Metabolic flexibility and endurance performance : a thesis submitted for the degree of Doctor of Philosophy, School of Sport and Exercise, College of Health, Massey University
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This thesis examined the sex-specific biochemical, physiological and physical performance responses of highly-trained endurance athletes to chronic moderate and low carbohydrate (CHO) training diets. In addition, a novel exogenous ketone supplement was studied to examine its effects on participants’ physiology and performance during the two contrasting diets. STUDY ONE: This study was designed to test whether adaptation to a low CHO diet affects physical capacity during prolonged exercise. Thirteen highly-trained endurance athletes (eight males, VO2max = 66.0 ± 9.5 ml/kg/min; five females, VO2max = 50.6 ± 8.4 ml/kg/min) consumed a moderate (>5 g CHO/kg/day) or low (<2 g CHO/kg/day) CHO training diet for four weeks, in a randomised cross-over design. Performance was measured, after a 24 h moderate CHO “loading” regime, through a self-paced time trial to complete a fixed workload, equivalent to five hours at a workload calculated to elicit 55% VO2max. Although time-to-complete was not significantly different between diets, the average absolute (watts) and relative (W/kg) power outputs were significantly better on the low CHO diet (p = 0.03 and 0.02 respectively). Both sexes responded similarly in terms of performance, whilst only women significantly improved body composition when CHO was restricted (p = 0.02). It was concluded that when CHO is restricted during training, trained endurance athletes show improved ultra-endurance performance relative to their body mass. STUDY TWO: This study was designed to test the sex specific response to a low CHO diet during fasted endurance exercise. The participants and dietary restrictions were the same as outlined in Study One. Physiological measures were collected before, during and after a two-hour ride at a fixed power output, equivalent to 60 % VO2max. The ride was undertaken after an overnight (>12 hours) fast and completed at three points throughout each dietary intervention (baseline, week two, week four). As expected there were a significant main effect of diet and time on substrate oxidation rates during fasted exercise (p < 0.05). The low CHO diet resulted in lower CHO oxidation and higher fat oxidation (FATox) in both sexes throughout the exercise. The degree of ‘adaptation’ to low CHO intake increased from baseline to week four, with significant interactions between trial and diet (p < 0.05). There was a sex specific negative correlation between the rate of CHO oxidation and perceived exertion (RPE) at the end of the fasted exercise (p = 0.001). Women consistently had a higher RPE at the end of the exercise (p = 0.04). These data show that both men and women can increase their rates of FATox, in a time-dependent manner, when CHO is restricted in the training diet. STUDY THREE: This study was designed to examine the differences in the blood metabolome of highly- trained male endurance athletes (VO2max = 66.0 ± 9.5 ml/kg/min) who each underwent two contrasting dietary interventions, in a randomised crossover design as follows: four weeks moderate (>5 g CHO/kg/day) or low (<2 g CHO/kg/day) CHO. Exercise training was controlled during both conditions. Fasting venous blood samples were collected before and after exercise at 60% VO2max and the plasma metabolome was analysed using 700 Hz H1 nuclear magnetic resonance (NMR) spectroscopy. Unsupervised (PCA) and supervised (PLSA-DA & OPLS-DA) multivariate statistical analysis models failed to statistically separate the sample groups in regards to the dietary intervention. However, both methods of supervised discriminant analysis (PLS-DA and OPLS-DA) could separate groups based on time (i.e. pre–post exercise). The variable influence on projection (VIP) was used to identify the individual metabolites causing the group separation within the discriminant analysis. Metabolites were analysed using two-way ANOVA and paired t-tests, with the only significant difference being the blood glucose response to exercise at the end of each dietary intervention (p = 0.006). In conclusion, neither the resting nor exercising metabolome is significantly influenced by the CHO content of the diet. This indicates that endurance-trained individuals possess the metabolic flexibility to counter changes in dietary CHO availability and maintain a normal circulating metabolic profile. STUDY FOUR: The aim of this case study was two-fold: to test the effectiveness of a proposed study, and to explore the validity of reports which have claimed that ingesting a ketone supplement can improve endurance performance. One highly-trained male triathlete (VO2max = 73.0 ml/kg/min) completed four time-to-exhaustion (TTE) cycling bouts, each preceded by two hours of cycling at 60% VO2max (power = 213 W). The exercise bouts were completed in a crossover design as follows: ketogenic diet (<1.5 g CHO/kg/day) and regular (non-ketogenic) sports drink (K), ketogenic diet with ketone-containing drink (K+KS), high CHO diet (>5 g CHO/kg/day) and regular sports drink (CHO), moderate CHO diet and ketone-containing drink (CHO+KS). Ketosis was confirmed with sustained resting blood β –hydroxybutyrate (β- HB) levels of >0.2 mM. Ketone supplementation was associated with better performance following both dietary interventions, with CHO+KS being better than K+KS (12:54 minutes vs 13:32 minutes, respectively). Ketone supplementation resulted in higher [β-HB] during exercise relative to the sports drink (0.63 & 0.78 mM vs 0.20 & 0.25 mM, respectively). VO2 and blood lactate did not noticeably differ during the fixed intensity ride, but differed greatly during the TTE, with VO2 beginning higher on the high CHO diet. The results from this study show the potential benefits of ingesting a ketone supplement on endurance performance and suggest that the moderate CHO status of the individual may have an additive effect. Based on these results, it was suggested that a full scientific study be carried out to further test the effectiveness of ketone supplementation on endurance performance. STUDY FIVE: The aim of this study was to test the effects of ingesting a ketone supplement on endurance performance in two different metabolic states, induced by dietary interventions. Six well- trained male endurance athletes (age: 29 ± 9 yrs, mass: 74.1 ± 7.7 kg, VO2max: 64.1 ± 5.8 ml/kg/min) underwent a randomised, double-blinded, placebo-controlled protocol, consisting of two dietary interventions, completed as a cross-over design. Following each dietary intervention, a performance session was carried out, during which, participants drank either a ketone-containing (KS) or placebo (PLB) drink. Thus, the performance session was carried out a total of six times; habitual diet (BASE1, BASE2), moderate-CHO diet + PLB (PLB+CHO), moderate-CHO diet + KS (KS+CHO), ketogenic diet + PLB (PLB+K), ketogenic diet + KS (KS+K). Physiological measures were taken during each performance session, which consisted of a 40-minute fixed intensity ride, followed by a self-paced time trial (TT), to complete a fixed workload equivalent to 20 minutes at 75% VO2max. There were no main effects or interactions between diet and KS on TT performance or body mass. The KS significantly increased the beta-hydroxybutyrate concentration [β-HB] in the blood at rest and during exercise (peak = 1.1 mM) (p = 0.001). The KS caused an attenuated blood lactate response during the TT compared to baseline and PLB. The respiratory exchange ratio (RER) was significantly lower on the ketogenic diet at rest and throughout fixed intensity exercise but did not differ during the TT. It is concluded that the circulating [β-HB] attained were not high enough to significantly contribute to muscular energy provision via oxidative phosphorylation and that future research into ketone supplements and exercise performance should ensure that a minimum of 2 mM [β-HB] is obtained. Further, the CHO status of the individual can be largely ignored as supplementation appears to be equally effective irrespective of the CHO status.
The following Figures were removed for copyright reasons but may be accessed via their sources: Figs 2-2 (=Abo Alrob & Lopaschuk, 2014 Fig 2), 2-3 (=van Loon, 2004 Fig 2), 2-4 (=Brooks & Mercier, 1994 Fig 1), Figs 2-6 & 2-7 (=McGarry & Foster, 1980 Figs 2 & 3), 2-10 (=Beckonert et al., 2007 Fig 2) & 2-11 (=Trygg et al., 2007 Fig 6).
Athletes, Nutrition, Low-carbohydrate diet, Endurance sports, Physiological aspects, Ketones, Physiological effect, Dietary supplements