Metabolic flexibility in postmenopausal women: Hormone replacement therapy is associated with higher mitochondrial content, respiratory capacity, and lower total fat mass.

AIM: To investigate effects of hormone replacement therapy in postmenopausal women on factors associated with metabolic flexibility related to whole-body parameters including fat oxidation, resting energy expenditure, body composition and plasma concentrations of fatty acids, glucose, insulin, cortisol, and lipids, and for the mitochondrial level, including mitochondrial content, respiratory capacity, efficiency, and hydrogen peroxide emission. METHODS: 22 postmenopausal women were included. 11 were undergoing estradiol and progestin treatment (HT), and 11 were matched non-treated controls (CONT). Peak oxygen consumption, maximal fat oxidation, glycated hemoglobin, body composition, and resting energy expenditure were measured. Blood samples were collected at rest and during 45 min of ergometer exercise (65% VO2peak). Muscle biopsies were obtained at rest and immediately post-exercise. Mitochondrial respiratory capacity, efficiency, and hydrogen peroxide emission in permeabilized fibers and isolated mitochondria were measured, and citrate synthase (CS) and 3-hydroxyacyl-CoA dehydrogenase (HAD) activity were assessed. RESULTS: HT showed higher absolute mitochondrial respiratory capacity and post-exercise hydrogen peroxide emission in permeabilized fibers and higher CS and HAD activities. All respiration normalized to CS activity showed no significant group differences in permeabilized fibers or isolated mitochondria. There were no differences in resting energy expenditure, maximal, and resting fat oxidation or plasma markers. HT had significantly lower visceral and total fat mass compared to CONT. CONCLUSION: Use of hormone therapy is associated with higher mitochondrial content and respiratory capacity and a lower visceral and total fat mass. Resting energy expenditure and fat oxidation did not differ between HT and CONT.

The influence of age, sex and cardiorespiratory fitness on maximal fat oxidation rate.

Fat oxidation decreases with age, yet no studies have previously investigated if aging affects the maximal fat oxidation rate (MFO) during exercise in men and women differently. We hypothesized that increased age would be associated with a decline in MFO and this would be more pronounced in women due to menopause, compared with men. In this cross-sectional study design, 435 (247/188, male/female) subjects of varying ages performed a DXA scan, a submaximal graded exercise test and a maximal oxygen uptake test, to measure MFO and cardiorespiratory fitness (CRF) by indirect calorimetry. Subjects were stratified into 12 groups according to sex (male/female), age (<45, 45-55 and >55 years), CRF (below average and above average). Women aged <45 years had a higher MFO relative to fat free mass (FFM) (mg/min/kg) compared with men, regardless of CRF. However, there were no differences in MFO (mg/min/kg FFM) between men and women, in the groups aged between 45-55 and >55 years. In summary, we found that women aged <45 years display a higher MFO (mg/min/kg FFM) compared with men and that this sexual divergence is abolished after the age of 45 years. Novelty: Maximal fat oxidation rate is higher in young women compared with men. This sex-related difference is attenuated after the age of 45 years. Cardiorespiratory fitness does not influence this sex-related difference.

Cardiovascular, muscular, and skeletal adaptations to recreational team handball training: a randomized controlled trial with young adult untrained men.

PURPOSE: The prevalence of lifestyle diseases has escalated, and effective exercise training programmes are warranted. This study tested the hypothesis that regular participation in small-sided team handball training could provide beneficial health effects on cardiovascular, skeletal, and muscular parameters in young adult untrained men. METHOD: Twenty-six untrained 20-30-year-old men were randomly allocated to either a team handball training group (HG; n = 14), which completed 1.9 ± 0.3 training sessions per week over 12 weeks, or an inactive control group (CG; n = 12). Physiological training adaptations were assessed pre- and post interventions by DXA scans, blood samples, muscle biopsies, and physical tests. RESULTS: The average heart rate during training was equivalent to 84 ± 4% of maximal heart rate. Compared to CG, HG displayed significant increases in VO2max (11 ± 6%), proximal femur bone mineral density (2 ± 1%), whole-body bone mineral content (2 ± 1%), intermittent endurance performance (32 ± 16%), incremental treadmill test performance (16 ± 7%) and muscle citrate synthase activity (22 ± 28%) as well as decreases in total fat mass (7 ± 7%) and total fat percentage (6 ± 7%) (all p < 0.05). There were no significant changes in muscle mass, blood pressure, resting heart rate, muscle hydroxyl-acyl-dehydrogenase activity, or blood lipids (all p > 0.05). CONCLUSION: Participation in regular recreational team handball training was associated with positive cardiovascular, skeletal, and muscular adaptations, including increased maximal oxygen uptake, increased muscle enzymatic activity, and improved bone mineralization as well as lower fat percentage. These findings suggest that recreational team handball training may be an effective health-promoting activity for young adult men.

Correction to: Cardiovascular, muscular, and skeletal adaptations to recreational team handball training: a randomized controlled trial with young adult untrained men.

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Football training over 5 years is associated with preserved femoral bone mineral density in men with prostate cancer.

This study investigated the association between long-term adherence to football training and retaining bone mineralization and physical capacity in men with prostate cancer (PCa) managed with androgen deprivation therapy (ADT). Patients completing follow-up at 32 weeks in the FC Prostate Randomized Controlled Trial (RCT) in 2012 or 2013 were invited to 5-year follow-up assessments in May 2017 (n = 30). Changes in physiological outcomes over time between the football participants (FTG) and nonparticipants (CON) were examined. Twenty-two men accepted the invitation of which 11, aged 71.3 ± 3.8 years, had continued to play self-organized football 1.7 (SD 0.5) times per week for 4½ years (±8 months). At 5 years, right femoral neck bone mineral density (BMD) had improved significantly in the FTG compared to CON (P = .028). No other significant between-group differences were observed. In FTG, RHR decreased by 4.3 bpm (P = .009) with no changes in CON. Muscle mass, knee-extensor muscle strength, VO2 max, and postural balance decreased in both groups. In FTG, the fraction of training time with HR between 80%-90% or >90% of HRmax was 29.9% (SD 20.6) and 22.8% (SD 28.7), respectively. Average distance covered during 3 × 20 minutes of football training was 2524 m (SD 525). Football training over a 5-year period was associated with preserved femoral neck BMD in elderly men with PCa managed on ADT. Intensity during football training was >80% of HRmax for 51% of training time after 5 years. Body composition and physical capacity deteriorated over 5 years regardless of football participation.

Obesity leads to impairments in the morphology and organization of human skeletal muscle lipid droplets and mitochondrial networks, which are resolved with gastric bypass surgery-induced improvements in insulin sensitivity.

AIMS: Skeletal muscle lipid stores and mitochondrial function have been appointed as key players in obesity-induced insulin resistance. However, there are conflicting reports in the literature based on in vitro quantitative measurements. Here, we test the hypothesis that it is not the quantity but the quality that matters. METHODS: This study combines quantitative and qualitative structural measurements of lipid stores and mitochondrial dynamics in skeletal muscle from lean subjects, and subjects with morbid obesity, with and without type 2 diabetes, before and after gastric bypass surgery. RESULTS: The structural organization of muscle mitochondrial networks in type II muscle fibres from subjects with morbid obesity is impaired. In addition, the amount of skeletal muscle perilipin 2 protein per intramyocellular lipid is reduced in subjects with morbid obesity, resulting in qualitative alterations in perilipin 2 coat around some lipid droplets. Gastric bypass surgery-induced weight loss and insulin resistance remission were associated with decreases in intramyocellular lipid stores and, qualitative improvements in lipid droplets' morphology, perilipin 2 coat and mitochondrial dynamics. CONCLUSION: Morbid obesity leads to severe qualitative alterations of both skeletal muscle lipid stores and mitochondrial networks. The degree of structural improvements after gastric bypass surgery was proportional to the improvements in whole body insulin sensitivity, suggesting an association between these events.

Correlates and predictors of obesity-specific quality of life of former participants of a residential intensive lifestyle intervention.

Introduction: The aim of this study was to investigate the relationship between weight loss during and after a unique type of weight loss intervention, namely, a residential intensive lifestyle intervention (ILI), and participants' obesity-specific health-related quality of life (HRQOL) several years after the intervention. In the residential ILI under investigation, participants attended a 10- to 12-week long course away from their daily living environment, namely, at Ubberup Folk High School located in Denmark. Methods: A total of 79 former participants (31 male, mean age 36.6; SD = 12.7 years) who had participated in the intervention on average 5.3 (SD = 3.2) years ago were recruited for this study. They completed a questionnaire on weight-related quality of life (IWQOL-lite) and physical activity, as well as measurements of VO2max, blood pressure, Homeostatic Model Assessment for Insulin Resistance, waist circumference and hand grip strength. Results: The study results showed that weight change after the end of the intervention could predict HRQOL whereas how much weight they lost during the intervention could not. Furthermore, almost all of the investigated physiological factors were related to participants' current HRQOL. Waist circumference showed relationships with four of the five aspects of HRQOL. Conclusion: Focusing on behavioural change, adhering to improved lifestyle and maintaining weight loss after the end of the intervention seem to be the key not only for cardio-metabolic risk factors but also for sustainable HRQOL.

Mitochondrial respiratory capacity remains stable despite a comprehensive and sustained increase in insulin sensitivity in obese patients undergoing gastric bypass surgery.

AIM: It has been proposed, but not yet demonstrated by convincing evidence in published articles, that insulin resistance and mitochondrial respiratory function are causally related physiological phenomena. Here, we tested the prediction that weight loss-induced increase in insulin sensitivity will correlate with a corresponding change in mitochondrial respiratory capacity over the same time period. METHODS: Insulin sensitivity was evaluated using the hyperinsulinaemic-euglycaemic clamp technique, and skeletal muscle mitochondrial respiratory capacity was evaluated by high-resolution respirometry in 26 patients with obesity. Each experiment was performed ~2 months and 1-2 weeks before, and ~4 and ~19 months after Roux-en-Y gastric bypass (RYGB) surgery. RESULTS: A substantial weight loss was observed in all patients, and insulin sensitivity increased in all patients over the 21-months time period of the study. In contrast, skeletal muscle mitochondrial respiratory capacity, intrinsic mitochondrial respiratory capacity and mitochondrial content remained unchanged over the same time period. CONCLUSION: Among obese patients with and without type 2 diabetes undergoing RYGB surgery, intrinsic mitochondrial respiratory capacity in skeletal muscle is not correlated with insulin sensitivity before or after the surgical intervention. Mitochondrial respiratory function may not be germane to the pathophysiology and/or aetiology of obesity and/or type 2 diabetes.

Effects of 6-month aerobic interval training on skeletal muscle metabolism in middle-aged metabolic syndrome patients.

Aerobic interval training (AIT) improves the health of metabolic syndrome patients (MetS) more than moderate intensity continuous training. However, AIT has not been shown to reverse all metabolic syndrome risk factors, possibly due to the limited duration of the training programs. Thus, we assessed the effects of 6 months of AIT on cardio-metabolic health and muscle metabolism in middle-aged MetS. Eleven MetS (54.5±0.7 years old) underwent 6 months of 3 days a week supervised AIT program on a cycle ergometer. Cardio-metabolic health was assessed, and muscle biopsies were collected from the vastus lateralis prior and at the end of the program. Body fat mass (-3.8%), waist circumference (-1.8%), systolic (-10.1%), and diastolic (-9.3%) blood pressure were reduced, whereas maximal fat oxidation rate and VO2peak were significantly increased (38.9% and 8.0%, respectively; all P<.05). The remaining components of cardio-metabolic health measured (body weight, blood cholesterol, triglycerides, and glucose) were not changed after the intervention, and likewise, insulin sensitivity (CSi) remained unchanged. Total AMPK (23.4%), GLUT4 (20.5%), endothelial lipase (33.3%) protein expression, and citrate synthase activity (26.0%) increased with training (P<.05). Six months of AIT in MetS raises capacity for fat oxidation during exercise and increases VO2peak in combination with skeletal muscle improvements in mitochondrial enzyme activity. Muscle proteins involved in glucose, fat metabolism, and energy cell balance improved, although this was not reflected by parallel improvements in insulin sensitivity or blood lipid profile.

High-intensity interval training improves insulin sensitivity in older individuals.

AIM: Metabolic health may deteriorate with age as a result of altered body composition and decreased physical activity. Endurance exercise is known to counter these changes delaying or even preventing onset of metabolic diseases. High-intensity interval training (HIIT) is a time efficient alternative to regular endurance exercise, and the aim of this study was to investigate the metabolic benefit of HIIT in older subjects. METHODS: Twenty-two sedentary male (n = 11) and female (n = 11) subjects aged 63 ± 1 years performed HIIT training three times/week for 6 weeks on a bicycle ergometer. Each HIIT session consisted of five 1-minute intervals interspersed with 1½-minute rest. Prior to the first and after the last HIIT session whole-body insulin sensitivity, measured by a hyperinsulinaemic-euglycaemic clamp, plasma lipid levels, HbA1c, glycaemic parameters, body composition and maximal oxygen uptake were assessed. Muscle biopsies were obtained wherefrom content of glycogen and proteins involved in muscle glucose handling were determined. RESULTS: Insulin sensitivity (P = .011) and maximal oxygen uptake increased (P < .05) in both genders, while plasma cholesterol (P < .05), low-density lipoprotein (P < .05), visceral fat mass (P < .05) and per cent body fat (P < .05) decreased after 6 weeks of HIIT. HbA1c decreased only in males (P = .001). Muscle glycogen content increased in both genders (P = .001) and in line GLUT4 (P < .05), glycogen synthase (P = .001) and hexokinase II (P < .05) content all increased. CONCLUSION: Six weeks of HIIT significantly improves metabolic health in older males and females by reducing age-related risk factors for cardiometabolic disease.

Leydig cell dysfunction, systemic inflammation and metabolic syndrome in long-term testicular cancer survivors.

BACKGROUND: Twenty to thirty percent of testicular cancer (TC) survivors have elevated serum levels of luteinising hormone (LH) with or without corresponding low testosterone levels (Leydig cell dysfunction) during clinical follow-up for TC. However, it remains to be clarified if this subgroup of TC survivors has an increased long-term risk of systemic inflammation and metabolic syndrome (MetS) when compared with TC survivors with normal Leydig cell function during follow-up. PATIENTS AND METHODS: TC survivors with Leydig cell dysfunction and a control group of TC survivors with normal Leydig cell function during follow-up were eligible for participation in the study. Markers of systemic inflammation and prevalence of MetS were compared between TC survivors with Leydig cell dysfunction and the control group. RESULTS: Of 158 included TC survivors, 28 (18%) had uncompensated Leydig cell dysfunction, 59 (37%) had compensated Leydig cell dysfunction and 71 (45%) had normal Leydig cell function during follow-up. MetS and markers of systemic inflammation were evaluated at a median follow-up of 9.7 years (interquartile range 4.1-17.1) after TC treatment. The prevalence of MetS was significantly lower among patients with compensated Leydig cell dysfunction during follow-up (12% versus 27%, p = 0.04), whereas there was no difference between TC survivors with uncompensated Leydig cell dysfunction and controls (33% versus 27%, p = 0.5). Apart from high-sensitivity C-reactive protein which was higher in TC survivors with uncompensated Leydig cell dysfunction during follow-up, there was no evidence of increased systemic inflammation in patients with Leydig cell dysfunction during clinical follow-up. Total testosterone at follow-up was significantly associated with MetS, whereas there was no association between LH and MetS. CONCLUSION: We did not find evidence that TC survivors with Leydig cell dysfunction during clinical follow-up had increased long-term risk of MetS. Total testosterone at follow-up was significantly associated with MetS. The study is registered at www.clinicaltrials.govNCT02240966.

Reproductive hormones and metabolic syndrome in 24 testicular cancer survivors and their biological brothers.

Testicular cancer survivors have impaired gonadal function and increased risk of metabolic syndrome when compared to healthy controls. However, because of the fetal etiology of testicular cancer, familial unrelated healthy men might not be an optimal control group. The objective of this study was to clarify if testicular cancer survivors have impaired gonadal function and increased risk of metabolic syndrome when compared to their biological brothers. A cross-sectional study of testicular cancer survivors (ClinicalTrials.gov number, NCT02240966) was conducted between 2014 and 2016. Of 158 testicular cancer survivors included, 24 had a biological brother who accepted to participate in the study. Serum levels of reproductive hormones and prevalence of metabolic syndrome according to International Diabetes Federation Criteria and National Cholesterol Education Program (Adult Treatment Panel III) criteria comprised the main outcome measures of the study. Median age was similar in testicular cancer survivors and their biological brothers [44 years (IQR 39-50) vs. 46 (40-53) years respectively (p = 0.1)]. In testicular cancer survivors, follow-up since treatment was 12 years (7-19). Serum levels of luteinizing hormone and follicle-stimulating hormone were elevated (p ≤ 0.001), while total testosterone, free testosterone, inhibin B and anti-Müllerian hormone were lower (p ≤ 0.001) in testicular cancer survivors than in their biological brothers. The prevalence of metabolic syndrome was similar and apart from HDL-cholesterol, which was lower in testicular cancer survivors (p = 0.01); there were no differences in the individual components of the metabolic syndrome between testicular cancer survivors and their brothers. In conclusion, gonadal function was impaired in testicular cancer survivors, while we did not detect any difference in the prevalence of metabolic syndrome between testicular cancer survivors and their biological brothers.

The effect of age and unilateral leg immobilization for 2 weeks on substrate utilization during moderate-intensity exercise in human skeletal muscle.

KEY POINTS: This study aimed to provide molecular insight into the differential effects of age and physical inactivity on the regulation of substrate metabolism during moderate-intensity exercise. Using the arteriovenous balance technique, we studied the effect of immobilization of one leg for 2 weeks on leg substrate utilization in young and older men during two-legged dynamic knee-extensor moderate-intensity exercise, as well as changes in key proteins in muscle metabolism before and after exercise. Age and immobilization did not affect relative carbohydrate and fat utilization during exercise, but the older men had higher uptake of exogenous fatty acids, whereas the young men relied more on endogenous fatty acids during exercise. Using a combined whole-leg and molecular approach, we provide evidence that both age and physical inactivity result in intramuscular lipid accumulation, but this occurs only in part through the same mechanisms. ABSTRACT: Age and inactivity have been associated with intramuscular triglyceride (IMTG) accumulation. Here, we attempt to disentangle these factors by studying the effect of 2 weeks of unilateral leg immobilization on substrate utilization across the legs during moderate-intensity exercise in young (n = 17; 23 ± 1 years old) and older men (n = 15; 68 ± 1 years old), while the contralateral leg served as the control. After immobilization, the participants performed two-legged isolated knee-extensor exercise at 20 ± 1 W (∼50% maximal work capacity) for 45 min with catheters inserted in the brachial artery and both femoral veins. Biopsy samples obtained from vastus lateralis muscles of both legs before and after exercise were used for analysis of substrates, protein content and enzyme activities. During exercise, leg substrate utilization (respiratory quotient) did not differ between groups or legs. Leg fatty acid uptake was greater in older than in young men, and although young men demonstrated net leg glycerol release during exercise, older men showed net glycerol uptake. At baseline, IMTG, muscle pyruvate dehydrogenase complex activity and the protein content of adipose triglyceride lipase, acetyl-CoA carboxylase 2 and AMP-activated protein kinase (AMPK)γ3 were higher in young than in older men. Furthermore, adipose triglyceride lipase, plasma membrane-associated fatty acid binding protein and AMPKγ3 subunit protein contents were lower and IMTG was higher in the immobilized than the contralateral leg in young and older men. Thus, immobilization and age did not affect substrate choice (respiratory quotient) during moderate exercise, but the whole-leg and molecular differences in fatty acid mobilization could explain the age- and immobilization-induced IMTG accumulation.

Higher muscle content of perilipin 5 and endothelial lipase protein in trained than untrained middle-aged men.

A high VO(2)max in middle-age is related to high metabolic flexibility and lowered risk of metabolic diseases. However, the influence of a high VO(2)max induced by years of regular training in middle-age on protein expression related to muscle metabolism is not well studied. This study measures key proteins involved in mitochondrial oxidation, glucose and lipid metabolism in skeletal muscle of trained and untrained middle-aged men. 16 middle-aged men, matched for lean body mass, were recruited into an endurance trained (TR, n=8) or an untrained (CON, n=8) group based on their VO(2)max. A muscle biopsy was obtained from m. vastus lateralis and protein levels were analyzed by Western blotting. The TR had higher protein levels of mitochondrial complex III-V, endothelial lipase (EL) and perilipin 5 compared to the CON. Glycogen synthase (P=0.05), perilipin 3 (P=0.09) and ATGL (P=0.09) tended to be higher in TR than CON, but there was no difference in AKT I/II, HKII, GLUT4 and LPL protein expression. Lastly, there was a positive correlation between plasma HDL and EL (R(2)=0.53, P<0.01). In conclusion, a high VO(2)max in middle-aged men was as expected is reflected in higher muscle oxidative capacity, but also in higher endothelial lipase and perilipin 5 expression and a borderline higher glycogen synthase protein expression, which may contribute to a higher metabolic flexibility.

Cardiorespiratory fitness in 16 025 adults aged 18-91 years and associations with physical activity and sitting time.

Our aim was to provide up-to-date cardiorespiratory fitness reference data for adults of all ages and to investigate associations between cardiores-piratory fitness and leisure time physical activity as well as sitting time. In the Danish Health Examination Survey 2007-2008, cardiorespiratory fitness was estimated in 16 025 individuals aged 18-91 years from validated cycle ergometer exercise tests. Level of leisure time physical activity (sedentary, light, moderate, and vigorous) and daily sitting time in hours was obtained from a self-administered questionnaire. Men had 20-33% higher cardiorespiratory fitness than women, depending on age, and cardiorespiratory fitness decreased by 0.26 and 0.23 mL/min/kg per year in men and women, respectively. Cardiorespiratory fitness was higher among participants who reported a high level of physical activity in leisure time compared with participants who were sedentary. Among sedentary or lightly physically active participants, inverse associations between total daily sitting time and cardiorespiratory fitness were found, while there was no association between sitting time and cardiorespiratory fitness among moderately or vigorously physically active participants. These data on cardiorespiratory fitness can serve as useful reference material. Although reluctant to conclude on causality, sitting time might impact cardiorespiratory fitness among individuals with low levels of leisure time physical activity.

Increased post-operative cardiopulmonary fitness in gastric bypass patients is explained by weight loss.

Roux-en-Y gastric bypass (RYGB) leads to a major weight loss in obese patients. However, given that most patients remain obese after the weight loss, regular exercise should be part of a healthier lifestyle. The primary aim of this study was to investigate the cardiopulmonary fitness in obese patients before and after RYGB. Thirty-four patients had body composition and cardiopulmonary fitness (VO2max ) assessed and completed questionnaires regarding physical activity and function twice before RYGB (time points A and B) and 4 and 18 months after surgery (time points C and D). Weight loss was 37 ± 2 kg during the study period. VO2max increased (A: 21 ± 1 vs D: 29 mL/min/kg, P < 0.001), but absolute VO2max decreased (A: 2713 ± 126 vs 2609 ± 187 mL/min, P = 0.02) and VO2max per kilogram fat free mass did not change. Self-perceived limitations to perform exercise decreased and self-perceived physical fitness increased after RYGB. Self-reported low- and high-intensity physical activity did not change. With weight loss, self-rated fitness level increased and the limitations to perform exercise decreased in RYGB patients. Nevertheless, as shown by the lower absolute VO2max , RYGB patients do not adopt new exercise habits following surgery.

Mitochondrial coupling and capacity of oxidative phosphorylation in skeletal muscle of Inuit and Caucasians in the arctic winter.

During evolution, mitochondrial DNA haplogroups of arctic populations may have been selected for lower coupling of mitochondrial respiration to ATP production in favor of higher heat production. We show that mitochondrial coupling in skeletal muscle of traditional and westernized Inuit habituating northern Greenland is identical to Danes of western Europe haplogroups. Biochemical coupling efficiency was preserved across variations in diet, muscle fiber type, and uncoupling protein-3 content. Mitochondrial phenotype displayed plasticity in relation to lifestyle and environment. Untrained Inuit and Danes had identical capacities to oxidize fat substrate in arm muscle, which increased in Danes during the 42 days of acclimation to exercise, approaching the higher level of the Inuit hunters. A common pattern emerges of mitochondrial acclimatization and evolutionary adaptation in humans at high latitude and high altitude where economy of locomotion may be optimized by preservation of biochemical coupling efficiency at modest mitochondrial density, when submaximum performance is uncoupled from VO2max and maximum capacities of oxidative phosphorylation.

Maintained peak leg and pulmonary VO2 despite substantial reduction in muscle mitochondrial capacity.

We recently reported the circulatory and muscle oxidative capacities of the arm after prolonged low-intensity skiing in the arctic (Boushel et al., 2014). In the present study, leg VO2 was measured by the Fick method during leg cycling while muscle mitochondrial capacity was examined on a biopsy of the vastus lateralis in healthy volunteers (7 male, 2 female) before and after 42 days of skiing at 60% HR max. Peak pulmonary VO2 (3.52 ± 0.18 L.min(-1) pre vs 3.52 ± 0.19 post) and VO2 across the leg (2.8 ± 0.4L.min(-1) pre vs 3.0 ± 0.2 post) were unchanged after the ski journey. Peak leg O2 delivery (3.6 ± 0.2 L.min(-1) pre vs 3.8 ± 0.4 post), O2 extraction (82 ± 1% pre vs 83 ± 1 post), and muscle capillaries per mm(2) (576 ± 17 pre vs 612 ± 28 post) were also unchanged; however, leg muscle mitochondrial OXPHOS capacity was reduced (90 ± 3 pmol.sec(-1) .mg(-1) pre vs 70 ± 2 post, P < 0.05) as was citrate synthase activity (40 ± 3 µmol.min(-1) .g(-1) pre vs 34 ± 3 vs P < 0.05). These findings indicate that peak muscle VO2 can be sustained with a substantial reduction in mitochondrial OXPHOS capacity. This is achieved at a similar O2 delivery and a higher relative ADP-stimulated mitochondrial respiration at a higher mitochondrial p50. These findings support the concept that muscle mitochondrial respiration is submaximal at VO2max , and that mitochondrial volume can be downregulated by chronic energy demand.