/or DHA Supplementation in Muscle of Obese Female Mice

Obesity and aging promote chronic low-grade systemic inflammation. The aim of the study was to analyze the effects of long-term physical exercise and/or omega-3 fatty acid Docosahexaenoic acid (DHA) supplementation on genes or proteins related to muscle metabolism, inflammation, muscle damage/regeneration and myokine expression in aged and obese mice. Two-month-old C57BL/6J female mice received a control or a high-fat diet for 4 months. Then, the diet-induced obese (DIO) mice were distributed into four groups: DIO, DIO + DHA, DIO + EX (treadmill training) and DIO + DHA + EX up to 18 months. Mice fed a control diet were sacrificed at 2, 6 and 18 months. Aging increased the mRNA expression of Tnf-α and decreased the expression of genes related to glucose uptake (Glut1, Glut4), muscle atrophy (Murf1, Atrogin-1, Cas-9) and myokines (Metrnl, Il-6). In aged DIO mice, exercise restored several of these changes. It increased the expression of genes related to glucose uptake (Glut1, Glut4), fatty acid oxidation (Cpt1b, Acox), myokine expression (Fndc5, Il-6) and protein turnover, decreased Tnf-α expression and increased p-AKT/AKT ratio. No additional effects were observed when combining exercise and DHA. These data suggest the effectiveness of long-term training to prevent the deleterious effects of aging and obesity on muscle dysfunction.

A Single Bout of Ultra-Endurance Exercise Reveals Early Signs of Muscle Aging in Master Athletes

Middle-aged and master endurance athletes exhibit similar physical performance and long-term muscle adaptation to aerobic exercise. Nevertheless, we hypothesized that the short-term plasticity of the skeletal muscle might be distinctly altered for master athletes when they are challenged by a single bout of prolonged moderate-intensity exercise. Six middle-aged (37Y) and five older (50Y) master highly-trained athletes performed a 24-h treadmill run (24TR). Vastus lateralis muscle biopsies were collected before and after the run and assessed for proteomics, fiber morphometry, intramyocellular lipid droplets (LD), mitochondrial oxidative activity, extracellular matrix (ECM), and micro-vascularisation. Before 24TR, muscle fiber type morphometry, intramyocellular LD, oxidative activity, ECM and micro-vascularisation were similar between master and middle-aged runners. For 37Y runners, 24TR was associated with ECM thickening, increased capillary-to-fiber interface, and an 89% depletion of LD in type-I fibers. In contrast, for 50Y runners, 24TR did not alter ECM and capillarization and poorly depleted LDs. Moreover, an impaired succinate dehydrogenase activity and functional class scoring of proteomes suggested reduced oxidative phosphorylation post-24TR exclusively in 50Y muscle. Collectively, our data support that middle-aged and master endurance athletes exhibit distinct transient plasticity in response to a single bout of ultra-endurance exercise, which may constitute early signs of muscle aging for master athletes.

Effects of neuromuscular electrical stimulation on energy expenditure and postprandial metabolism in healthy men

It is unclear whether neuromuscular electrical stimulation (NMES) has meaningful metabolic effects when users have the opportunity to self-select the intensity to one that can be comfortably tolerated. Nine healthy men aged 28 ± 9 y (mean ± SD) with a body mass index 22.3 ± 2.3 kg/m2 completed 3 trials involving a 2-h oral glucose tolerance test whilst, in a randomised counterbalanced order, (1) sitting motionless (SIT), (2) standing motionless (STAND); and (3) sitting motionless with NMES of quadriceps and calves at a self-selected tolerable intensity. The mean (95% confidence interval [CI]) total energy expenditure was greater in the NMES trial (221 [180-262] kcal/2 h) and STAND trial (178 [164-191] kcal/2 h) than during SIT (159 [150-167] kcal/2 h) (both, p < 0.05). This was primarily driven by an increase in carbohydrate oxidation in the NMES and STAND trials compared with the SIT trial (p < 0.05). Postprandial insulin iAUC was lower in both NMES and STAND compared with SIT (16.4 [7.7-25.1], 17 [7-27] and 22.6 [10.8-34.4] nmol·120 min/L, respectively; both, p < 0.05). Compared with sitting, both NMES and STAND increased energy expenditure and whole-body carbohydrate oxidation and reduced postprandial insulin concentrations in healthy men, with more pronounced effects seen with NMES. Self-selected NMES is a potential strategy for improving metabolic health. This trial is registered at ClinicalTrials.gov (ID: NCT04389736). Novelty: NMES at a comfortable intensity enhances energy expenditure and carbohydrate oxidation, and reduces postprandial insulinemia. Thus, self-selected NMES represents a potential strategy to improve metabolic health.

Skeletal Muscle Function, Structure, and Metabolism in Patients With Heart Failure With Reduced Ejection Fraction and Heart Failure With Preserved Ejection Fraction

BACKGROUND

Reduced exercise capacity in patients with heart failure (HF) could be partially explained by skeletal muscle dysfunction. We compared skeletal muscle function, structure, and metabolism among clinically stable outpatients with HF with preserved ejection fraction, HF with reduced ejection fraction, and healthy controls (HC). Furthermore, the molecular, metabolic, and clinical profile of patients with reduced muscle endurance was described.

METHODS

Fifty-five participants were recruited prospectively at the University Hospital Jena (17 HF with preserved ejection fraction, 18 HF with reduced ejection fraction, and 20 HC). All participants underwent echocardiography, cardiopulmonary exercise testing, 6-minute walking test, isokinetic muscle function, and skeletal muscle biopsies. Expression levels of fatty acid oxidation, glucose metabolism, atrophy genes, and proteins as well as inflammatory biomarkers were assessed. Mitochondria were evaluated using electron microscopy.

RESULTS

Patients with HF with preserved ejection fraction showed compared with HF with reduced ejection fraction and HC reduced muscle strength (eccentric extension: 13.3±5.0 versus 18.0±5.9 versus 17.9±5.1 Nm/kg, P=0.04), elevated levels of MSTN-2 (myostatin-2), FBXO-32 (F-box only protein 32 [Atrogin1]) gene and protein, and smaller mitochondrial size (P<0.05). Mitochondrial function and fatty acid and glucose metabolism were impaired in HF-patients compared with HC (P<0.05). In a multiple regression analysis, GDF-15 (growth and differentiation factor 15), CPT1B (carnitine palmitoyltransferase IB)-protein and oral anticoagulation were independent factors for predicting reduced muscle endurance after adjusting for age (log10 GDF-15 [pg/mL] [B, -54.3 (95% CI, -106 to -2.00), P=0.043], log10 CPT1B per fold increase [B, 49.3 (95% CI, 1.90-96.77), P=0.042]; oral anticoagulation present [B, 44.8 (95% CI, 27.90-61.78), P<0.001]).

CONCLUSIONS

Patients with HF with preserved ejection fraction have worse muscle function and predominant muscle atrophy compared with those with HF with reduced ejection fraction and HC. Inflammatory biomarkers, fatty acid oxidation, and oral anticoagulation were independent factors for predicting reduced muscle endurance.

Effects of a Ketogenic Diet on Muscle Fatigue in Healthy, Young, Normal-Weight Women: A Randomized Controlled Feeding Trial

Ketogenic low-carbohydrate high-fat (LCHF) diets are increasingly popular in broad sections of the population. The main objective of this study was to evaluate the effects of a non-energy-restricted ketogenic LCHF diet on muscle fatigue in healthy, young, and normal-weight women. Twenty-four women were randomly allocated to a 4-week ketogenic LCHF diet followed by a 4-week control diet (a National Food Agency recommended diet), or the reverse sequence due to the crossover design. Treatment periods were separated by a 15 week washout period. Seventeen women completed the study and were included in the analyses. Treatment effects were evaluated using mixed models. The ketogenic LCHF diet had no effect on grip strength or time to fatigue, measured with handgrip test (day 24–26). However, cycling time to fatigue decreased with almost two minutes (−1.85 min 95% CI:[−2.30;−1.40]; p < 0.001) during incremental cycling (day 25–27), accommodated with higher ratings of perceived exertion using the Borg scale (p < 0.01). Participants’ own diary notes revealed experiences of muscle fatigue during daily life activities, as well as during exercise. We conclude that in young and healthy women, a ketogenic LCHF diet has an unfavorable effect on muscle fatigue and might affect perceived exertion during daily life activities.

Muscle wasting and cachexia in heart failure: mechanisms and therapies

Body wasting is a serious complication that affects a large proportion of patients with heart failure. Muscle wasting, also known as sarcopenia, is the loss of muscle mass and strength, whereas cachexia describes loss of weight. After reaching guideline-recommended doses of heart failure therapies, the most promising approach to treating body wasting seems to be combined therapy that includes exercise, nutritional counselling, and drug treatment. Nutritional considerations include avoiding excessive salt and fluid intake, and replenishment of deficiencies in trace elements. Administration of omega-3 polyunsaturated fatty acids is beneficial in selected patients. High-calorific nutritional supplements can also be useful. The prescription of aerobic exercise training that provokes mild or moderate breathlessness has good scientific support. Drugs with potential benefit in the treatment of body wasting that have been tested in clinical studies in patients with heart failure include testosterone, ghrelin, recombinant human growth hormone, essential amino acids, and β₂-adrenergic receptor agonists. In this Review, we summarize the pathophysiological mechanisms of muscle wasting and cachexia in heart failure, and highlight the potential treatment strategies. We aim to provide clinicians with the relevant information on body wasting to understand and treat these conditions in patients with heart failure.

Physiological effects of increased foraging effort in a small passerine

Foraging to obtain food, either for self-maintenance or at presumably elevated rates to provision offspring, is thought to be an energetically demanding activity but one that is essential for fitness (higher reproductive success and survival). Nevertheless, the physiological mechanisms that allow some individuals to support higher foraging performance, and the mechanisms underlying costs of high workload, remain poorly understood. We experimentally manipulated foraging behaviour in zebra finches (Taeniopygia guttata) using the technique described by Koetsier and Verhulst (2011). Birds in the “high foraging effort” (HF) group had to obtain food either while flying/hovering or by making repeated hops or jumps from the ground up to the feeder, behaviour typical of the extremely energetically-expensive foraging mode observed in many free-living small passerines. HF birds made significantly more trips to the feeder per 10min whereas control birds spent more time (perched) at the feeder. Despite this marked change in foraging behaviour we documented few short- or long-term effects of “training” (3 days and 90 days of “training” respectively) and some of these effects were sex-specific. There were no effects of treatment on BMR, hematocrit, hemoglobin, or plasma glycerol, triglyceride, glucose levels, and masses of kidney, crop, large intestine, small intestine, gizzard and liver. HF females had higher masses of flight muscle, leg muscle, heart and lung compared to controls. In contrast, HF males had lower heart mass than controls and there were no differences for other organs. When both sexes were pooled, there were no effects of treatment on body composition. Finally, birds in the HF treatment had higher levels of reactive oxygen metabolites (dROMs) and, consequently, although treatment did not affect total antioxidant capacity (OXY), birds in the HF treatment had higher oxidative stress.

Altered Energetics of Exercise Explain Risk of Rhabdomyolysis in Very Long-Chain Acyl-CoA Dehydrogenase Deficiency

Rhabdomyolysis is common in very long-chain acyl-CoA dehydrogenase deficiency (VLCADD) and other metabolic myopathies, but its pathogenic basis is poorly understood. Here, we show that prolonged bicycling exercise against a standardized moderate workload in VLCADD patients is associated with threefold bigger changes in phosphocreatine (PCr) and inorganic phosphate (Pi) concentrations in quadriceps muscle and twofold lower changes in plasma acetyl-carnitine levels than in healthy subjects. This result is consistent with the hypothesis that muscle ATP homeostasis during exercise is compromised in VLCADD. However, the measured rates of PCr and Pi recovery post-exercise showed that the mitochondrial capacity for ATP synthesis in VLCADD muscle was normal. Mathematical modeling of oxidative ATP metabolism in muscle composed of three different fiber types indicated that the observed altered energy balance during submaximal exercise in VLCADD patients may be explained by a slow-to-fast shift in quadriceps fiber-type composition corresponding to 30% of the slow-twitch fiber-type pool in healthy quadriceps muscle. This study demonstrates for the first time that quadriceps energy balance during exercise in VLCADD patients is altered but not because of failing mitochondrial function. Our findings provide new clues to understanding the risk of rhabdomyolysis following exercise in human VLCADD.

The influence of iron deficiency on the functioning of skeletal muscles: experimental evidence and clinical implications

Skeletal and respiratory myopathy not only constitutes an important pathophysiological feature of heart failure and chronic obstructive pulmonary disease, but also contributes to debilitating symptomatology and predicts worse outcomes in these patients. Accumulated evidence from laboratory experiments, animal models, and interventional studies in sports medicine suggests that undisturbed systemic iron homeostasis significantly contributes to the effective functioning of skeletal muscles. In this review, we discuss the role of iron status for the functioning of skeletal muscle tissue, and highlight iron deficiency as an emerging therapeutic target in chronic diseases accompanied by a marked muscle dysfunction.

Exercise training at the intensity of maximal fat oxidation in obese boys

The objectives of this study were to explore the effects of 10 weeks of exercise training at the intensity of maximal fat oxidation rate (FATmax) on body composition, cardiovascular fitness, and functional capacity in 8- to 10-year-old obese boys. This is a school-based interventional study. Twenty-six obese boys and 20 lean boys were randomly allocated into the exercise and control groups. Measurements of body composition, FATmax through gas analyses, predicted maximal oxygen uptake, and functional capacity (run, jump, abdominal muscle function, and body flexibility) were conducted at baseline and at the end of experiments. Two exercise groups participated in 10 weeks of supervised exercise training at individualized FATmax intensities, for 1 h per day and 5 days per week. FATmax training decreased body mass (-1.0 kg, p < 0.05), body mass index (-1.2 kg/m(2), p < 0.01), fat mass (-1.2 kg, p < 0.01), and abdominal fat (-0.13 kg, p < 0.01) of the trained obese boys. Their cardiovascular fitness (p < 0.05) and body flexibility (p < 0.05) were also improved after training. The lean boys showed improvements in cardiovascular fitness after training (p < 0.05). FATmax training increased the FATmax in obese boys from 0.35 ± 0.12 g/min to 0.38 ± 0.13 g/min, but this change was not statistically significant. In addition, there was no change in daily energy intake for all participants before and after the experimental period. Results of this study suggest that FATmax is an effective exercise training intensity for the treatment of childhood obesity.

Comparative histology of muscle in free ranging cetaceans: shallow versus deep diving species

Different marine mammal species exhibit a wide range of diving behaviour based on their breath-hold diving capabilities. They are classically categorized as long duration, deep-diving and short duration, shallow-diving species. These abilities are likely to be related to the muscle characteristics of each species. Despite the increasing number of publications on muscle profile in different cetacean species, very little information is currently available concerning the characteristics of other muscle components in these species. In this study, we examined skeletal muscle fiber type, fiber size (cross sectional area and lesser diameter), intramuscular substrates, and perimysium-related structures, by retrospective study in 146 stranded cetaceans involving 15 different species. Additionally, we investigated diving profile-specific histological features. Our results suggest that deep diving species have higher amount of intramyocyte lipid droplets, and evidence higher percentage of intramuscular adipose tissue, and larger fibre sizes in this group of animals.

Exercise training at the maximal fat oxidation intensity improved health-related physical fitness in overweight middle-aged women

Background/Objective

The purpose of this study was to test the hypothesis that exercise training at the maximal fat oxidation (FATmax) intensity would improve the health-related physical fitness in overweight middle-aged women.

Methods

Thirty women (45-59 years old and BMI 28.2 ± 1.8 kg/m²) were randomly allocated into the Exercise and Control groups. Body composition, FATmax, predicted maximal oxygen uptake, heart function during submaximal exercise, stroke volume, left ventricular ejection fraction, trunk muscle strength, and body flexibility were measured before and after the experimental period.

Results

Following the 10 weeks of supervised exercise training, the Exercise group achieved significant improvements in body composition, cardiovascular function, skeletal muscle strength, and body flexibility; whereas there were no changes in these variables of the Control group. There was also no significant change in daily energy intake for all participants before and after the interventions.

Conclusion

The 10-week FATmax intensity training is an effective treatment to improve health-related physical fitness in overweight middle-aged women.

The wasting continuum in heart failure: from sarcopenia to cachexia

Sarcopenia (muscle wasting) and cachexia share some pathophysiological aspects. Sarcopenia affects approximately 20 %, cachexia <10 % of ambulatory patients with heart failure (HF). Whilst sarcopenia means loss of skeletal muscle mass and strength that predominantly affects postural rather than non-postural muscles, cachexia means loss of muscle and fat tissue that leads to weight loss. The wasting continuum in HF implies that skeletal muscle is lost earlier than fat tissue and may lead from sarcopenia to cachexia. Both tissues require conservation, and therapies that stop the wasting process have tremendous therapeutic appeal. The present paper reviews the pathophysiology of muscle and fat wasting in HF and discusses potential treatments, including exercise training, appetite stimulants, essential amino acids, growth hormone, testosterone, electrical muscle stimulation, ghrelin and its analogues, ghrelin receptor agonists and myostatin antibodies.

The genetics of human running: ACTN3 polymorphism as an evolutionary tool improving the energy economy during locomotion

BACKGROUND

Covering long distances was an important trait to human evolution and continues to be highlighted for health and athletic status. This ability is benefitted by a low cost of locomotion (CoL), meaning that the individuals who are able to expend less energy would be able to cover longer distances. The CoL has been shown to be influenced by distinct and even 'opposite' factors, such as physiological and muscular characteristics, which are genetically inherited. In this way, DNA alterations could be important determinants of the characteristics associated with the CoL. A polymorphism in the ACTN3 gene (R577X) has been related to physical performance, associating the X allele with endurance and the R allele with strength/power abilities.

AIM

To investigate the influence of ACTN3 genotypes on the CoL.

SUBJECTS AND METHODS

One hundred and fifty healthy male individuals performed two constant speed tests (at 10 and 12 km/h) to determine the CoL.

RESULTS

Interestingly, the results showed that heterozygous individuals (RX genotype) presented significantly lower CoL compared to RR and XX individuals.

CONCLUSIONS

It is argued that RX genotype might generate an intermediate strength-to-endurance phenotype, leading to a better phenotypic profile for energy economy during running and, consequently, for long-term locomotion.

Increasing exercise intensity reduces heterogeneity of glucose uptake in human skeletal muscles

Proper muscle activation is a key feature of survival in different tasks in daily life as well as sports performance, but can be impaired in elderly and in diseases. Therefore it is also clinically important to better understand the phenomenon that can be elucidated in humans non-invasively by positron emission tomography (PET) with measurements of spatial heterogeneity of glucose uptake within and among muscles during exercise. We studied six healthy young men during 35 minutes of cycling at relative intensities of 30% (low), 55% (moderate), and 75% (high) of maximal oxygen consumption on three separate days. Glucose uptake in the quadriceps femoris muscle group (QF), the main force producing muscle group in recreational cycling, and its four individual muscles, was directly measured using PET and 18F-fluoro-deoxy-glucose. Within-muscle heterogeneity was determined by calculating the coefficient of variance (CV) of glucose uptake in PET image voxels within the muscle of interest, and among-muscles heterogeneity of glucose uptake in QF was expressed as CV of the mean glucose uptake values of its separate muscles. With increasing intensity, within-muscle heterogeneity decreased in the entire QF as well as within its all four individual parts. Among-muscles glucose uptake heterogeneity also decreased with increasing intensity. However, mean glucose uptake was consistently lower and heterogeneity higher in rectus femoris muscle that is known to consist of the highest percentage of fast twitch type II fibers, compared to the other three QF muscles. In conclusion, these results show that in addition to increased contribution of distinct muscle parts, with increases in exercise intensity there is also an enhanced recruitment of muscle fibers within all of the four heads of QF, despite established differences in muscle-part specific fiber type distributions. Glucose uptake heterogeneity may serve as a useful non-invasive tool to elucidate muscle activation in aging and diseased populations.

Diet-induced obesity up-regulates the abundance of GPR43 and GPR120 in a tissue specific manner

BACKGROUND/AIMS

GPR43 and GPR120 have recently been deorphanised as receptors for fatty acids. Fatty acids mediate a variety of metabolic processes in the body, however, the effect these receptors have on metabolism is not fully understood. Here, we characterise the effect of diet-induced obesity on the expression of GPR43 and GPR120 in tissues important in maintaining metabolic health.

METHODS

Six-week old male Sprague Dawley rats were fed either a high fat diet (HFD; 22% fat) or control diet (5% fat; n = 8-9/group) for 12 weeks. Rats were euthanized and the heart, liver, soleus and extensor digitorum longus (EDL) skeletal muscles were excised. GPR43 and GPR120 receptor abundance was quantified by 'real-time' PCR.

RESULTS

GPR43 mRNA abundance was significantly up-regulated by a HFD in liver and soleus and EDL skeletal muscles compared to control (p ≤ 0.05). Whilst a HFD significantly up-regulated GPR120 gene transcripts in cardiac tissue and EDL skeletal muscle when compare to control (p ≤ 0.05).

CONCLUSION

We have shown for the first time that up-regulation of GPR43 and GPR120 in response to a HFD, is tissue specific. This suggests these receptors have different roles in mediating metabolic function in a number of tissues in the human body.

Beneficial metabolic adaptations due to endurance exercise training in the fasted state

Training with limited carbohydrate availability can stimulate adaptations in muscle cells to facilitate energy production via fat oxidation. Here we investigated the effect of consistent training in the fasted state, vs. training in the fed state, on muscle metabolism and substrate selection during fasted exercise. Twenty young male volunteers participated in a 6-wk endurance training program (1-1.5 h cycling at ∼70% Vo(₂max), 4 days/wk) while receiving isocaloric carbohydrate-rich diets. Half of the subjects trained in the fasted state (F; n = 10), while the others ingested ample carbohydrates before (∼160 g) and during (1 g·kg body wt⁻¹·h⁻¹) the training sessions (CHO; n = 10). The training similarly increased Vo(₂max) (+9%) and performance in a 60-min simulated time trial (+8%) in both groups (P < 0.01). Metabolic measurements were made during a 2-h constant-load exercise bout in the fasted state at ∼65% pretraining Vo(₂max). In F, exercise-induced intramyocellular lipid (IMCL) breakdown was enhanced in type I fibers (P < 0.05) and tended to be increased in type IIa fibers (P = 0.07). Training did not affect IMCL breakdown in CHO. In addition, F (+21%) increased the exercise intensity corresponding to the maximal rate of fat oxidation more than did CHO (+6%) (P < 0.05). Furthermore, maximal citrate synthase (+47%) and β-hydroxyacyl coenzyme A dehydrogenase (+34%) activity was significantly upregulated in F (P < 0.05) but not in CHO. Also, only F prevented the development exercise-induced drop in blood glucose concentration (P < 0.05). In conclusion, F is more effective than CHO to increase muscular oxidative capacity and at the same time enhances exercise-induced net IMCL degradation. In addition, F but not CHO prevented drop of blood glucose concentration during fasting exercise.

Distinct skeletal muscle fiber characteristics and gene expression in diet-sensitive versus diet-resistant obesity

Inter-individual variability in weight gain and loss under energy surfeit and deficit conditions, respectively, are well recognized but poorly understood phenomena. We documented weight loss variability in an intensively supervised clinical weight loss program and assessed skeletal muscle gene expression and phenotypic characteristics related to variable response to a 900 kcal regimen. Matched pairs of healthy, diet-compliant, obese diet-sensitive (ODS) and diet-resistant (ODR) subjects were defined as those in the highest and lowest quintiles for weight loss rate. Physical activity energy expenditure was minimal and comparable. Following program completion and weight stabilization, skeletal muscle biopsies were obtained. Gene expression analysis of rectus femoris and vastus lateralis indicated upregulation of genes and gene sets involved in oxidative phosphorylation and glucose and fatty acid metabolism in ODS compared with ODR. In vastus lateralis, there was a higher proportion of oxidative (type I) fibers in ODS compared with ODR women and lean controls, fiber hypertrophy in ODS compared with ODR women and lean controls, and lower succinate dehydrogenase in oxidative and oxidative-glycolytic fibers in all obese compared with lean subjects. Intramuscular lipid content was generally higher in obese versus lean, and specifically higher in ODS vs. lean women. Altogether, our findings demonstrate differences in muscle gene expression and fiber composition related to clinical weight loss success.

Effect of extracellular osmolality on metabolism in contracting mammalian skeletal muscle in vitro

Extensive research has been conducted on hepatocyte metabolism perturbed under the influence of anisosmotic stress. However, much less is known about the behaviour of skeletal muscle metabolism under similar conditions. After establishing a working model to study anisosmotic stress in resting mammalian skeletal muscle, the current study tested the hypothesis that hyperosmotic (HYPER) stress would lead to increased creatine, lactate, and measured enzyme activity, whereas hypo-osmotic (HYPO) stress would lead to decreased metabolites and enzyme activity vs. iso-osmotic (ISO) stress post contraction. Rat soleus (SOL) and extensor digitorum longus (EDL) were isolated and incubated in an organ bath (95% O2, 5% CO2, pH 7.4, 25 °C) altered to targeted osmotic conditions (ISO, 290 osmol·L–1; HYPO, 180 osmol·L–1; HYPER, 400 osmol·L–1). Muscle samples were flash frozen after 10 min of contraction. Post contraction, muscle water content in the SOL HYPO condition was 18% greater than ISO, and HYPER had approximately 14% less water content than ISO (p < 0.05). In the HYPO condition, EDL had 21% greater water content than ISO, and HYPER had 17% less water content than ISO (p < 0.05). SOL HYPO resulted in higher phosphocreatine and lower lactate and creatine vs. HYPER (p < 0.05) but there were no differences in EDL between HYPO and HYPER. Pyruvate dehydrogenase activity increased in SOL HYPER vs. HYPO, whereas glycogen phosphorylase a increased in EDL HYPER vs. HYPO. In conclusion, fibre-type-specific responses exist after contraction such that when SOL muscle is perturbed in HYPER, as compared with HYPO, media, metabolic activity increases. Future work should focus on glucose uptake–regulation during anisosmotic conditions.

Metabolic and phenotypic characteristics of human skeletal muscle fibers as predictors of glycogen utilization during electrical stimulation

Characteristics of skeletal muscle such as fiber type composition and activities of key metabolic enzymes have been purported to affect glycogen utilization. However, the relative importance individual factors may have in predicting glycogen utilization of individual muscle fibers has not been addressed. Thus, we sought to determine the relative importance that metabolic characteristics and phenotypic expression of individual fibers have in predicting fiber specific glycogen utilization during neuromuscular electrical stimulation (NMES) exercise. Biopsies were taken from the m, vastus lateralis (VL) of eight recreationally active males before and immediately after 30 min of non-fatiguing NMES and analyzed for type (I, IIa and IIx), succinate dehydrogenase activity (SDH), glycerol-phosphate dehydrogenase activity (GPDH), quantitative-actomyosin adenosine triphosphatase activity (qATPase), and glycogen content. Our results demonstrate that a ratio of enzyme activities representing pathways for energy supply and energy demand (SDH: qATPase) accounted for more of the variance in glycogen utilization (y=0.2091 e(-0.0329x ), R2=0.622, P< or = 0.0001) than SDH (R2=0.321) or qATPase (R2=0.365) alone. Fiber phenotype was also a significant predictor of glycogen utilization, but to a lesser extent than the other variables studied (R2=0.201). A ratio of the activities of enzymes representing pathways of energy supply and energy demand, represented by SDH:qATPase, is a better predictor of glycogen utilization than either of its components independently while fiber phenotype, although a statistically significant predictor of glycogen utilization, may not be the most appropriate determinate of the functional characteristics of an individual fiber.

Use of intramuscular triacylglycerol as a substrate source during exercise in humans

Fat and carbohydrate are the principal substrates that fuel aerobic ATP synthesis in skeletal muscle. Most endogenous fat is stored as triacylglycerol in subcutaneous and deep visceral adipose tissue. Smaller quantities of triacylglycerol are deposited as lipid droplets inside skeletal muscle fibers. The potential role of intramyocellular triacylglycerol (IMTG) as a substrate source during exercise in humans has recently regained much of its interest because of the proposed functional relationship between IMTG accumulation and the development of skeletal muscle insulin resistance. Exercise likely represents an effective means to prevent excess IMTG accretion by stimulating its rate of oxidation. However, there is much controversy on the actual contribution of the IMTG pool as a substrate source during exercise. The apparent discrepancy in the literature likely stems from methodological difficulties that have been associated with the methods used to estimate IMTG oxidation during exercise. However, recent studies using stable isotope methodology, 1H-magnetic resonance spectroscopy, and electron and/or immunofluorescence microscopy all support the contention that the IMTG pool can function as an important substrate source during exercise. Although more research is warranted, IMTG mobilization and/or oxidation during exercise seem to be largely determined by exercise intensity, exercise duration, macronutrient composition of the diet, training status, gender, and/or age. In addition, indirect evidence suggests that the capacity to mobilize and/or oxidize IMTG is substantially impaired in an obese and/or Type 2 diabetic state. As we now become aware that skeletal muscle has an enormous capacity to oxidize IMTG stores during exercise, more research is warranted to develop combined exercise, nutritional, and/or pharmacological interventions to effectively stimulate IMTG oxidation in sedentary, obese, and/or Type 2 diabetes patients.

Optimal exercise intensities for fat metabolism in handbike cycling and cycling

STUDY DESIGN

Energy expenditure (EE) and fat oxidation in handbike cycling compared to cycling in order to determine the intensity that elicits maximal fat oxidation in handbike cycling.

OBJECTIVE

To establish the exercise intensity with the highest fat oxidation rate in handbike cycling compared with cycling (control group) in order to give training recommendations for spinal cord-injured (SCI) athletes performing handbike cycling.

SETTING

Institute of Sports Medicine, Swiss Paraplegic Centre, Nottwil, Switzerland.

METHODS

Eight endurance-trained handbike cyclists (VO2 peak(handbike cycling) 37.5+/-7.8 ml/kg/min) and eight endurance trained cyclists (VO2 peak(cycling) 62.5+/-4.5 ml/kg/min) performed three 20-min exercise blocks at 55, 65 and 75% VO2 peak in handbike cycling on a treadmill or in cycling on a cycling ergometer, respectively, in order to find the intensity with the absolutely highest fat oxidation.

RESULTS

The contribution of fat to total EE was highest (39.1+/-16.3% EE) at 55% VO2 peak in handbike cycling compared to cycling, where highest contribution of fat to EE (50.8+/-13.8%) was found at 75% VO2 peak. In handbike cycling, the highest absolute fat oxidation (0.28+/-0.10 g/min) was found at 55% VO2 peak compared to cycling, where highest fat oxidation (0.67+/-0.20 g/min) was found at 75% VO2 peak.

CONCLUSION

Well-trained handbike cyclists have their highest fat oxidation at 55% VO2 peak(handbike cycling) compared to well-trained cyclists at 75% VO2 peak(cycling). Handbike cyclists should perform endurance exercise training at 55% VO2 peak(handbike cycling), whereas well-trained cyclists should be able to exercise at 75% VO2 peak(cycling). For training recommendations, the heart rate at 55% VO2 peak(handbike cycling) lies at 135+/-6 bpm in handbike cycling in SCI compared to 147+/-14 bpm at 75% VO2 peak(cycling) in well-trained cyclists. We presume that the reduced muscle mass involved in exercise during handbike cycling is the most important factor for impaired fat oxidation compared to cycling. But also other factors as fitness level and haemodynamic differences should be considered. Our results are only applicable to well-trained handbike cyclists with SCI and not for the general SCI population.

Effects of aerobic training on pyruvate dehydrogenase and pyruvate dehydrogenase kinase in human skeletal muscle

This study examined the effects of short- and long-term aerobic training on the stable up-regulation of pyruvate dehydrogenase (PDH) and PDH kinase (PDK) in human skeletal muscle. We hypothesized that 8 weeks, but not 1 week, of aerobic training would increase total PDH (PDHt) and PDK activities compared to pretraining, and this would be detectable at the level of gene transcription (mRNA) and/or gene translation (protein). Resting muscle biopsies were taken before and after 1 and 8 weeks of aerobic cycle exercise training. PDHt and PDK activities, and their respective protein and mRNA expression, did not differ after 1 week of aerobic training. PDHt activity increased 31% after 8 weeks and this may be partially due to a 1.3-fold increase in PDH-E(1)alpha protein expression. PDK activity approximately doubled after 8 weeks of aerobic training and this was attributed to a 1.3-fold increase in PDK2 isoform protein expression. Similar to 1 week, no changes were observed at the mRNA level after 8 weeks of training. These findings suggest that aerobically trained human skeletal muscle has an increased maximal capacity to utilize carbohydrates, evident by increased PDHt, but increased metabolic control sensitivity to pyruvate through increased contribution of PDK2 to total PDK activity.

Endurance training in humans leads to fiber type-specific increases in levels of peroxisome proliferator-activated receptor-gamma coactivator-1 and peroxisome proliferator-activated receptor-alpha in skeletal muscle

The peroxisome proliferator-activated receptor (PPAR)-gamma coactivator-1 (PGC-1) can induce mitochondria biogenesis and has been implicated in the development of oxidative type I muscle fibers. The PPAR isoforms alpha, beta/delta, and gamma control the transcription of genes involved in fatty acid and glucose metabolism. As endurance training increases skeletal muscle mitochondria and type I fiber content and fatty acid oxidative capacity, our aim was to determine whether these increases could be mediated by possible effects on PGC-1 or PPAR-alpha, -beta/delta, and -gamma. Seven healthy men performed 6 weeks of endurance training and the expression levels of PGC-1 and PPAR-alpha, -beta/delta, and -gamma mRNA as well as the fiber type distribution of the PGC-1 and PPAR-alpha proteins were measured in biopsies from their vastus lateralis muscle. PGC-1 and PPAR-alpha mRNA expression increased by 2.7- and 2.2-fold (P < 0.01), respectively, after endurance training. PGC-1 expression was 2.2- and 6-fold greater in the type IIa than in the type I and IIx fibers, respectively. It increased by 2.8-fold in the type IIa fibers and by 1.5-fold in both the type I and IIx fibers after endurance training (P < 0.015). PPAR-alpha was 1.9-fold greater in type I than in the II fibers and increased by 3.0-fold and 1.5-fold in these respective fibers after endurance training (P < 0.001). The increases in PGC-1 and PPAR-alpha levels reported in this study may play an important role in the changes in muscle mitochondria content, oxidative phenotype, and sensitivity to insulin known to be induced by endurance training.

Influence of prolonged endurance cycling and recovery diet on intramuscular triglyceride content in trained males

Intramuscular triglycerides (IMTG) are assumed to form an important substrate source during prolonged endurance exercise in trained males. This study investigated the effects of endurance exercise and recovery diet on IMTG content in vastus lateralis muscle. Nine male cyclists were provided with a standardized diet for 3 days, after which they performed a 3-h exercise trial at a 55% maximum workload. Before and immediately after exercise and after 24 and 48 h of recovery, magnetic resonance spectroscopy (MRS) was performed to quantitate IMTG content. Muscle biopsies were taken after 48 h of recovery to determine IMTG content by using quantitative fluorescence microscopy. The entire procedure was performed two times; in one trial, a normal diet containing 39% energy (En%) as fat was provided (NF) and in the other a typical carbohydrate-rich athlete's diet (LF: 24 En% fat) was provided. During exercise, IMTG content decreased by 21.4 +/- 3.1%. During recovery, IMTG content increased significantly in the NF trial only, reaching preexercise levels within 48 h. In accord with MRS, fluorescence microscopy showed significantly higher IMTG content in the NF compared with the LF trial, with differences restricted to the type I muscle fibers (2.1 +/- 0.2 vs. 1.4 +/- 0.2% area lipid staining, respectively). In conclusion, IMTG content in the vastus lateralis muscle declines significantly during prolonged endurance exercise in male cyclists. When a normal diet is used, IMTG contents are subsequently repleted within 48 h of postexercise recovery. In contrast, IMTG repletion is impaired substantially when a typical, carbohydrate-rich athlete's diet is used. Data obtained by quantitative fluorescence microscopy correspond well with MRS results, implying that both are valid methods to quantify IMTG content.

Intramyocellular lipids form an important substrate source during moderate intensity exercise in endurance-trained males in a fasted state

Both stable isotope methodology and fluorescence microscopy were applied to define the use of intramuscular triglyceride (IMTG) stores as a substrate source during exercise on a whole-body as well as on a fibre type-specific intramyocellular level in trained male cyclists. Following an overnight fast, eight subjects were studied at rest, during 120 min of moderate intensity exercise (60 % maximal oxygen uptake capacity (VO2,max)) and 120 min of post-exercise recovery. Continuous infusions of [U-13C]palmitate and [6,6-2H2]glucose were administered at rest and during subsequent exercise to quantify whole-body plasma free fatty acid (FFA) and glucose oxidation rates and the contribution of other fat sources (sum of muscle- plus lipoprotein-derived TG) and muscle glycogen to total energy expenditure. Fibre type-specific intramyocellular lipid content was determined in muscle biopsy samples collected before, immediately after and 2 h after exercise. At rest, fat oxidation provided 66 +/- 5 % of total energy expenditure, with FFA and other fat sources contributing 48 +/- 6 and 17 +/- 3 %, respectively. FFA oxidation rates increased during exercise, and correlated well with the change in plasma FFA concentrations. Both the use of other fat sources and muscle glycogen declined with the duration of exercise, whereas plasma glucose production and utilisation increased (P < 0.001). On average, FFA, other fat sources, plasma glucose and muscle glycogen contributed 28 +/- 3, 15 +/- 2, 12 +/- 1 and 45 +/- 4 % to total energy expenditure during exercise, respectively. Fluorescence microscopy revealed a 62 +/- 7 % net decline in muscle lipid content following exercise in the type I fibres only, with no subsequent change during recovery. We conclude that IMTG stores form an important substrate source during moderate intensity exercise in endurance-trained male athletes following an overnight fast, with the oxidation rate of muscle- plus lipoprotein-derived TG being decreased with the duration of exercise.

Luteal and follicular glucose fluxes during rest and exercise in 3-h postabsorptive women

We examined the effects of exercise intensity and menstrual cycle phase on glucose flux rates during rest and exercise in rested and fed (3-h postabsorptive) women. Eight moderately active, eumenorrheic women were studied under conditions of rest (90 min) and exercise (60 min, leg ergometer cycling at 45 and 65% peak oxygen consumption) during follicular and luteal phases. In both menstrual phases, an effect of exercise intensity was evident with glucose rates of appearance and disappearance and metabolic clearance rates: rest < 45% intensity < 65% intensity (P < 0.05). In addition, we observed no significant effect of menstrual phase on glucose rates of appearance and disappearance and metabolic clearance rate during rest or exercise at either intensity. These results are interpreted to mean that in women fed several hours before study 1) glucose flux is directly related to exercise intensity, 2) menstrual cycle phase does not alter glucose flux during rest and exercise, and 3) the subtle effects of endogenous ovarian hormones on glucose kinetics are subordinate to the much larger effects of exercise and recent carbohydrate nutrition.

Brief intense exercise followed by passive recovery modifies the pattern of fuel use in humans during subsequent sustained intermittent exercise

The role of work period duration as the principal factor influencing carbohydrate metabolism during intermittent exercise has been investigated. Fuel oxidation rates and muscle glycogen and free carnitine content were compared between two protocols of sustained intermittent intense exercise with identical treadmill speed and total work duration. In the first experiment subjects (n=6) completed 40 min of intermittent treadmill running involving a work : recovery cycle of 6 : 9 s or 24 : 36 s on separate days. With 24 : 36 s exercise a higher rate of carbohydrate oxidation approached significance (P=0.057), whilst fat oxidation rate was lower (P < or = 0.01) and plasma lactate concentration higher (P < or = 0.01). Muscle glycogen was lower post-exercise with 24 : 36 s (P < or = 0.05). Muscle free carnitine decreased (P < or = 0.05), but there was no difference between protocols. In the second experiment a separate group of subjects (n=5) repeated the intermittent exercise protocols with the addition of a 10-min bout of intense exercise, followed by 43 +/- 5 min passive recovery, prior to sustained (40 min) intermittent exercise. For this experiment the difference in fuel use observed previously between 6 : 9 s and 24 : 36 s was abolished. Carbohydrate and fat oxidation, plasma lactate and muscle glycogen levels were similar in 6 : 9 s and 24 : 36 s. When compared with the first experiment, this result was because of reduced carbohydrate oxidation in 24 : 36 s (P < or = 0.05). There was no difference, and no change, in muscle free carnitine between protocols. A 10-min bout of intense exercise, followed by 43 +/- 5 min of passive recovery, substantially modifies fuel use during subsequent intermittent intense exercise.

Determinants of the variability in respiratory exchange ratio at rest and during exercise in trained athletes

We examined the variability and determinants of the respiratory exchange ratio (RER) at rest and during exercise in 61 trained cyclists. Fasting (10-12 h) RER was measured at rest and during exercise at 25, 50, and 70% of peak power output (W(peak)), during which blood samples were drawn for [lactate] and [free fatty acid] ([FFA]). Before these measurements, training volume, dietary intake and muscle fiber composition, [substrate], and enzyme activities were determined. There was large interindividual variability in resting RER (0.718-0.927) that persisted during exercise of increasing intensity. The major determinants of resting RER included muscle glycogen content, training volume, proportion of type 1 fibers, [FFA] and [lactate], and %dietary fat intake (adjusted r(2) = 0.59, P < 0.001). Except for muscle fiber composition, these variables also predicted RER at 25, 50, and 70% W(peak) to different extents. The key determinant at 25% W(peak) was blood-borne [substrate], at 50% was muscle [substrate] and glycolytic enzyme activities, and at 70% was [lactate]. Resting RER was also a significant determinant of RER at 25 (r = 0.60) and 50% (r = 0.44) W(peak).

Non-invasive observation of acetyl-group buffering by 1H-MR spectroscopy in exercising human muscle

The observation of a previously unidentified peak in localized 1H magnetic resonance (MR) spectra of human muscle during and after a work load is reported. Basic NMR properties of this resonance, as well as physiologic circumstances of its observation, suggest that it is due to the acetyl group of acetylcarnitine. The relatively large pool of muscular carnitine acts as a buffering system stabilizing the ratio of acetylated to free coenzyme A. Free carnitine can be acetylated to a large extent whenever a mismatch occurs between the fluxes through pyruvate dehydrogenase and the TCA cycle. Results of initial applications of 1H MR spectroscopy in several muscles and under different exercise regimens are in agreement with earlier invasive measurements of acetylcarnitine. It is demonstrated that the detailed dynamics of acetyl group formation are now likely to be observable non-invasively in humans by localized 1H magnetic resonance spectroscopy on standard MR imaging systems, and that acetylcarnitine buffering as a function of exercise type, oxygenation states, diet and pathology could thus be studied repeatedly and in various muscle groups with much improved temporal resolution.

Endurance training increases fatty acid turnover, but not fat oxidation, in young men

We examined the effects of exercise intensity and a 10-wk cycle ergometer training program [5 days/wk, 1 h, 75% peak oxygen consumption (VO2 peak)] on plasma free fatty acid (FFA) flux, total fat oxidation, and whole body lipolysis in healthy male subjects (n = 10; age = 25.6 +/- 1.0 yr). Two pretraining trials (45 and 65% of VO2 peak) and two posttraining trials (same absolute workload, 65% of old VO2 peak; and same relative workload, 65% of new VO2 peak) were performed by using an infusion of [1-13C]palmitate and [1,1,2,3, 3-2H]glycerol. An additional nine subjects (age 25.4 +/- 0.8 yr) were treated similarly but were infused with [1,1,2,3,3-2H]glycerol and not [1-13C]palmitate. Subjects were studied postabsorptive for 90 min of rest and 1 h of cycling exercise. After training, subjects increased VO2 peak by 9.4 +/- 1.4%. Pretraining, plasma FFA kinetics were inversely related to exercise intensity with rates of appearance (Ra) and disappearance (Rd) being significantly higher at 45 than at 65% VO2 peak (Ra: 8.14 +/- 1.28 vs. 6.64 +/- 0.46, Rd: 8. 03 +/- 1.28 vs. 6.42 +/- 0.41 mol. kg-1. min-1) (P </= 0.05). After training, when measured at the same absolute and relative intensities, FFA Ra increased to 8.84 +/- 1.1, 8.44 +/- 1.1 and Rd to 8.82 +/- 1.1, 8.35 +/- 1.1 mol. kg-1. min-1, respectively (P </= 0.05). Total fat oxidation determined from respiratory exchange ratio was elevated during exercise compared with rest, but did not differ among the four conditions. Glycerol Ra was elevated during exercise compared with rest but did not demonstrate significant intensity or training effects during exercise. Thus, in young men, plasma FFA flux is increased during exercise after endurance training, but total fat oxidation and whole-body lipolysis are unaffected when measured at the same absolute or relative exercise intensities.

Effects of exercise intensity and training on lipid metabolism in young women

We examined the effects of exercise intensity and training [12 wk, 5 days/wk, 1 h, 75% peak oxygen consumption (VO2 peak)] on lipolysis and plasma free fatty acid (FFA) flux in women (n = 8; 24.3 +/- 1.6 yr). Two pretraining trials (45 and 65% of VO2 peak) and two posttraining trials [same absolute workload (65% of old VO2 peak; ABT) and same relative workload (65% of new VO2 peak; RLT)] were performed using infusions of [1,1,2,3,3-2H]glycerol and [1-13C]palmitate. Pretraining rates of FFA appearance (Ra), disappearance (Rd), and oxidation (Rox p) were similar between the 65% (6.8 +/- 0.6, 6.2 +/- 0.7, 3.1 +/- 0.3 micromol. kg-1. min-1, respectively) and the 45% of VO2 peak trials. At ABT and RLT training increased FFA Ra to 8.4 +/- 1.0 and 9.7 +/- 1.1 micromol. kg-1. min-1, Rd to 8.3 +/- 1.0 and 9.5 +/- 1.1 micromol. kg-1. min-1, and Rox p to 4.8 +/- 0.4 and 6.7 +/- 0.7 micromol. kg-1. min-1, respectively (P </= 0.05). Total FFA oxidation from respiratory exchange ratio was also elevated after training at ABT and RLT, with all of the increase attributed to plasma FFA sources. Pretraining, glycerol Ra was higher during exercise at 65 than 45% of VO2 peak (6. 9 +/- 0.9 vs. 4.7 +/- 0.6 micromol. kg-1. min-1) but was not changed by training. In young women 1) plasma FFA kinetics and oxidation are not linearly related to exercise intensity before training, 2) training increases FFA Ra, Rd, and Rox p whether measured at given absolute or relative exercise intensities, 3) whole body lipolysis (glycerol Ra) during exercise is not significantly impacted by training, and 4) training-induced increases in plasma FFA oxidation are the main contributor to elevated total FFA oxidation during exercise exertion after training.

Intramuscular mechanisms regulating fatty acid oxidation during exercise

Fatty acid oxidation increases in response to exercise, but at high intensities the contribution of fatty acid oxidation to energy production decreases. The carnitine palmitoyl transferase (CPT) complex appears to be a plausible site of regulation of fatty acid oxidation at rest and during exercise. Muscle malonyl-CoA, a potent inhibitor of CPT I, decreases during muscle contraction as a consequence of phosphorylation and inactivation of acetyl-CoA carboxylase (ACC) by AMP-activated protein kinase (AMPK). When AMPK is artificially activated with AICA riboside, ACC is inactivated, malonyl-CoA decreases, and fatty acid oxidation increases. Factors other than the muscle malonyl-CoA concentration are likely responsible for the decline in fatty acid oxidation during exercise at high intensities.

Mammalian fuel utilization during sustained exercise

The 'crossover' and 'lactate shuttle' concepts of substrate utilization in humans during exercise are extended to describe metabolic responses on other mammalian species. The 'crossover concept' is that lipid plays a predominant role in sustaining efforts requiring half or less aerobic capacity (VO2max); however, greater relative efforts depend increasingly on blood glucose and muscle glycogen as substrates. Thus, as exercise intensity increases from mild to moderate and hard, fuel selection switches (crosses over) from lipid to carbohydrate dependence. Glycogen and glucose catabolic rates are best described as exponential functions of exercise intensity, but with a greater gain in slope of the glycogen than glucose response. In contrast, plasma free fatty acid flux is described as an inverted hyperbola with vertex at approximately 50% VO2max. Both endocrine and intra-cellular factors play critical roles in determining substrate balance during sustained exercise. Moreover, genotypic adaptation for aerobic capacity as well as phenotypic adaptations to short- and long-term chronic activity affect the balance of substrate utilization during exercise. The concept of a 'lactate shuttle' is that during hard exercise, as well as other conditions of accelerated glycolysis, glycolytic flux in muscle involves lactate formation regardless of the state of oxygenation. Further, according to the lactate shuttle concept, lactate represents a major means of distributing carbohydrate potential energy for oxidation and gluconeogenesis. In humans and other mammals, the formation, distribution and disposal of lactate (not pyruvate) represent key steps in the regulation of intermediary metabolism during sustained exercise.

Importance of the 'crossover' concept in exercise metabolism

1. The 'crossover' concept is a model of substrate supply during exercise which makes the following predictions. 2. Lipid is the major fuel (approximately 60%) for non-contracting skeletal muscle and the body at rest. 3. Energy flux, as determined by exercise intensity, is the major factor in determining the balance of substrate utilization during exercise. Thus, moderate and greater exercise intensities increase contraction-induced muscle glycogenolysis and glycolysis, increase recruitment of fast-twitch muscle fibres, increase sympathetic nervous system activity and down-regulate mitochondrial fatty acid uptake. 4. Glycogen and glucose utilization scales exponentially to relative exercise power output with a greater gain in glycogen than in glucose use at high power. The relationship between free fatty acid flux and power output is an inverted hyperbola. Consequently, at high power outputs, the role of lipid oxidation is diminished. 5. Factors such as endurance training, energy supply, as influenced by dietary manipulation, and prior exercise play secondary roles in determining the balance of substrate utilization during exercise. 6. Comparisons of the metabolic responses in subjects engaged in activities requiring vastly different metabolic rates or comparisons of subjects of different gender, age or training status require normalization of data to total energy flux.

Training-induced alterations of glucose flux in men

We examined the hypothesis that glucose flux was directly related to relative exercise intensity both before and after a 10-wk cycle ergometer training program in 19 healthy male subjects. Two pretraining trials [45 and 65% of peak O2 consumption (VO2peak)] and two posttraining trials (same absolute and relative intensities as 65% pretraining) were performed for 90 min of rest and 1 h of cycling exercise. After training, subjects increased VO2peak by 9.4 +/- 1.4%. Pretraining, the intensity effect on glucose kinetics was evident with rates of appearance (R(a); 5.84 +/- 0.23 vs. 4.73 +/- 0.19 mg x kg(-1) x min(-1)), disappearance (R(d); 5.78 +/- 0.19 vs. 4.73 +/- 0.19 mg x kg(-1) x min(-1) x min(-1)), oxidation (R(ox); 5.36 +/- 0.15 vs. 3.41 +/- 0.23 mg x kg(-1) x min(-1)), and metabolic clearance (7.03 +/- 0.56 vs. 5.20 +/- 0.28 ml x kg(-1) x min(-1)) of glucose being significantly greater (P < or = 0.05) in the 65% than the 45% VO2peak trial. When R(d) was expressed as a percentage of total energy expended per minute (R(dE)), there was no difference between the 45 and 65% intensities. Training did reduce R(a) (4.63 +/- 0.25), R(d) (4.65 +/- 0.24), R(ox) (3.77 +/- 0.43), and R(dE) (15.30 +/- 0.40 to 12.85 +/- 0.81) when subjects were tested at the same absolute workload (P < or = 0.05). However, when they were tested at the same relative workload, R(a), R(d), and R(dE) were not different, although R(ox) was lower posttraining (5.36 +/- 0.15 vs. 4.41 +/- 0.42, P < or = 0.05). These results show 1) glucose use is directly related to exercise intensity; 2) training decreases glucose flux for a given power output; 3) when expressed as relative exercise intensity, training does not affect the magnitude of blood glucose use during exercise; 4) training alters the pathways of glucose disposal.