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

Field and laboratory correlates of performance in competitive cross-country mountain bikers

We designed a laboratory test with variable fixed intensities to simulate cross-country mountain biking and compared this to more commonly used laboratory tests and mountain bike performance. Eight competitive male mountain bikers participated in a cross-country race and subsequently did six performance tests: an individual outdoor time trial on the same course as the race and five laboratory tests. The laboratory tests were as follows: an incremental cycle test to fatigue to determine peak power output; a 26-min variable fixed-intensity protocol using an electronically braked ergometer followed immediately by a 1-km time trial using the cyclist's own bike on an electronically braked roller ergometer; two 52-min variable fixed-intensity protocols each followed by a 1-km time trial; and a 1-km time trial done on its own. Outdoor competition time and outdoor time trial time correlated significantly (r = 0.79, P < 0.05). Both outdoor tests correlated better with peak power output relative to body mass (both r = -0.83, P < 0.05) than absolute peak power output (outdoor competition: r = -0.65; outdoor time trial: r = -0.66; non-significant). Outdoor performance times did not correlate with the laboratory tests. We conclude that cross-country mountain biking is similar to uphill or hilly road cycling. Further research is required to design sport-specific tests to determine the remaining unexplained variance in performance.

Effects of obesity phenotype on fat metabolism in obese men during endurance exercise

OBJECTIVE

The effects of obesity phenotype on fat metabolism during endurance exercise are unclear. This study aimed to investigate in obese men whether body fat distribution would influence plasma fat availability and oxidation during endurance exercise.

DESIGN

Fourteen sedentary men (body mass index (BMI) > 25 kg/m2) were divided into two groups by visceral fat (VF) area: VF obese (VF-Ob) (n = 7, age; 52.0 +/- 2.5 (s.e.) years) and abdominal subcutaneous fat obese (SF-Ob) (n = 7, age; 57.3 +/- 2.8 (s.e.) years). All participants performed stationary cycling exercise for 60 min at 50% of peak oxygen uptake.

MEASUREMENTS

Blood and respiratory gas samples were taken for analysis of hormone, metabolite and substrate oxidation in each participant at rest and during exercise.

RESULTS

There is a significant group x time interaction in the plasma concentration of free fatty acid (FFA) (P < 0.05) and glycerol (P < 0.05) during the exercise bout. In addition, total plasma concentration of FFA (area under the curve) was 59.2% higher in VF-Ob compared with SF-Ob men during endurance exercise (1.99 +/- 0.24 and 1.25 +/- 0.13 mEq/l/min, respectively; P < 0.05). Total plasma concentration of glycerol (area under the curve) was 102.3% higher in VF-Ob than SF-Ob men during the exercise (69.6 +/- 12.5 and 34.4 +/- 5.1 mg/dl/min, respectively; P < 0.05). However, fat oxidation was not different throughout the exercise between VF-Ob and SF-Ob men (176.5 +/- 25.7 and 183.0 +/- 12.8 kcal/60 min, respectively).

CONCLUSION

During moderate endurance exercise, plasma fat availability may be higher in men with VF obesity compared to men with SF obesity. However, total fat oxidation is similar between obesity phenotype.

Measurement of exogenous carbohydrate oxidation: a comparison of [U-14C]glucose and [U-13C]glucose tracers

The purpose of this study was to assess the level of agreement between two techniques commonly used to measure exogenous carbohydrate oxidation (CHO(EXO)). To accomplish this, seven healthy male subjects (24 +/- 3 yr, 74.8 +/- 2.1 kg, V(O2(max)) 62 +/- 4 ml x kg(-1) x min(-1)) exercised at 50% of their peak power for 120 min on two occasions. During these exercise bouts, subjects ingested a solution containing either 144 g glucose (8.7% wt/vol glucose) or water. The glucose solution contained trace amounts of both [U-13C]glucose and [U-14C]glucose to allow CHO(EXO) to be quantified simultaneously. The water trial was used to correct for background 13C enrichment. 13C appearance in the expired air was measured using isotope ratio mass spectrometry, whereas 14C appearance was quantified by trapping expired CO(2) in solution (using hyamine hydroxide) and adding a scintillator before counting radioactivity. CHO(EXO) measured with [13C]glucose ([13C]CHO(EXO)) was significantly greater than CHO(EXO) measured with [14C]glucose ([14C]CHO(EXO)) from 30 to 120 min. There was a 15 +/- 4% difference between [13C]CHO(EXO) and [14C]CHO(EXO) such that the absolute difference increased with the magnitude of CHO(EXO). Further investigations suggest that the difference is not because of losses of CO2 from the trapping solution before counting or an underestimation of the "strength" of the trapping solution. Previous research suggests that the degree of isotopic fractionation is small (S. C. Kalhan, S. M. Savin, and P. A. Adam. J Lab Clin Med89: 285-294, 1977). Therefore, the explanation for the discrepancy in calculated CHO(EXO) remains to be fully understood.

Are the effects of training on fat metabolism involved in the improvement of performance during high-intensity exercise?

The objective of the present study was to relate changes in certain muscle characteristics and indicators of metabolism in response to endurance training to the concomitant changes in time to exhaustion (T(lim)) at a work rate corresponding to maximal oxygen uptake VO(2speak). Eight healthy sedentary subjects pedalled on a cycle ergometer 2 h a day, 6 days a week, for 4 weeks. Training caused increases in VO(2peak) (by 8%), T(lim) (from 299 +/- 23 s before to 486 +/- 63 s after training), citrate synthase and 3-hydroxyl-acyl-CoA dehydrogenase (HAD) activities (by 54% and 16%, respectively) and capillary density (by 31%). Decreases in activity of lactate dehydrogenase (LDH) and muscle type of LDH (by 24% and 28%, respectively) and the phosphofructokinase/citrate synthase ratio (by 37%) were also observed. Respiratory exchange ratio (RER) tended to be lower (P < 0.1) at all relative work rates after training while the corresponding ventilation rates (VE) were unchanged. At the same absolute work rate, RER and (VE) were lower after training (P < 0.05). The improvement of T(lim) with training was related to the increases in HAD activity (r = 0.91, P = 0.0043), and to the decreases in RER calculated for Pa(peak) (r = 0.71, P = 0.0496). The present results suggest that the training-induced adaptations in fat metabolism might influence T(lim) at a work rate corresponding to VO(2peak) and stimulate the still debated and incompletely understood role of fat metabolism during short high-intensity exercise.

Determinants of fat oxidation during exercise in healthy men and women: a cross-sectional study

The aim of the present study was to establish fat oxidation rates over a range of exercise intensities in a large group of healthy men and women. It was hypothesised that exercise intensity is of primary importance to the regulation of fat oxidation and that gender, body composition, physical activity level, and training status are secondary and can explain part of the observed interindividual variation. For this purpose, 300 healthy men and women (157 men and 143 women) performed an incremental exercise test to exhaustion on a treadmill [adapted from a previous protocol (Achten J, Venables MC, and Jeukendrup AE. Metabolism 52: 747–752, 2003)]. Substrate oxidation was determined using indirect calorimetry. For each individual, maximal fat oxidation (MFO) and the intensity at which MFO occurred (Fatmax) were determined. On average, MFO was 7.8 ± 0.13 mg·kg fat-free mass (FFM)−1·min−1 and occurred at 48.3 ± 0.9% maximal oxygen uptake (V̇o2 max), equivalent to 61.5 ± 0.6% maximal heart rate. MFO (7.4 ± 0.2 vs. 8.3 ± 0.2 mg·kg·FFM−1·min−1; P < 0.01) and Fatmax (45 ± 1 vs. 52 ± 1% V̇o2 max; P < 0.01) were significantly lower in men compared with women. When corrected for FFM, MFO was predicted by physical activity (self-reported physical activity level), V̇o2 max, and gender ( R2 = 0.12) but not with fat mass. Men compared with women had lower rates of fat oxidation and an earlier shift to using carbohydrate as the dominant fuel. Physical activity, V̇o2 max, and gender explained only 12% of the interindividual variation in MFO during exercise, whereas body fatness was not a predictor. The interindividual variation in fat oxidation remains largely unexplained.

Excess body fat in men decreases plasma fatty acid availability and oxidation during endurance exercise

The effect of relative body fat mass on exercise-induced stimulation of lipolysis and fatty acid oxidation was evaluated in 15 untrained men (5 lean, 5 overweight, and 5 obese with body mass indexes of 21 +/- 1, 27 +/- 1, and 34 +/- 1 kg/m2, respectively, and %body fat ranging from 12 to 32%). Palmitate and glycerol kinetics and substrate oxidation were assessed during 90 min of cycling at 50% peak aerobic capacity (VO2 peak) by use of stable isotope-labeled tracer infusion and indirect calorimetry. An inverse relationship was found between %body fat and exercise-induced increase in glycerol appearance rate relative to fat mass (r2 = 0.74; P < 0.01). The increase in total fatty acid uptake during exercise [(micromol/kg fat-free mass) x 90 min] was approximately 50% smaller in obese (181 +/- 70; P < 0.05) and approximately 35% smaller in overweight (230 +/- 71; P < 0.05) than in lean (354 +/- 34) men. The percentage of total fatty acid oxidation derived from systemic plasma fatty acids decreased with increasing body fat, from 49 +/- 3% in lean to 39 +/- 4% in obese men (P < 0.05); conversely, the percentage of nonsystemic fatty acids, presumably derived from intramuscular and possibly plasma triglycerides, increased with increasing body fat (P < 0.05). We conclude that the lipolytic response to exercise decreases with increasing adiposity. The blunted increase in lipolytic rate in overweight and obese men compared with lean men limits the availability of plasma fatty acids as a fuel during exercise. However, the rate of total fat oxidation was similar in all groups because of a compensatory increase in the oxidation of nonsystemic fatty acids.

Substrate oxidation, obesity and exercise training

Regular physical exercise is of the utmost importance in the treatment of obesity because exercise is one of the factors determining long-term weight maintenance in weight reduction programmes and because exercise has been associated with a reduced risk for developing type 2 diabetes mellitus and cardiovascular disease. Obesity is associated with an impaired utilization of fat as a fuel during post-absorptive conditions, during beta-adrenergic stimulation and possibly during exercise, although the latter data are controversial. One of the underlying mechanisms for the positive effect of exercise training in obesity may be related to its effects on fat utilization because exercise training has been shown to increase basal fat oxidation and exercise fat oxidation in lean volunteers. Data on the effect of aerobic exercise training on exercise fat oxidation are controversial, whereas the available data indicate that exercise training may not be able to increase resting fat oxidation or 24-hour fat oxidation in obese subjects. Because disturbed muscle fat oxidation may be a primary event in the aetiology of obesity it is of the utmost importance to obtain more information on how and whether exercise training may be able to compensate for these impairments.

Effects of obesity on substrate utilization during exercise

OBJECTIVE

The capacity for lipid and carbohydrate (CHO) oxidation during exercise is important for energy partitioning and storage. This study examined the effects of obesity on lipid and CHO oxidation during exercise.

RESEARCH METHODS AND PROCEDURES

Seven obese and seven lean [body mass index (BMI), 33 +/- 0.8 and 23.7 +/- 1.2 kg/m(2), respectively] sedentary, middle-aged men matched for aerobic capacity performed 60 minutes of cycle exercise at similar relative (50% VO(2max)) and absolute exercise intensities.

RESULTS

Obese men derived a greater proportion of their energy from fatty-acid oxidation than lean men (43 +/- 5% 31 +/- 2%; p = 0.02). Plasma fatty-acid oxidation determined from recovery of infused [0.15 micromol/kg fat-free mass (FFM) per minute] [1-(13)C]-palmitate in breath CO(2) was similar for obese and lean men (8.4 +/- 1.1 and 29 +/- 15 micromol/kg FFM per minute). Nonplasma fatty-acid oxidation, presumably, from intramuscular sources, was 50% higher in obese men than in lean men (10.0 +/- 0.6 versus 6.6 +/- 0.8 micromol/kg FFM per minute; p < 0.05). Systemic glucose disposal was similar in lean and obese groups (33 +/- 8 and 29 +/- 15 micromol/kg FFM per minute). However, the estimated rate of glycogen-oxidation was 50% lower in obese than in lean men (61 +/- 12 versus 90 +/- 6 micromol/kg FFM per minute; p < 0.05).

DISCUSSION

During moderate exercise, obese sedentary men have increased rates of fatty-acid oxidation from nonplasma sources and reduced rates of CHO oxidation, particularly muscle glycogen, compared with lean sedentary men.

The effect of a 3-month low-intensity endurance training program on fat oxidation and acetyl-CoA carboxylase-2 expression

Endurance training has been shown to increase fat oxidation both at rest and during exercise. However, most exercise training studies have been performed at high exercise intensity in well-trained athletes, and not much is known about the effect of a low-intensity training program on fat oxidation capacity in lean sedentary humans. Here, we examine the effect of 3-month low-intensity training program on total and intramuscular triglyceride (IMTG)- and/or VLDL-derived fat oxidation capacity and skeletal muscle mRNA expression. Six healthy untrained subjects (aged 43 +/- 2 years, BMI 22.7 +/- 1.1 kg/ m(2), V(O)(2max) 3.2 +/- 0.2 l/min) participated in a supervised 12-week training program at 40% V(O)(2max) three times weekly. Total and plasma-derived fatty acid oxidation at rest and during 1 h exercise was measured using [(13)C]palmitate, and in a separate test, [(13)C]acetate recovery was determined. Muscle biopsies were taken after an overnight fast. Total fat oxidation during exercise increased from 1,241 +/- 93 to 1,591 +/- 130 micromol/min (P = 0.06), and IMTG- and/or VLDL-derived fatty acid oxidation increased from 236 +/- 84 to 639 +/- 172 micromol/min (P = 0.09). Acetyl-CoA carboxylase-2 mRNA expression was significantly decreased after training (P = 0.005), whereas lipoprotein lipase mRNA expression tended to increase (P = 0.07). In conclusion, a minimal amount of physical activity tends to increase fat oxidation and leads to marked changes in the expression of genes encoding for key enzymes in fat metabolism.

Effect of gender on lipid kinetics during endurance exercise of moderate intensity in untrained subjects

We evaluated lipid metabolism during 90 min of moderate-intensity (50% VO(2) peak) cycle ergometer exercise in five men and five women who were matched on adiposity (24 +/- 2 and 25 +/- 1% body fat, respectively) and aerobic fitness (VO(2) peak: 49 +/- 2 and 47 +/- 1 ml x kg fat-free mass(-1) x min(-1), respectively). Substrate oxidation and lipid kinetics were measured by using indirect calorimetry and [(13)C]palmitate and [(2)H(5)]glycerol tracer infusion. The total increase in glycerol and free fatty acid (FFA) rate of appearance (R(a)) in plasma during exercise (area under the curve above baseline) was approximately 65% greater in women than in men (glycerol R(a): 317 +/- 40 and 195 +/- 33 micromol/kg, respectively; FFA R(a): 652 +/- 46 and 453 +/- 70 micromol/kg, respectively; both P < 0.05). Total fatty acid oxidation was similar in men and women, but the relative contribution of plasma FFA to total fatty acid oxidation was higher in women (76 +/- 5%) than in men (46 +/- 5%; P < 0.05). We conclude that lipolysis of adipose tissue triglycerides during moderate-intensity exercise is greater in women than in men, who are matched on adiposity and fitness. The increase in plasma fatty acid availability leads to a greater rate of plasma FFA tissue uptake and oxidation in women than in men. However, total fat oxidation is the same in both groups because of a reciprocal decrease in the oxidation rate of fatty acids derived from nonplasma sources, presumably intramuscular and possibly plasma triglycerides, in women.

Effect of exercise training at different intensities on fat metabolism of obese men

The present study investigated the effect of exercise training at different intensities on fat oxidation in obese men. Twenty-four healthy male obese subjects were randomly divided in either a low- [40% maximal oxygen consumption (VO(2 max))] or high-intensity exercise training program (70% VO(2 max)) for 12 wk, or a non-exercising control group. Before and after the intervention, measurements of fat metabolism at rest and during exercise were performed by using indirect calorimetry, [U-(13)C]palmitate, and [1,2-(13)C]acetate. Furthermore, body composition and maximal aerobic capacity were measured. Total fat oxidation did not change at rest in any group. During exercise, after low-intensity exercise training, fat oxidation was increased by 40% (P < 0.05) because of an increased non-plasma fatty acid oxidation (P < 0.05). High-intensity exercise training did not affect total fat oxidation during exercise. Changes in fat oxidation were not significantly different among groups. It was concluded that low-intensity exercise training in obese subjects seemed to increase fat oxidation during exercise but not at rest. No effect of high-intensity exercise training on fat oxidation could be shown.

Addition of protein and amino acids to carbohydrates does not enhance postexercise muscle glycogen synthesis

Ingestion of a protein-amino acid mixture (Pro; wheat protein hydrolysate, leucine, and phenylalanine) in combination with carbohydrate (CHO; 0.8 g x kg(-1) x h(-1)) has been shown to increase muscle glycogen synthesis after exercise compared with the same amount of CHO without Pro. The aim of this study was to investigate whether coingestion of Pro also increases muscle glycogen synthesis when 1.2 g CHO. kg(-1). h(-1) is ingested. Eight male cyclists performed two experimental trials separated by 1 wk. After glycogen-depleting exercise, subjects received either CHO (1.2 g x kg(-1) x h(-1)) or CHO+Pro (1.2 g CHO x kg(-1) x h(-1) + 0.4 g Pro x kg(-1) x h(-1)) during a 3-h recovery period. Muscle biopsies were obtained immediately, 1 h, and 3 h after exercise. Blood samples were collected immediately after the exercise bout and every 30 min thereafter. Plasma insulin was significantly higher in the CHO+Pro trial compared with the CHO trial (P < 0.05). No difference was found in plasma glucose or in rate of muscle glycogen synthesis between the CHO and the CHO+Pro trials. Although coingestion of a protein amino acid mixture in combination with a large CHO intake (1.2 g x kg(-1) x h(-1)) increases insulin levels, this does not result in increased muscle glycogen synthesis.

Oxidation of carbohydrate feedings during prolonged exercise: current thoughts, guidelines and directions for future research

Although it is known that carbohydrate (CHO) feedings during exercise improve endurance performance, the effects of different feeding strategies are less clear. Studies using (stable) isotope methodology have shown that not all carbohydrates are oxidised at similar rates and hence they may not be equally effective. Glucose, sucrose, maltose, maltodextrins and amylopectin are oxidised at high rates. Fructose, galactose and amylose have been shown to be oxidised at 25 to 50% lower rates. Combinations of multiple transportable CHO may increase the total CHO absorption and total exogenous CHO oxidation. Increasing the CHO intake up to 1.0 to 1.5 g/min will increase the oxidation up to about 1.0 to 1.1 g/min. However, a further increase of the intake will not further increase the oxidation rates. Training status does not affect exogenous CHO oxidation. The effects of fasting and muscle glycogen depletion are less clear. The most remarkable conclusion is probably that exogenous CHO oxidation rates do not exceed 1.0 to 1.1 g/min. There is convincing evidence that this limitation is not at the muscular level but most likely located in the intestine or the liver. Intestinal perfusion studies seem to suggest that the capacity to absorb glucose is only slightly in excess of the observed entrance of glucose into the blood and the rate of absorption may thus be a factor contributing to the limitation. However, the liver may play an additional important role, in that it provides glucose to the bloodstream at a rate of about 1 g/min by balancing the glucose from the gut and from glycogenolysis/gluconeogenesis. It is possible that when large amounts of glucose are ingested absorption is a limiting factor, and the liver will retain some glucose and thus act as a second limiting factor to exogenous CHO oxidation.