Several studies have shown that, in response to prolonged exercise at a given relative workload (%V˙O2max), when compared with men, women rely more on fat and less on CHO oxidation (32,36). The separate and combined effects of CHO supplementation before and during exercise on fuel selection in men (26,40) or in women (1) have been investigated in a limited number of studies with no direct comparison between sexes. However, changes in fuel selection during prolonged exercise could be slightly different in men and women after a high-CHO diet (33), when CHO are ingested during exercise (24,28,38), and when both CHO supplementation procedures are combined (1). In the study by Tarnopolsky et al. (33), when compared with a mixed diet, the increased contribution of CHO oxidation to the energy yield (%En) after a high-CHO diet was larger in women (from 69 to 75 %En, vs 84 to 83 %En in men). In the few studies that reported exogenous glucose oxidation in women during exercise (24,28,38), the %En from glucose ingestion was consistently ∼2%-4% higher in women than in men, and when compared with a control situation with water ingestion, both a larger increase in total CHO oxidation (38) and a greater reduction in endogenous CHO oxidation (28) were reported in women. Finally, the increase in %En from CHO oxidation attributable to the combined effect of a high-CHO diet and CHO ingestion during exercise was lower in the study by Andrews et al. (1) in women (+12 %En, from 59 to 71 %En) than in the study by Widrick et al. (40) in men (+22 %En, from 62 to 84 %En). Taken together, these results suggest that women might respond differently than men to a high-CHO preexercise diet and/or glucose supplementation during exercise when these procedures are used separately or in combination. A better understanding of sex differences in fuel selection during prolonged exercise with CHO supplementation could help refine nutritional recommendations for recreationally active women and female athletes.
The purpose of the present experiment was thus to compare changes in fuel selection during exercise in response to a high-CHO preexercise diet, glucose ingestion during exercise, and the combined effects of both procedures in men and women. We hypothesized that both the high-CHO diet and glucose ingestion during exercise would increase CHO oxidation and that the effects of CHO supplementation on CHO oxidation could be higher in women than in men. We also hypothesized that the %En from exogenous glucose oxidation could be higher in women than in men with a greater associated reduction in endogenous CHO oxidation after both diets. Finally, consistent data show that the sex difference in fuel selection is partly attributable to the effect of estrogens, which favor fat oxidation during exercise (12,13,15,20). However, there are only limited data on fuel selection during prolonged exercise in women taking oral contraceptives (OC) and thus supplemented with synthetic estrogens (see Burrows and Peters (5) for a recent review), and we are not aware of any study comparing the effect of a high-CHO diet and/or glucose ingestion on fuel selection during prolonged exercise in women taking OC. For this reason, in the present experiment, the effects of the CHO supplementation procedures, separately and combined, were compared in women regularly taking triphasic OC (W+OC) or not (W−OC) and in men. On the basis of data concerning the effect of estrogens on fuel selection during exercise, we hypothesized that, when compared with W−OC, W+OC would rely more on fat and less on CHO oxidation in the control (water ingestion after a mixed diet) and the three experimental situations.
Six men and 12 nulliparous women (Table 1) gave their informed written consent to participate in this study, which was approved by the University of Montreal ethics committee on the use of human subjects in research. The six women in the W−OC group had never taken any OC or had stopped for at least 1 yr and, at the time of experiment, had regular menstrual cycles lasting between 25 and 35 d (29 ± 1 d; mean ± SEM). The six women in the W+OC group had been taking triphasic OC for at least 1 yr. All subjects were moderately active (2-6 h·wk−1) and, as shown in Table 1, had normal plasma glucose concentrations after a 12-h fast as well as 120 min after ingestion of 75 g of glucose in 300 mL of water. Subjects were asked to maintain their regular activity level and to notify the investigators of any alteration in their habitual exercise. None of the subjects was a smoker, heavy drinker (<3 drinks per week), under medication (except for OC), or taking recreational drugs.
Fat-free mass (FFM) was measured using bioimpedancemetry (SBF-521; Tanita, Tokyo, Japan). Maximal oxygen consumption (V˙O2max) and experimental workload on the cycle ergometer (GP400; Ergomeca, La Bayette, France) were determined for each subject using open-circuit spirometry (MOXUS Metabolic Cart, AEI Technologies, Naperville, IL) during a preliminary test session. The subjects were then studied four times (at approximately 2-wk intervals for men and at 1-month intervals for women) during 120-min exercise periods at 50% of the maximal power output (Table 1) corresponding to ∼57% V˙O2max. The women were studied between 6 and 10 d after the onset of menses, depending on the length of the cycle, i.e., during the midfollicular phase (17). During the exercise period, the subjects ingested either 2 g·kg−1 body mass of glucose dissolved in 20 mL·kg−1 of water or 20 mL·kg−1 of water only. The drinks were given in six doses (5.7 mL·kg−1 20 min before the beginning of exercise and 2.85 mL·kg−1 every 20 min thereafter up to the 80th minute, i.e., in the glucose trial: 0.570 g·kg−1 of glucose 20 min before the beginning of exercise and 0.285 g·kg−1 every 20 min thereafter). The two trials (water and glucose ingestion) were conducted after either 2 d of a mixed diet (45 and 40 kcal·kg−1·d−1 for men and women, respectively; ∼55% CHO, 30% fat, and 15% protein) or 2 d of a hypercaloric (53 and 48 kcal·kg−1·d−1, i.e., a 8-kcal·kg−1·d−1 supplementation) and high-CHO diet (80% CHO, 10% fat, and 10% protein). As shown by Tarnopolsky et al. (34), in women, a hypercaloric diet is needed to increase muscle glycogen stores. The high-CHO diet was introduced immediately after a 90-min exercise bout at ∼70% V˙O2max on the cycle ergometer, conducted between ∼9:00 and ∼11:00 a.m., 48 h before the experimental protocol to favor CHO storage. The subjects were provided with prepackaged meals that did not contain any food with a high 13C content (e.g., corn, sugarcane), which may modify background 13C enrichment of expired CO2. In addition, during the 2 d preceding the tests, the subjects refrained from exercising and from drinking alcohol. The experiments, which were presented in a balanced random order among the subjects, were performed between 9:00 and 11:30 a.m. after a standard breakfast (mixed diet: ∼15.0 and 13.3 kcal·kg−1; high-CHO diet: ∼17.5 and 16.2 kcal·kg−1, for men and women, respectively) ingested between 7:00 and 8:00 a.m.
The glucose ingested during the exercise period (Biopharm; Laval, Quebec, Canada) was derived from corn (13C/12C = −11.03‰ δ13C PDB) and was artificially enriched with U-13C-glucose (13C/12C > 99%; Isotec, Miamisburg, OH) to achieve a final isotopic composition close to 70‰ δ13C PDB; the actual value measured by mass spectrometry was 68.4‰ δ13C PDB.
Measures and computations.
Measurements were made at rest before ingestion of exogenous glucose and every 20 min during the exercise period. Fat and CHO oxidations were computed from indirect respiratory calorimetry corrected for protein oxidation. For this purpose, V˙O2 and CO2 production (V˙CO2) was measured using open-circuit spirometry (5-min collection period), and urea production was estimated during the exercise period as previously described (22). Briefly, urea excretion during the 120 min of exercise was estimated from its concentration in urine and sweat and from urine and sweat loss. Sweat loss was estimated from changes in body mass, taking into account fluid intake, mass loss through CO2 production, and water loss by the lungs. For the measurement of 13C/12C in expired CO2, 10-mL samples of expired gases were collected in vacutainers (Becton Dickinson, Franklin Lakes, NJ). Finally, 10-mL blood samples were withdrawn through a catheter (Baxter Health Care Corp., Valencia, CA) inserted into an antecubital vein at the beginning of the experiment, for the measurement of plasma glucose, free fatty acid, and insulin concentrations at the 60th and 120th minutes. Plasma, urine, and sweat samples were stored at −80°C until analysis.
Protein oxidation and the associated amount of energy provided were computed from the amount of urea excreted, taking into account that 1 g of urea excreted corresponds to 2.9 g of protein oxidized and that the energy potential of protein is 4.70 kcal·g−1 (22). CHO and fat oxidation were then computed from V˙O2 and V˙CO2 (in L·min−1) corrected for the volumes of O2 and CO2 corresponding to protein oxidation (20):
Measurement of 13C/12C in expired CO2 was performed by mass spectrometry (Prism, VG, Manchester, UK), and the values were expressed in per mill difference by comparison with the PDB Chicago Standard: ‰ δ13C PDB = [(Rspl / Rstd) − 1] × 1000, where Rspl and Rstd are the 13C/12C ratio in the sample and standard (1.1237%), respectively.
The oxidation rate of exogenous glucose (g·min−1) was computed as follows:
where V˙CO2 (not corrected for protein oxidation) is in liters per minute (L·min−1), Rexp is the observed isotopic composition of expired CO2, Rref is the isotopic composition of expired CO2 with water ingestion after the corresponding diet, Rexo is the isotopic composition of the exogenous glucose ingested, and k (0.7462 L·g−1) is the volume of CO2 provided by the complete oxidation of glucose. This computation is made based on the observation that, in response to exercise, 13C provided from 13C-glucose is not irreversibly lost in pools of tricarboxylic acid cycle intermediates and/or bicarbonate and that 13CO2 recovery in expired gases is thus complete or almost complete (29,35). However, the 13C/12C in expired CO2 only slowly equilibrates with the 13C/12C in the CO2 produced in tissues (27). To take this delay between 13CO2 production in tissues and at the mouth into account, the computations were only made during the last 80 min of the observation period, thus allowing for an equilibration period of 40 min from the start of exercise.
Energy expenditure, that is, the amount of energy provided by the oxidation of protein, CHO, and fat, and the respective contributions of these substrates were computed from the amounts oxidized and their respective energy potential (20).
Plasma glucose and free fatty acid concentrations were measured using spectrophotometric automated assays (Boehringer, Mannheim, Germany), whereas plasma insulin concentration was measured using an automated radioimmunoassay (KTSP-11001; Immunocorp Sciences, Montreal, Quebec, Canada). Urine and sweat urea concentrations were measured using a Synchron Clinical System (CX7; Beckman, Anaheim, CA).
Data are presented as mean ± SEM. Intergroup differences in physical and fitness characteristics of subjects, as well as fuel selection in the control situation (mixed diet and water ingestion), were compared using one-way ANOVA (Statistica package; StatSoft, Tulsa, OK). The effects of diet and glucose ingestion during exercise were compared using three-way ANOVA for independent (group) and repeated measurements (diet and ingestion). The combined effect of the high-CHO diet and glucose ingestion during exercise was compared with the three other trials using a two-way ANOVA for independent (group) and repeated measurements (trial). Tukey HSD post hoc tests were used to identify the location of significant differences (P < 0.05) when the ANOVA yielded a significant F-ratio. Cohen's f was used as a measure of effect size in the ANOVA with three or more comparisons (small, medium, and large effect sizes: 0.1, 0.25, or 0.40, respectively), and Cohen's d was reported for pairwise comparisons (small, medium, and large effect sizes: 0.20, 0.50, and 0.80, respectively) (10). Post hoc power analyses were also performed to estimate the probability of type II error when failing to reject the null hypothesis and to compute the sample size required to achieve a power of 0.80.
As shown in Table 1, the height and body mass were significantly larger in men than in both W−OC and W+OC, whereas percent body fat was significantly lower. Absolute maximal workload (W) and V˙O2max (L·min−1) were also significantly higher in men than in women. When corrected for body mass, the V˙O2max (mL·kg−1·min−1) was also significantly (∼28%) higher in men than in women. As generally reported (25), the sex difference in V˙O2max was smaller (∼10%) but remained significant when corrected for FFM (P = 0.025). No significant difference was found for any variable between W−OC and W+OC.
Oxygen consumption (V˙O2), which was stable during the exercise period in the three groups and in the four experimental situations, and urea excretion were not different in any group, and no difference was observed after either the high-CHO diet or the glucose ingestion during exercise (Table 2). The RER was significantly lower (P = 0.037, f = 0.477) in women than in men in the control situation (water ingestion after the mixed diet), and there were significant main effects for diet (P < 0.001, f = 1.228) and glucose ingestion (P < 0.001, f = 1.482) because RER increased in all groups both after the high-CHO diet and when glucose was ingested during exercise (Table 2). Furthermore, after both diets, the increase in RER with glucose ingestion was larger in women than in men (ingestion × group interaction, P = 0.034, f = 0.358).
When corrected for FFM, no significant difference in protein oxidation was observed in any group and was not modified by either the high-CHO diet or the glucose ingestion during exercise. The %En from protein oxidation was not significantly different among the three groups in the control situation and was not modified in any of the three experimental situations (2.5-4.0 %En; Fig. 1). As shown in Figure 2, the oxidation rate of CHO was significantly higher in men than in women (main effect, P = 0.049, f = 0.532). The high-CHO diet increased the oxidation rate of CHO (main effect, P = 0.001, f = 0.963) without any sex difference. Glucose ingestion during exercise also increased the oxidation rate of CHO (main effect, P < 0.001, f = 1.254), but the increase was larger in women than in men (group × ingestion interaction, P = 0.033, f = 0.330; Fig. 2). Changes in the oxidation rate of fat were the mirror image of that of total CHO (data not shown).
The isotopic enrichment of expired CO2 immediately before ingestion of the first dose of 13C-glucose was slightly but significantly higher after the high-CHO diet than after the mixed diet (−22.1‰ ± 0.8‰ vs −24.4‰ ± 1.1‰ δ13C PDB, pooled data, n = 18) but was not significantly different among the three groups (Fig. 3). This value increased markedly after ingestion of 13C-labeled glucose during exercise (main effects of time and ingestion). No significant difference was observed among the three groups for the response of 13C/12C in expired CO2 during the exercise period, and the values were not significantly different after the high-CHO and the mixed diet.
The oxidation rate of exogenous glucose during the last 80 min of exercise was not significantly different in any group and was not modified by either the high-CHO diet or the glucose ingestion during exercise (Fig. 2). During the last 80 min of exercise, the cumulative amounts of exogenous glucose oxidized after the mixed and high-CHO diets were 64.0 and 63.5 g, respectively, in men (∼47% of the amount ingested) and were 45.1 and 46.0 g, respectively, in women (not significantly different in W−OC and W+OC; ∼43% of the amount ingested), contributing between 24.9% and 27.4% to the energy yield (Fig. 1). The high-CHO diet significantly increased endogenous CHO oxidation (main effect, P < 0.001, f = 1.147). Glucose ingestion during exercise significantly reduced endogenous glucose oxidation in the three groups (main effect, P < 0.001, f = 2.463; Fig. 2), but the reduction observed when glucose was ingested was larger in men than in women (ingestion × group interaction, P = 0.024, f = 0.203). In the control situation, the %En from fat and CHO oxidation were not significantly different in W−OC and W+OC, but these values were significantly different for both groups of women (pooled data) than in men (%En from fat: P = 0.037, d = 1.412; %En from CHO: P = 0.018, d = 1.624; Fig. 1). Both the high-CHO diet and glucose ingestion during exercise significantly decreased and increased, respectively, the %En from fat (P < 0.001, f = 1.201; P < 0.001, f = 1.360, respectively, for the main effect of diet and ingestion) and CHO oxidation (P < 0.001, f = 1.250; P < 0.001, f = 1.456, respectively) in all groups. These changes in the %En from fat and CHO oxidation were significantly larger in women than in men (ingestion × group interactions, P = 0.008 and f = 0.42; P = 0.028 and f = 0.37, respectively, for the %En from fat and CHO oxidation). The high-CHO diet significantly increased the %En from total and endogenous CHO oxidation in all groups (main effects, P values < 0.001, f values ∼1.27). The %En from endogenous CHO oxidation significantly decreased in all groups when glucose was ingested during exercise (main effect, P < 0.001, f = 2.55), and this decrease was larger in men than in women (ingestion × group interaction, P = 0.046, f = 0.22). Finally, no difference in the %En from exogenous glucose oxidation was observed among the three groups (P = 0.592, f = 0.27) or between the two diets (P = 0.793, f = 0.07).
Table 3 shows the average plasma glucose, free fatty acid, and insulin concentration during the last hour of exercise. Plasma glucose concentration was significantly higher when glucose was ingested during exercise in all groups (main effect, P = 0.001, f = 1.02). In response to exercise, plasma free fatty acid concentration was significantly higher in women than in men (main effect, P = 0.041, f = 0.73) and was significantly reduced after the high-CHO diet (main effect, P = 0.001, f = 0.95) and the glucose ingestion (main effect, P < 0.001, f = 1.79) in the three groups. Plasma free fatty acid concentration was significantly higher in women than in men when glucose was ingested during exercise (group × ingestion interaction, P = 0.042, f = 0.30). The significant difference in plasma free fatty acid concentration observed between the mixed and high-CHO diet when water was ingested disappeared when glucose was ingested during exercise (diet × ingestion interaction, P = 0.019, f = 0.59). Plasma insulin concentration was not significantly different in men and in women during exercise in the control situation and was not significantly modified by the high-CHO diet with water ingestion. In contrast, insulin concentration was significantly higher when glucose was ingested during exercise after both diets (main effect, P < 0.001, f = 3.03).
In the present experiment, the difference in the %En from CHO oxidation in the control situation between men and women (62 vs 53 %En) was similar to the average value compiled from 25 studies reviewed by Tarnopolsky (32) (69 vs 59 %En, at an average of 57% V˙O2max) or to that reported by Venables et al. (36) (63 vs 54 %En, at 56% V˙O2max). Several mechanisms could explain this sex difference in fuel selection during exercise. For example, in some studies reporting sex differences in fuel selection during exercise, a higher fitness level in women could explain the higher reliance on fat oxidation than in men for a given percent V˙O2max (see Tarnopolsky (32) for a review). However, in the present experiment, the V˙O2max expressed in either milliliters per kilogram per minute (mL·kg−1·min−1) or milliliters per kilogram of FFM per minute (mL·kg FFM−1·min−1) suggests that a sex difference in fitness level, if any, would favor fat oxidation in men. As reviewed by Kiens (21), higher intramyocellular lipid (IMCL) content and higher mRNA and protein levels of muscle lipoprotein lipase and of several lipid-binding proteins have been reported in women than in men. This has, however, not been consistently shown to translate to higher IMCL utilization in women (14,21). Burke et al. (4) and Loucks (23) have also shown that the amount of CHO in the diet is lower in women than in men. This could partly explain the higher reliance on fat oxidation generally observed in women. However, in the present experiment, with an amount of CHO in the mixed diet (control situation) corresponding to the recommendation by Burke et al. (4) (5-7 g of CHO·kg−1·d−1) in both men and women (6.4 and 5.7 g of CHO·kg−1·d−1, respectively), the sex difference in fuel selection during prolonged exercise remained. This provides experimental support to the hypothesis that the sex difference in fuel selection is due in part to the effect of estrogens. It is well established that estrogens favor fat utilization and reduce muscle glycogenolysis during exercise (2,3,11,32). In this respect, Suh et al. (31) have suggested that synthetic estrogens, as contained in OC, could have a larger effect than naturally occurring estrogens on fuel selection during exercise. However, the few studies in which fuel selection was described in women regularly taking triphasic OC did not report any difference with W−OC (8,18,31) (see also Burrows and Peters (5) for review). In the present experiment, the %En from CHO oxidation was slightly lower in W+OC than in W−OC (51 and 54 %En), but this difference did not reach statistical significance (F-ratio = 3.305, P = 0.215, df = 10) and the effect size was small (d = 0.398). A post hoc analysis showed that this comparison was underpowered (power = 0.096) but that the number of subjects required to rule out a type II error would be difficult to achieve in this type of experiment (α = 0.05, power = 0.8, n = 202).
Effect of the high-CHO diet.
In men, CHO oxidation during prolonged exercise increases with the amount of CHO in the diet (see, e.g., Spriet and Peters (30), for review). In line with this observation, in the present experiment in men, CHO oxidation significantly increased when the CHO content in the diet increased from 55% to 80% of the energy intake (main effect, Fig. 2; e.g., from 30.0 to 33.7 mg·kg FFM−1·min−1) when no glucose was ingested. In women, the effect of a preexercise high-CHO diet on fuel selection during prolonged exercise has been described in only two studies (33,37). In the study by Walker et al. (37), no comparison was made with men, but a significant increase in CHO oxidation (from 51.6 to 59.7 mg·kgFFM−1·min−1) was also observed during ∼110-min time trials at ∼80% V˙O2max after a high-CHO diet. In the study by Tarnopolsky et al. (33), the %En from CHO oxidation, in women, increased from 69% to 75% during a 60-min exercise at 75% V˙O2max after 4 d of a high-CHO diet. In contrast, this diet did not modify the %En from CHO oxidation in men (84% and 83%, respectively, after the diet low and high in CHO). On the basis of these observations, we expected that, in the present experiment, the effect of the high-CHO diet on fuel selection during exercise could be higher in women than in men, but the results observed do not support this hypothesis. Indeed, the effect of the high-CHO diet on the increase in %En from CHO oxidation was not significantly different (main effect) in men and in women both when water (from 62 to 70 %En in men and 53 to 62 %En in women [pooled data]) and glucose were ingested during exercise (from 68 to 73 %En in men and 65 to 75 %En in women [pooled data]). The high-CHO diet also resulted in similar increases in %En from CHO oxidation in W−OC and W+OC with water and glucose ingestion. Taken together, these observations indicate that when compared with fuel selection after a mixed diet, the increase in %En from CHO oxidation after a high-CHO diet is similar in men, W−OC and W+OC, both when water and glucose are ingested during exercise. It should be recognized that the high-CHO diet (but not the mixed diet) was ingested after a prolonged exercise period to favor subsequent muscle glycogen storage. This could partly explain the difference in fuel selection observed after the mixed and high-CHO diets, but the bias introduced by the glycogen depleting exercise, if any, would be similar in the three groups of subjects.
Effect of glucose ingestion during exercise.
Ingestion of CHO during exercise also increases CHO oxidation in men (see Coggan and Coyle (9) for review) and in W−OC subjects (1,6,16,28,38,39). A direct comparison of CHO ingestion during exercise on the %En from total CHO oxidation between men and women was made only by Harger-Domitrovich et al. (16), Riddell et al. (28), and Wallis et al. (38). In the study by Riddell et al. (28), no sex difference was observed for the increase in %En from CHO oxidation when glucose was ingested during exercise (73 to 79 %En and 70 to 75 %En in men and women, respectively). In contrast, this increase was larger in men than in women in the study by Harger-Domitrovich et al. (16) (46 to 69 %En and 55 to 70 %En, respectively) but was smaller in the study by Wallis et al. (38) (50 to 60 %En and 40 to 54 %En in men and women, respectively). In the present experiment, the increase in %En from CHO oxidation when glucose was ingested during exercise was also higher in women (both in W−OC and W+OC) than in men (group × ingestion interaction). These differences between results from Harger-Domitrovich et al. (16) and Riddell et al. (28) on one hand, and those from Wallis et al. (38) and the present experiment on the other hand, may be attributable to differences in the amount of CHO ingested. When corrected for FFM, in the study by Wallis et al. (38) (21.5 and 28.0 mg·kg FFM−1·min−1 in men and women, respectively), as well as in the current study (18.0 and 21.1 mg·kg FFM−1·min−1 in men and women, respectively), the amount of CHO ingested was about 17%-30% larger in women than in men. The ingestion rates were relatively similar or equal in the study by Riddell et al. (28) (14.5 and 15.5 mg·kg FFM−1·min−1 in men and women, respectively) and by Harger-Domitrovich et al. (16) (10.0 mg·kg FFM−1·min−1 for both sexes).
Exogenous CHO oxidation.
A large number of studies have described exogenous glucose oxidation during exercise in men (see, e.g., Jeukendrup (19) for review), but a comparison between men and women was made only by M'Kaouar et al. (24), Riddell et al. (28), and Wallis et al. (38). In these three studies, when compared with the value observed in men (11-24 %En), the %En from exogenous glucose oxidation was consistently higher in women (14-26 %En). Although this sex difference did not reach statistical significance, the effect size computed from the results reported by Wallis et al. (38) and Riddell et al. (28) is moderate to large (Cohen d = 0.76 and d = 0.94, respectively). Results from the present experiment are in line with these observations. The %En from exogenous glucose oxidation was also slightly but not significantly higher in women (pooled) than in men with small to large effect sizes, both after the mixed (27 vs 26 %En, d = 0.34) and high-CHO diet (27 vs 25 %En, d = 0.80). On the basis of data from Riddell et al. (28), Wallis et al. (38), M'Kaouar et al. (24), and the present experiment, post hoc analyses revealed that none of these studies had sufficient statistical power to firmly conclude that exogenous glucose oxidation during exercise is not different in men and women. The number of subjects required to rule out a type II error, again, largely exceeds the actual sample size in this type of study (∼50-300 vs 6-8 subjects). It should be recognized, as already discussed for total CHO oxidation, that the consistently higher %En from exogenous glucose oxidation observed in women than in men (15% vs 11% in Riddell et al. (28), 26% vs 24% in M'Kaouar et al. (24), 22% vs 18% in Wallis et al. (38), and 27% vs 26% in the present study) could be attributable to the higher dose ingested when expressed in milligrams per kilograms of FFM per minute (mg·kg−1 FFM·min−1).
Finally, all the studies reporting exogenous glucose oxidation during exercise in women have been conducted in W−OC. Estrogens, particularly synthetic estrogens in OC (31), have been shown to decrease the rate of plasma glucose appearance and disappearance during exercise (7,13,31). Thus, it could have been suspected that the uptake and oxidation rate of exogenous glucose would be lower in W+OC. Results from the present experiment do not confirm this hypothesis and suggest that triphasic OC have only a limited effect, if any, on the metabolic fate of exogenous glucose during exercise.
Endogenous CHO oxidation.
In men, when CHO are ingested during exercise, the increase in total CHO oxidation (see above) is entirely attributable to the oxidation of exogenous CHO, which reduces endogenous CHO oxidation (19). In the study by Riddell et al. (28), when glucose was ingested during exercise, there was a trend for a greater reduction in endogenous CHO oxidation in women than in men (−10 vs −4 %En; P = 0.05). This trend for a sex difference in endogenous CHO oxidation was not confirmed in the study by Wallis et al. (38) (−8 %En in both men and women) and cannot be ascertained from the study by M'Kaouar et al. (24) in which no situation with water ingestion was included. Results from the present experiment show that, as expected, glucose ingestion significantly reduced the %En from endogenous CHO oxidation. In addition, a significant group × ingestion interaction was present, and post hoc comparisons indicated that the effect of glucose ingestion on the %En from endogenous CHO oxidation was larger in men than in women after both diets. As a result, the significant difference in %En from endogenous CHO oxidation observed between men and women when water was ingested (62 %En in men vs. 54 and 51 %En in W−OC and W+OC, respectively, after the mixed diet; 70 %En in men vs 63 and 60 %En in W−OC and W+OC, respectively, after the high-CHO diet) disappeared when glucose was ingested during exercise (38-42 and 47-50 %En after the mixed and high-CHO diets, respectively).
Taken together, these observations indicate that when compared with fuel selection when water is ingested during exercise, the decrease in %En from endogenous CHO oxidation when glucose is ingested is lower in W−OC and W+OC than in men, both after the mixed and high-CHO diets.
High-CHO diet combined with glucose ingestion.
The combined effects of a high-CHO diet and of CHO ingestion during exercise on fuel selection have only been described by Widrick et al. (40) in men (∼120 min of cycling at 75% V˙O2max) and by Andrews et al. (1) in women (∼135 min of cycling at 69% V˙O2max). In these studies, as observed in the present study, when compared with the control situation, the %En from CHO oxidation was markedly and significantly increased when the two procedures of CHO supplementation were combined: from 62 to 84 %En (40) and from 59 to 71 %En (1). However, in the study by Widrick et al. (40), this effect was entirely attributable to the high-CHO diet: no further increase in the %En from CHO oxidation was observed when CHO ingestion was combined to the high-CHO diet. In the study by Andrews et al. (1), although the %En from CHO oxidation, when both the high-CHO diet and glucose ingestion were combined, was higher than when glucose was ingested after the mixed diet (67 and 71 %En), this did not reach statistical significance. These results could suggest that the %En from CHO oxidation is more sensitive to changes in the diet in men and to CHO ingestion during exercise in women.
In the present study, as discussed above, when compared with the control situation, both the high-CHO diet and the glucose ingestion during exercise separately increased the %En from CHO oxidation (main effects). A further significant increase was observed when these two procedures were combined without any difference among the three groups (main effect of trial without interaction). Differences between the results from studies by Widrick et al. (40), Andrews et al. (1), and the present experiment could be attributable to differences in the diet and the rate of CHO ingestion. In the study by Widrick et al. (40), the comparison was made between a low- and a high-CHO diet, and the amount of glucose ingested during exercise was low (12.5 mg·kg−1·min−1). This could explain that the effect of CHO supplementation on fuel selection was entirely attributable to the high-CHO diet with no additional effect of glucose ingestion. In contrast, in the study by Andrews et al. (1) as well as in the present study, the comparison was made between mixed and a high-CHO diet. However, the glucose ingestion rate was much larger in the study by Andrews et al. (1) than in the present experiment (25.0 and 16.6 mg·kg−1·min−1, respectively). This could explain why, in the present experiment, but not in that from Andrews et al. (1), a cumulative effect of the two modes of CHO supplementation was observed.
In the present experiment, after a mixed diet with water ingestion during exercise, the %En from CHO oxidation was higher in men than in both W−OC and W+OC. Supplementation with CHO, either through a preexercise high-CHO diet or through glucose ingestion during exercise, increased the %En from CHO in both men and women, and the sex difference observed in the control situation disappeared. However, the increase in the %En from total CHO oxidation observed when glucose was ingested during exercise and when combined with a high-CHO diet was larger in women than in men. This was not attributable to a higher %En from exogenous glucose oxidation in women, for which no sex difference was observed, but was attributable to a smaller decrease in endogenous glucose oxidation. Finally, no difference in fuel selection was observed between the two groups of women, except for a slightly but not significantly lower reliance on CHO oxidation in W+OC than in W−OC in the control situation.
This work was funded by the Natural Sciences and Engineering Research Council of Canada.
The results from the present study do not constitute endorsement by the American College of Sports Medicine.
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