Fuel kinetics during intense running and cycling when fed carbohydrate

Recovery after Running an "Everesting" Mountain Ultramarathon

Blood markers of muscle microdamage and systemic inflammation do not adequately explain the reduced performance observed over a prolonged recovery after running a mountain ultramarathon. This case study aimed to determine whether the reduced performance after the Everesting mountain ultramarathon can be further assessed by considering cardiorespiratory and metabolic alterations determined via repeated incremental and continuous running tests. A single runner (age: 24 years, BM: 70 kg, BMI: 22, Vo2peak: 74 mL∙min-1∙kg-1) was observed over a preparatory period of two months with a one-month recovery period. The Everesting consisted of nine ascents and descents of 9349 vertical metres completed in 18:22 (h:min). During the first phase of the recovery, enhanced peak creatine kinase (800%) and C-reactive protein (44%) levels explained the decreased performance. In contrast, decreased performance during the second, longer phase was associated with a decreased lactate threshold and Vo2 (21% and 17%, respectively), as well as an increased energetic cost of running (15%) and higher endogenous carbohydrate oxidation rates (87%), lactate concentrations (170%) and respiratory muscle fatigue sensations that remained elevated for up to one month. These alterations may represent characteristics that can explain the second phase of the recovery process after Everesting.

Melatonin Ingestion Prevents Liver Damage and Improves Biomarkers of Renal Function Following a Maximal Exercise

Background: While the promotion of the beneficial effects of melatonin (MEL) ingestion on the modulation of oxidative stress is widespread, less attention is given to the biological influence that it could exert on the results of hematology and clinical chemistry parameters. This study was undertaken to assess the effects of acute MEL ingestion on these parameters during a maximal running exercise. Methods: In double blind randomized design, 12 professional soccer players [age: 17.54 ± 0.78 yrs, body mass: 70.31 ± 3.86 kg, body height: 1.8 ± 0.08 m; maximal aerobic speed (MAS): 16.85 ± 0.63 km/h; mean ± standard deviation], all males, performed a diurnal (17:00 h ± 30 h) running exercise test (RET) at 100% of their MAS following either MEL or placebo ingestion. Blood samples were obtained at rest and following the RET. Results: Compared to placebo, MEL intake decreased post-exercise biomarkers of liver damage (aspartate aminotransferase, p<0.001; alanine aminotransferase, p<0.001; gamma-glutamyltransferase; p<0.05) and improved post-exercise renal function markers (i.e., creatinine, p<0.001). However, lipid profile, glucose, lactate and leukocyte were not affected by MEL ingestion. Regarding the time to exhaustion, no difference was found between MEL (362.46 ± 42.06 s) and PLA (374.54 ± 57.97 s) conditions. Conclusion: The results of this investigation clearly attest that MEL ingestion before a maximal running exercise might protect athletes from liver damage and perturbation in renal function biomarkers. However, this study comprises an acute MEL supplementation and no assessment on chronic effects or circadian rhythm the day before was done.

Effects of melatonin ingestion on physical performance and biochemical responses following exhaustive running exercise in soccer players

Antioxidant supplementation has become a common practice among athletes to boost sport achievement. Likewise, melatonin (MEL) has been ingested as an ergogenic aid to improve physical performance. To date, no study has checked whether the multiple beneficial effects of MEL have an outcome during a maximum running exercise until exhaustion. Therefore, the present study aimed to evaluate the effect of MEL ingestion on physical performance and biochemical responses (i.e., oxidative stress) during exhaustive exercise. In a double blind randomized study, thirteen professional soccer players [age: 17.5 ± 0.8 years, body mass: 70.3 ± 3.9 kg, body height: 1.80 ± 0.08 m; maximal aerobic speed (MAS): 16.85 ± 0.63 km/h; mean ± standard deviation], members of a first league squad, performed a running exercise until exhaustion at 100% of MAS, after either MEL or placebo ingestion. Physical performance was assessed, and blood samples were obtained at rest and following the exercise. Compared to placebo, MEL intake prevented the increase in oxidative stress markers (i.e., malondialdehyde), alleviated the alteration of antioxidant status (i.e., glutathione peroxidase, uric acid and total bilirubin) and decreased post-exercise biomarkers of muscle damage (i.e., creatine kinase and lactate dehydrogenase) (p < 0.05). However, physical performance was not affected by MEL ingestion (p > 0.05). In conclusion, acute MEL intake before a maximal running exercise protected athletes from oxidative stress and cellular damage but without an effect on physical performance.

The Effect of a Single Bout of Continuous Aerobic Exercise on Glucose, Insulin and Glucagon Concentrations Compared to Resting Conditions in Healthy Adults: A Systematic Review, Meta-Analysis and Meta-Regression

Background

Elevated glucose and insulin levels are major risk factors in the development of cardiometabolic disease. Aerobic exercise is widely recommended to improve glycaemic control, yet its acute effect on glycaemia and glucoregulatory hormones has not been systematically reviewed and analysed in healthy adults.

Objective

To determine the effect of a single bout of continuous aerobic exercise on circulating glucose, insulin, and glucagon concentrations in healthy adults.

Methods

CENTRAL, CINAHL, Embase, Global Health, HMIC, Medline, PubMed, PsycINFO, ScienceDirect, Scopus and Web of Science databases were searched from inception to May 2020. Papers were included if they reported a randomised, crossover study measuring glucose and/or insulin and/or glucagon concentrations before and immediately after a single bout of continuous aerobic exercise (≥ 30 min) compared to a time-matched, resting control arm in healthy adults. The risk of bias and quality of evidence were assessed using the Cochrane Risk of Bias Tool and GRADE approach, respectively. Random-effects meta-analyses were performed for glucose, insulin, and glucagon. Sub-group meta-analyses and meta-regression were performed for categorical (metabolic state [postprandial or fasted], exercise mode [cycle ergometer or treadmill]) and continuous (age, body mass index, % males, maximal aerobic capacity, exercise duration, exercise intensity) covariates, respectively.

Results

42 papers (51 studies) were considered eligible: glucose (45 studies, 391 participants), insulin (38 studies, 377 participants) and glucagon (5 studies, 47 participants). Acute aerobic exercise had no significant effect on glucose concentrations (mean difference: − 0.05 mmol/L; 95% CI, − 0.22 to 0.13 mmol/L; P = 0.589; I²: 91.08%, large heterogeneity; moderate-quality evidence). Acute aerobic exercise significantly decreased insulin concentrations (mean difference: − 18.07 pmol/L; 95% CI, − 30.47 to − 5.66 pmol/L; P = 0.004; I²: 95.39%, large heterogeneity; moderate-quality evidence) and significantly increased glucagon concentrations (mean difference: 24.60 ng/L; 95% CI, 16.25 to 32.95 ng/L; P < 0.001; I²: 79.36%, large heterogeneity; moderate-quality evidence). Sub-group meta-analyses identified that metabolic state modified glucose and insulin responses, in which aerobic exercise significantly decreased glucose (mean difference: − 0.27 mmol/L; 95% CI, − 0.55 to − 0.00 mmol/L; P = 0.049; I²: 89.72%, large heterogeneity) and insulin (mean difference: − 42.63 pmol/L; 95% CI, − 66.18 to − 19.09 pmol/L; P < 0.001; I²: 81.29%, large heterogeneity) concentrations in the postprandial but not fasted state. Meta-regression revealed that the glucose concentrations were also moderated by exercise duration and maximal aerobic capacity.

Conclusions

Acute aerobic exercise performed in the postprandial state decreases glucose and insulin concentrations in healthy adults. Acute aerobic exercise also increases glucagon concentrations irrespective of metabolic state. Therefore, aerobic exercise undertaken in the postprandial state is an effective strategy to improve acute glycaemic control in healthy adults, supporting the role of aerobic exercise in reducing cardiometabolic disease incidence.

PROSPERO registration number

CRD42020191345.

Supplementary Information

The online version contains supplementary material available at 10.1007/s40279-021-01473-2.

Time-Restricted Eating as a Nutrition Strategy for Individuals with Type 2 Diabetes: A Feasibility Study

Individuals with type 2 diabetes (T2D) require a long-term dietary strategy for blood glucose management and may benefit from time-restricted eating (TRE, where the duration between the first and last energy intake is restricted to 8–10 h/day). We aimed to determine the feasibility of TRE for individuals with T2D. Participants with T2D (HbA1c >6.5 to <9%, eating window >12 h/day) were recruited to a pre-post, non-randomised intervention consisting of a 2-week Habitual period to establish baseline dietary intake, followed by a 4-weeks TRE intervention during which they were instructed to limit all eating occasions to between 10:00 and 19:00 h on as many days of each week as possible. Recruitment, retention, acceptability, and safety were recorded throughout the study as indicators of feasibility. Dietary intake, glycaemic control, psychological well-being, acceptability, cognitive outcomes, and physiological measures were explored as secondary outcomes. From 594 interested persons, and 27 eligible individuals, 24 participants enrolled and 19 participants (mean ± SD; age: 50 ± 9 years, BMI: 34 ± 5 kg/m², HbA1c: 7.6 ± 1.1%) completed the 6-week study. Overall daily dietary intake did not change between Habitual (~8400 kJ/d; 35% carbohydrate, 20% protein, 41% fat, 1% alcohol) and TRE periods (~8500 kJ/d; 35% carbohydrate, 19% protein, 42% fat, 1% alcohol). Compliance to the 9 h TRE period was 72 ± 24% of 28 days (i.e., ~5 days/week), with varied adherence (range: 4–100%). Comparisons of adherent vs. non-adherent TRE days showed that adherence to the 9-h TRE window reduced daily energy intake through lower absolute carbohydrate and alcohol intakes. Overall, TRE did not significantly improve measures of glycaemic control (HbA1c −0.2 ± 0.4%; p = 0.053) or reduce body mass. TRE did not impair or improve psychological well-being, with variable effects on cognitive function. Participants described hunger, daily stressors, and emotions as the main barriers to adherence. We demonstrate that 4-weeks of TRE is feasible and achievable for these individuals with T2D to adhere to for at least 5 days/week. The degree of adherence to TRE strongly influenced daily energy intake. Future trials may benefit from supporting participants to incorporate TRE in regular daily life and to overcome barriers to adherence.

Metabolic responses to exercise on land and in water following glucose ingestion

Although aerobic exercise after a meal decreases postprandial blood glucose, the differences in glucose response between land and aquatic exercise are unclear. Thus, we examined the effect of different modes of exercise with same energy expenditure following glucose ingestion on carbohydrate metabolism. Ten healthy sedentary men (age, 22 ± 1 years) participated in this study. All subjects performed each of three exercise modes (cycling, walking and aquatic exercise) for 30 min after ingestion of a 75-g glucose solution with 1-2 weeks between trials. The exercise intensity was set at 40% of the maximum oxygen uptake that occurred during cycling. The velocity during walking and the target heart rate during aquatic exercise were predetermined in a pretest. The plasma glucose concentration at 30 min after exercise was significantly lower with aquatic exercise compared to that with cycling and walking (P<0·05). However, there were no significant differences among the three exercise modes in respiratory exchange ratio. On the other hand, serum free fatty acid concentration with aquatic exercise was significantly higher at 120 min after exercise compared with that after walking (P<0·05). These results suggest that aquatic exercise reduces postprandial blood glucose compared with both cycling and walking with the same energy expenditure. Aquatic exercise shows potential as an exercise prescription to prevent postprandial hyperglycaemia.

Does Carbohydrate Intake During Endurance Running Improve Performance? A Critical Review

Wilson, PB. Does carbohydrate intake during endurance running improve performance? A critical review. J Strength Cond Res 30(12): 3539-3559, 2016-Previous review articles assessing the effects of carbohydrate ingestion during prolonged exercise have not focused on running. Given the popularity of distance running and the widespread use of carbohydrate supplements, this article reviewed the evidence for carbohydrate ingestion during endurance running. The criteria for inclusion were (a) experimental studies reported in English language including a performance task, (b) moderate-to-high intensity exercise >60 minutes (intermittent excluded), and (c) carbohydrate ingestion (mouth rinsing excluded). Thirty studies were identified with 76 women and 505 men. Thirteen of the 17 studies comparing a carbohydrate beverage(s) with water or a placebo found a between-condition performance benefit with carbohydrate, although heterogeneity in protocols precludes clear generalizations about the expected effect sizes. Additional evidence suggests that (a) performance benefits are most likely to occur during events >2 hours, although several studies showed benefits for tasks lasting 90-120 minutes; (b) consuming carbohydrate beverages above ad libitum levels increases gastrointestinal discomfort without improving performance; (c) carbohydrate gels do not influence performance for events lasting 16-21 km; and (d) multiple saccharides may benefit events >2 hours if intake is ≥1.3 g·min Given that most participants were fasted young men, inferences regarding women, adolescents, older runners, and those competing in fed conditions are hampered. Future studies should address these limitations to further elucidate the role of carbohydrate ingestion during endurance running.

Physiological differences between cycling and running: lessons from triathletes

The purpose of this review was to provide a synopsis of the literature concerning the physiological differences between cycling and running. By comparing physiological variables such as maximal oxygen consumption (V O(2max)), anaerobic threshold (AT), heart rate, economy or delta efficiency measured in cycling and running in triathletes, runners or cyclists, this review aims to identify the effects of exercise modality on the underlying mechanisms (ventilatory responses, blood flow, muscle oxidative capacity, peripheral innervation and neuromuscular fatigue) of adaptation. The majority of studies indicate that runners achieve a higher V O(2max) on treadmill whereas cyclists can achieve a V O(2max) value in cycle ergometry similar to that in treadmill running. Hence, V O(2max) is specific to the exercise modality. In addition, the muscles adapt specifically to a given exercise task over a period of time, resulting in an improvement in submaximal physiological variables such as the ventilatory threshold, in some cases without a change in V O(2max). However, this effect is probably larger in cycling than in running. At the same time, skill influencing motor unit recruitment patterns is an important influence on the anaerobic threshold in cycling. Furthermore, it is likely that there is more physiological training transfer from running to cycling than vice versa. In triathletes, there is generally no difference in V O(2max) measured in cycle ergometry and treadmill running. The data concerning the anaerobic threshold in cycling and running in triathletes are conflicting. This is likely to be due to a combination of actual training load and prior training history in each discipline. The mechanisms surrounding the differences in the AT together with V O(2max) in cycling and running are not largely understood but are probably due to the relative adaptation of cardiac output influencing V O(2max) and also the recruitment of muscle mass in combination with the oxidative capacity of this mass influencing the AT. Several other physiological differences between cycling and running are addressed: heart rate is different between the two activities both for maximal and submaximal intensities. The delta efficiency is higher in running. Ventilation is more impaired in cycling than in running. It has also been shown that pedalling cadence affects the metabolic responses during cycling but also during a subsequent running bout. However, the optimal cadence is still debated. Central fatigue and decrease in maximal strength are more important after prolonged exercise in running than in cycling.

Fat and carbohydrate metabolism during submaximal exercise in children

During exercise, the contribution of fat and carbohydrate to energy expenditure is largely modulated by the intensity of exercise. Age, a short- or long-term diet enriched in carbohydrate or fat substrate stores, training and gender are other factors that have also been found to affect this balance. These factors have been extensively studied in adults from the perspective of improving performance in athletes, or from a health perspective in people with diseases. During the last decade, lifestyle changes associated with high-energy diets rich in lipid and reduced physical activity have contributed to the increase in childhood obesity. This lifestyle change has emerged as a serious health problem favouring the early development of cardiovascular diseases, insulin resistance or type 2 diabetes mellitus. Increasing physical activity levels in young people is important to increase energy expenditure and promote muscle oxidative capacity. Therefore, it is surprising that the regulation of balance between carbohydrate and lipid use during exercise has received much less attention in children than in adults. In this review, we have focused on the factors that affect carbohydrate and lipid metabolism during exercise and have identified areas that may be relevant in explaining the higher contribution of lipid to energy expenditure in children when compared with adults. Low muscle glycogen content is possibly associated with a low activity of glycolytic enzymes and high oxidative capacity, while lower levels of sympathoadrenal hormones are likely to favour lipid metabolism in children. Changes in energetic metabolism occurring during adolescence are also dependent on pubertal events with an increase in testosterone in boys and estrogen and progesterone in girls. The profound effects of ovarian hormones on carbohydrate and fat metabolism along with their effects on oxidative enzymes could explain that differences in substrate metabolism have not always been observed between girls and women. Finally, although the regulatory mechanisms of fat and carbohydrate balance during exercise are quite well identified, there are a lack of data specific to children and most of the evidences reported in this review were drawn from studies in adults. Isotope tracer techniques and nuclear magnetic resonance will allow non-invasive investigation of the metabolism of the different substrate sources in skeletal muscle.

Substrate source utilization during moderate intensity exercise with glucose ingestion in Type 1 diabetic patients

Substrate oxidation and the respective contributions of exogenous glucose, glucose released from the liver, and muscle glycogen oxidation were measured by indirect respiratory calorimetry combined with tracer technique in eight control subjects and eight diabetic patients (5 men and 3 women in both groups) of similar age, height, body mass, and maximal oxygen uptake, over a 60-min exercise period on cycle ergometer at 50.8% (SD 4.0) maximal oxygen uptake [131.0 W (SD 38.2)]. The subjects and patients ingested a breakfast (containing ∼80 g of carbohydrates) 3 h before and 30 g of glucose (labeled with 13C) 15 min before the beginning of exercise. The diabetic patients also received their usual insulin dose [Humalog = 9.1 U (SD 0.9); Humulin N = 13.9 U (SD 4.4)] immediately before the breakfast. Over the last 30 min of exercise, the oxidation of carbohydrate [1.32 g/min (SD 0.48) and 1.42 g/min (SD 0.63)] and fat [0.33 g/min (SD 0.10) and 0.30 g/min (SD 0.10)] and their contribution to the energy yield were not significantly different in the control subjects and diabetic patients. Exogenous glucose oxidation was also not significantly different in the control subjects and diabetic patients [6.3 g/30 min (SD 1.3) and 5.2 g/30 min (SD 1.6), respectively]. In contrast, the oxidation of plasma glucose and oxidation of glucose released from the liver were significantly lower in the diabetic patients than in control subjects [14.5 g/30 min (SD 4.3) and 9.3 g/30 min (SD 2.8) vs. 27.9 g/30 min (SD 13.3) and 21.6 g/30 min (SD 12.8), respectively], whereas that of muscle glycogen was significantly higher [28.1 g/30 min (SD 15.5) vs. 11.6 g/30 min (SD 8.1)]. These data indicate that, compared with control subjects, in diabetic patients fed glucose before exercise, substrate oxidation and exogenous glucose oxidation overall are similar but plasma glucose oxidation is lower; this is associated with a compensatory higher utilization of muscle glycogen.

Muscle glycogen oxidation during prolonged exercise measured with oral [13C]glucose: comparison with changes in muscle glycogen content

Plasma glucose and muscle glycogen oxidation during prolonged exercise [75-min at 48 and 76% maximal O2 uptake (V̇o2 max)] were measured in eight well-trained male subjects [V̇o2 max = 4.50 l/min (SD 0.63)] using a simplified tracer technique in which a small amount of glucose highly enriched in 13C was ingested: plasma glucose oxidation was computed from 13C/12C in plasma glucose (which was stable beginning at minute 30 and minute 15 during exercise at 48 and 76% V̇o2 max, respectively) and 13CO2 production, and muscle glycogen oxidation was estimated by subtracting plasma glucose oxidation from total carbohydrate oxidation. Consistent data from the literature suggest that this small dose of exogenous glucose does not modify muscle glycogen oxidation and has little effect, if any, on plasma glucose oxidation. The percent contributions of plasma glucose and muscle glycogen oxidation to the energy yield at 48% V̇o2 max [15.1% (SD 3.8) and 45.9% (SD 5.8)] and at 76% V̇o2 max [15.4% (SD 3.6) and 59.8% (SD 9.2)] were well in line with data previously reported for similar work loads and exercise durations using conventional tracer techniques. The significant reduction in glycogen concentration measured from pre- and postexercise vastus lateralis muscle biopsies paralleled muscle glycogen oxidation calculated using the tracer technique and was larger at 76% than at 48% V̇o2 max. However, the correlation coefficients between these two estimates of muscle glycogen utilization were not different from zero at each of the two work loads. The simplified tracer technique used in the present experiment appears to be a valid alternative approach to the traditional tracer techniques for computing plasma glucose and muscle glycogen oxidation during prolonged exercise.

Dose-Response Effects of Ingested Carbohydrate on Exercise Metabolism in Women

PURPOSE

The effect of different quantities of carbohydrate (CHO) intake on CHO metabolism during prolonged exercise was examined in endurance-trained females.

METHOD

On four occasions, eight females performed 2 h of cycling at approximately 60% .VO2max with ingestion of beverages containing low (LOW, 0.5 g.min(-1)), moderate (MOD, 1.0 g.min(-1)), or high (HIGH, 1.5 g.min(-1)) amounts of CHO, or water only (WAT). Test solutions contained trace amounts of [U-13C] glucose. Indirect calorimetry combined with measurement of expired 13CO2 and plasma 13C enrichment enabled calculation of exogenous CHO, liver-derived glucose, and muscle glycogen oxidation during the last 30 min of exercise.

RESULTS

The highest rates of exogenous CHO oxidation were observed in MOD, with no further increases in HIGH (peak rates of 0.33 +/- 0.02, 0.50 +/- 0.03, and 0.48 +/- 0.05 g.min(-1) for LOW, MOD, and HIGH, respectively; P < 0.05 for LOW vs MOD and HIGH). Endogenous CHO oxidation was lowest in MOD (0.99 +/- 0.06, 0.82 +/- 0.08, 0.70 +/- 0.07, and 0.89 +/- 0.09 g.min(-1); P < 0.05 for MOD vs all other trials). Compared with WAT, CHO ingestion reduced liver glucose oxidation during exercise by approximately 30% (P < 0.05 for WAT vs all CHO). Differential rates of muscle glycogen oxidation were observed with different CHO doses (0.57 +/- 0.07, 0.53 +/- 0.08, 0.41 +/- 0.07, and 0.60 +/- 0.09 g.min(-1) for WAT, LOW, MOD, and HIGH respectively; P < 0.05 for MOD vs HIGH).

CONCLUSION

In endurance-trained women, the highest rates of exogenous CHO oxidation and greatest endogenous CHO sparing was observed when CHO was ingested at moderate rates (1.0 g.min(-1), 60 g.h(-1)) during exercise.

Fiber type-specific muscle glycogen sparing due to carbohydrate intake before and during exercise

The effect of carbohydrate intake before and during exercise on muscle glycogen content was investigated. According to a randomized crossover study design, eight young healthy volunteers (n = 8) participated in two experimental sessions with an interval of 3 wk. In each session subjects performed 2 h of constant-load bicycle exercise ( approximately 75% maximal oxygen uptake). On one occasion (CHO), they received carbohydrates before ( approximately 150 g) and during (1 g.kg body weight(-1).h(-1)) exercise. On the other occasion they exercised after an overnight fast (F). Fiber type-specific relative glycogen content was determined by periodic acid Schiff staining combined with immunofluorescence in needle biopsies from the vastus lateralis muscle before and immediately after exercise. Preexercise glycogen content was higher in type IIa fibers [9.1 +/- 1 x 10(-2) optical density (OD)/microm(2)] than in type I fibers (8.0 +/- 1 x 10(-2) OD/microm(2); P < 0.0001). Type IIa fiber glycogen content decreased during F from 9.6 +/- 1 x 10(-2) OD/microm(2) to 4.5 +/- 1 x 10(-2) OD/microm(2) (P = 0.001), but it did not significantly change during CHO (P = 0.29). Conversely, in type I fibers during CHO and F the exercise bout decreased glycogen content to the same degree. We conclude that the combination of carbohydrate intake both before and during moderate- to high-intensity endurance exercise results in glycogen sparing in type IIa muscle fibers.

Substrate utilization during prolonged exercise with ingestion of 13C-glucose in acute hypobaric hypoxia (4,300 m)

Energy substrate oxidation was measured using indirect respiratory calorimetry combined with tracer technique in five healthy young male subjects, during a 80-min exercise period on ergocycle with ingestion of 140 g of (13)C-labelled glucose, in normoxia and acute hypobaric hypoxia (445 mmHg or 4,300 m), at the same relative [77% V(.-)((O)(2)(max))] and absolute workload (161+/-8 W, corresponding to 77 and 54% V(.-)((O)(2)(max)) in hypoxia and normoxia). The oxidation rate of exogenous glucose was not significantly different in the three experimental situations: 21.4+/-2.9, 20.2+/-1.2 and 17.2+/-0.6 g over the last 40 min of exercise at approximately 77 and approximately 54% V(.-)((O)(2)(max)) in normoxia and in hypoxia, respectively, providing 12.5+/-1.5, 16.8+/-1.1 and 14.9+/-1.1% of the energy yield, although ingestion of glucose during exercise resulted in a higher plasma glucose concentration in hypoxia than normoxia. The contribution of carbohydrate (CHO) oxidation to the energy yield was significantly higher in hypoxia (92.0+/-2.1%) than in normoxia for both a given absolute (75.3+/-5.2%) and relative workload (78.1+/-1.8%). This greater reliance on CHO oxidation in hypoxia was entirely due to the significantly larger contribution of endogenous glucose oxidation to the energy yield: 75.9+/-1.7% versus 66.6+/-3.3 and 55.2+/-3.7% in normoxia at the same relative and absolute workload.

Partitioning oxidative fuels during cold exposure in humans: muscle glycogen becomes dominant as shivering intensifies

The effects of changes in shivering intensity on the relative contributions of plasma glucose, muscle glycogen, lipids and proteins to total heat production are unclear in humans. The goals of this study were: (1) to determine whether plasma glucose starts playing a more prominent role as shivering intensifies, (2) to quantify overall changes in fuel use in relation to the severity of cold exposure, and (3) to establish whether the fuel selection pattern of shivering is different from the classic fuel selection pattern of exercise. Using a combination of indirect calorimetry and stable isotope methodology, fuel metabolism was monitored in non-acclimatized adult men exposed for 90 mins to 10 degrees C (low-intensity shivering (L)) or 5 degrees C (moderate-intensity shivering (M)). Results show that plasma glucose oxidation is strongly stimulated by moderate shivering (+122% from L to M), but the relative contribution of this pathway to total heat generation always remains minor (< 15% of total heat production). Instead, muscle glycogen is responsible for most of the increase in heat production between L and M. By itself, the increase in CHO oxidation is responsible for the 100 W increase in metabolic rate observed between L and M, because rates of lipid and protein oxidation remain constant. This high reliance on CHO is not compatible with the well known fuel selection pattern of exercise, when considering the relatively low metabolic rates elicited by shivering (approximately 30% for M). We conclude that shivering and exercise of similar energy requirements appear to be supported by different fuel mixtures. Investigating the physiological mechanisms underlying why a muscle producing only heat (shivering), or significant movement (exercise), shows a different pattern of fuel selection at the same power output strikes us as a fascinating area for future research.

Effects of carbohydrate availability on sustained shivering I. Oxidation of plasma glucose, muscle glycogen, and proteins

Carbohydrates (CHO) can play an important thermogenic role during shivering, but the effect of their availability on the use of other oxidative fuels is unclear. Using indirect calorimetry and tracer methods ([U-13C]glucose ingestion), we have determined the specific contributions of plasma glucose, muscle glycogen, proteins, and lipids to total heat production (Hprod) in men exposed to cold for 2-h (liquid-conditioned suit perfused with 10 degrees C water). Measurements were made after low-CHO diet and exercise (Lo) and high-CHO diet without exercise (Hi). The size of CHO reserves had no effect on Hprod but a major impact on fuel selection before and during shivering. In the cold, a complete shift from lipid oxidation for Lo (53, 28, and 19% Hprod for lipids, CHO, and proteins, respectively) to CHO-based metabolism for Hi (23, 65, and 12% Hprod for lipids, CHO, and proteins, respectively) was observed. Plasma glucose oxidation remains a minor fuel under all conditions (<13% Hprod), falling to 7% Hprod for Lo. Therefore, adjusting plasma glucose oxidation to compensate for changes in muscle glycogen oxidation is not a strategy used for maintaining heat production. Instead, proteins and lipids share responsibility for this compensation. We conclude that humans can show remarkable flexibility in oxidative fuel selection to ensure that heat production is not compromised during sustained cold exposure.

Effect of exercise mode on blood glucose disposal during physiological hyperinsulinaemia in humans

The aim of this study was to compare whole-body glucose uptake in cycling and running performed during physiological hyperinsulinaemia. On three occasions, seven male subjects underwent a hyperinsulinaemic (30 mU m(-2) min(-1)), euglycaemic (5 mmol l(-1)) clamp for 120 min. On one occasion, subjects rested for the duration of the trial (CON). On the other two occasions, after an initial resting period of 30 min, subjects either cycled (CYC) or ran (RUN) for 90 min at 65% of maximal O(2) uptake (VO(2max)). Insulin infusion resulted in physiological hyperinsulinaemia that was maintained for the duration of each trial [CON: 61 (3) mU l(-1); CYC: 77 (7) mU l(-1); RUN: 77 (5) mU l(-1)]. The rate of glucose uptake was greater during RUN than during CYC [last 30 min of exercise: 140 (4) vs 109 (8) micromol kg(-1) min(-1), respectively; P <0.01]. A differential amount of active muscle mass and/or muscle fibre type recruitment might account for the observed differences in glucose disposal between cycling and running.

Effect of cold exposure on fuel utilization in humans: plasma glucose, muscle glycogen, and lipids

The relative roles of circulatory glucose, muscle glycogen, and lipids in shivering thermogenesis are unclear. Using a combination of indirect calorimetry and stable isotope methodology ([U-13C]glucose ingestion), we have quantified the oxidation rates of these substrates in men acutely exposed to cold for 2 h (liquid conditioned suit perfused with 10 degrees C water). Cold exposure stimulated heat production by 2.6-fold and increased the oxidation of plasma glucose from 39.4 +/- 2.4 to 93.9 +/- 5.5 mg/min (+138%), of muscle glycogen from 126.6 +/- 7.8 to 264.2 +/- 36.9 mg glucosyl units/min (+109%), and of lipids from 46.9 +/- 3.2 to 176.5 +/- 17.3 mg/min (+376%). Despite the observed increase in plasma glucose oxidation, this fuel only supplied 10% of the energy for heat generation. The major source of carbohydrate was muscle glycogen (75% of all glucose oxidized), and lipids produced as much heat as all other fuels combined. During prolonged, low-intensity shivering, we conclude that total heat production is unequally shared among lipids (50%), muscle glycogen (30%), plasma glucose (10%), and proteins (10%). Therefore, future research should focus on lipids and muscle glycogen that provide most of the energy for heat production.

Oral [(13)C]glucose and endogenous energy substrate oxidation during prolonged treadmill running

Six male subjects were studied during running exercise (120 min, 69% maximal oxygen consumption) with ingestion of a placebo or 3.5 g/kg of [(13)C]glucose (approximately 2 g/min). Indirect respiratory calorimetry corrected for urea excretion in urine and sweat, production of (13)CO(2) at the mouth, and changes in plasma glucose (13)C/(12)C were used to compute energy substrate oxidation. The oxidation rate of exogenous glucose increased from 1.02 at minute 60 to 1.22 g/min at minute 120 providing approximately 24 and 33% of the energy yield (%En). Glucose ingestion did not modify protein oxidation, which provided approximately 4-5%En, but significantly increased glucose oxidation by approximately 7%, reduced lipid oxidation by approximately 16%, and markedly reduced endogenous glucose oxidation (1.25 vs. 2.21 g/min between minutes 80 and 120, respectively). The oxidation rate of glucose released from the liver (0.38 and 0.47 g/min, or 10-13%En at minutes 60 and 120, respectively), and of plasma glucose (1.30-1.69 g/min, or 34 and 45%En and 50 and 75% of glucose oxidation) significantly increased from minutes 60 to 120, whereas the oxidation of muscle glycogen significantly decreased (1.28 to 0.58 g of glucose/min, or 34 and 16%En and 50 and 25% of glucose oxidation). These results indicate that, during moderate prolonged running exercise, ingestion of a very large amount of glucose significantly reduces endogenous glucose oxidation, thus sparing muscle and/or liver glycogen stores.

Effect of carbohydrate ingestion on metabolism during running and cycling

The effects of carbohydrate or water ingestion on metabolism were investigated in seven male subjects during two running and two cycling trials lasting 60 min at individual lactate threshold using indirect calorimetry, U-14C-labeled tracer-derived measures of the rates of oxidation of plasma glucose, and direct determination of mixed muscle glycogen content from the vastus lateralis before and after exercise. Subjects ingested 8 ml/kg body mass of either a 6.4% carbohydrate-electrolyte solution (CHO) or water 10 min before exercise and an additional 2 ml/kg body mass of the same fluid after 20 and 40 min of exercise. Plasma glucose oxidation was greater with CHO than with water during both running (65 +/- 20 vs. 42 +/- 16 g/h; P < 0.01) and cycling (57 +/- 16 vs. 35 +/- 12 g/h; P < 0.01). Accordingly, the contribution from plasma glucose oxidation to total carbohydrate oxidation was greater during both running (33 +/- 4 vs. 23 +/- 3%; P < 0.01) and cycling (36 +/- 5 vs. 22 +/- 3%; P < 0.01) with CHO ingestion. However, muscle glycogen utilization was not reduced by the ingestion of CHO compared with water during either running (112 +/- 32 vs. 141 +/- 34 mmol/kg dry mass) or cycling (227 +/- 36 vs. 216 +/- 39 mmol/kg dry mass). We conclude that, compared with water, 1) the ingestion of carbohydrate during running and cycling enhanced the contribution of plasma glucose oxidation to total carbohydrate oxidation but 2) did not attenuate mixed muscle glycogen utilization during 1 h of continuous submaximal exercise at individual lactate threshold.

Effects of fat adaptation and carbohydrate restoration on prolonged endurance exercise

We determined the effect of fat adaptation on metabolism and performance during 5 h of cycling in seven competitive athletes who consumed a standard carbohydrate (CHO) diet for 1 day and then either a high-CHO diet (11 g. kg(-1)x day(-1) CHO, 1 g x kg(-1) x day(-1) fat; HCHO) or an isoenergetic high-fat diet (2.6 g x kg(-1) x day(-1) CHO, 4.6 g x kg(-1) x day(-1) fat; fat-adapt) for 6 days. On day 8, subjects consumed a high-CHO diet and rested. On day 9, subjects consumed a preexercise meal and then cycled for 4 h at 65% peak O(2) uptake, followed by a 1-h time trial (TT). Compared with baseline, 6 days of fat-adapt reduced respiratory exchange ratio (RER) with cycling at 65% peak O(2) uptake [0.78 +/- 0.01 (SE) vs. 0.85 +/- 0.02; P < 0.05]. However, RER was restored by 1 day of high-CHO diet, preexercise meal, and CHO ingestion (0.88 +/- 0.01; P < 0.05). RER was higher after HCHO than fat-adapt (0.85 +/- 0.01, 0.89 +/- 0.01, and 0.93 +/- 0.01 for days 2, 8, and 9, respectively; P < 0.05). Fat oxidation during the 4-h ride was greater (171 +/- 32 vs. 119 +/- 38 g; P < 0.05) and CHO oxidation lower (597 +/- 41 vs. 719 +/- 46 g; P < 0.05) after fat-adapt. Power output was 11% higher during the TT after fat-adapt than after HCHO (312 +/- 15 vs. 279 +/- 20 W; P = 0.11). In conclusion, compared with a high-CHO diet, fat oxidation during exercise increased after fat-adapt and remained elevated above baseline even after 1 day of a high-CHO diet and increased CHO availability. However, this study failed to detect a significant benefit of fat adaptation to performance of a 1-h TT undertaken after 4 h of cycling.

Effect of training status on fuel selection during submaximal exercise with glucose ingestion

In this study, an oral glucose load was enriched with a [U-(13)C]glucose tracer to determine differences in substrate utilization between endurance-trained (T) and untrained (UT) subjects during submaximal exercise at the same relative and absolute workload when glucose is ingested. Six highly trained cyclists/triathletes [maximal workload (Wmax), 400 +/- 9 W] and seven UT subjects (Wmax, 296 +/- 8 W) were studied during 120 min of cycling exercise at 50% Wmax ( approximately 55% maximal O(2) consumption). The T subjects performed a second trial at the mean workload of the UT group (148 +/- 4 W). Before exercise, 8.0 ml/kg of a (13)C-enriched glucose solution (80 g/l) was ingested. During exercise, boluses of 2.0 ml/kg of the same solution were administered every 15 min. Measurements were made in the 90- to 120-min period when a steady state was present in breath (13)CO(2) and plasma glucose (13)C enrichment. Energy expenditure was higher in T than in UT subjects (58 vs. 47 kJ/min, respectively; P < 0.001) at the same relative intensity. This was completely accounted for by an increased fat oxidation (0.57 vs. 0.40 g/min; P < 0.01). At the same absolute intensity, fat oxidation contributed more to energy expenditure in the T compared with the UT group (44 vs. 33%, respectively; P < 0.01). The reduction in carbohydrate oxidation in the T group was explained by a diminished oxidation rate of muscle glycogen (indirectly assessed by using tracer methodology at 0.72 +/- 0.1 and 1.03 +/- 0.1 g/min, respectively; P < 0.01) and liver-derived glucose (0.15 +/- 0.03 and 0.22 +/- 0.02 g/min, respectively; P < 0.05). Exogenous glucose oxidation rates were similar during all trials (+/-0.70 g/min).

Oxidation of an oral [13C]glucose load at rest and prolonged exercise in trained and sedentary subjects

The purpose of this study was to compare the oxidation of [13C]glucose (100 g) ingested at rest or during exercise in six trained (TS) and six sedentary (SS) male subjects. The oxidation of plasma glucose was also computed from the volume of 13CO2 and 13C/12C in plasma glucose to compute the oxidation rate of glucose released from the liver and from glycogen stores in periphery (mainly muscle glycogen stores during exercise). At rest, oxidative disposal of both exogenous (8.3 +/- 0.3 vs. 6.6 +/- 0.8 g/h) and liver glucose (4.4 +/- 0.5 vs. 2.6 +/- 0.4 g/h) was higher in TS than in SS. This could contribute to the better glucose tolerance observed at rest in TS. During exercise, for the same absolute workload [140 +/- 5 W: TS = 47 +/- 2.5; SS = 68 +/- 3 %maximal oxygen uptake (VO2 max)], [13C]glucose oxidation was higher in TS than in SS (39.0 +/- 2.6 vs. 33.6 +/- 1.2 g/h), whereas both liver glucose (16.8 +/- 2.4 vs. 24.0 +/- 1.8 g/h) and muscle glycogen oxidation (36.0 +/- 3.0 vs. 51.0 +/- 5.4 g/h) were lower. For the same relative workload (68 +/- 3% VO2 max: TS = 3.13 +/- 0.96; SS = 2.34 +/- 0.60 l O2/min), exogenous glucose (44.4 +/- 1.8 vs. 33.6 +/- 1.2 g/h) and muscle glycogen oxidation (73.8 +/- 7.2 vs. 51.0 +/- 5.4 g/h) were higher in TS. However, despite a higher energy expenditure in TS, liver glucose oxidation was similar in both groups (22.2 +/- 3.0 vs. 24.0 +/- 1.8 g/h). Thus exogenous glucose oxidation was selectively favored in TS during exercise, reducing both liver glucose and muscle glycogen oxidation.