Effect of short-term macronutrient manipulation on endurance capacity of long-distance runners
Introduction: The influence of specific nutrition programmes on optimal endurance performance enjoys wide interest. However, limited knowledge in this regard accentuates the need for further research on optimal nutrition for individual endurance performance optimisation. Objectives: (i) To investigate differences in the effects of a short-term (48-hour) highcarbohydrate (high-CHO) versus a high-FAT diet on indirect respiratory indices of long-distance runners, namely maximal oxygen consumption (V̇ O₂max), oxygen consumption (V̇ O₂), carbon dioxide output (V̇ CO₂), respiratory exchange ratio (RER), minute ventilation (V̇ E), and substrate utilisation (CHO oxidation and fat oxidation), as well as on physiological and perceptual measurements such as time to exhaustion, absolute (W) and relative power output (W/kg) and work output (kJ), during a treadmill graded exercise test (GXT) to exhaustion. (ii) To determine certain threshold points that occurred during the GXT, including ventilatory threshold 1 (VT1), ventilatory threshold 2 (VT2), lactate lhreshold (LT), peak oxygen uptake (V̇ O₂peak) and maximal oxygen consumption (V̇ O₂max) after the high-CHO and high-FAT trials, respectively. (iii) To explore individual preferential fuel source use over a short-term period to enhance performance. Methods: This was a randomised controlled cross-over trial assessing the effects of a 48-hour high-CHO (67%CHO, 17%fat, 16%Prot) or 48-hour high-FAT (65%fat, 21%CHO, 14%prot) diet amongst 24 well-trained male endurance runners. After each 48-hour diet period and an overnight fast, the participants completed a GXT consisting of 3-minute stages with 1 km/h increments until exhaustion. The two dietary treatment periods were parted by a two-week washout period. The study treatments were compared with respect to the various measurements using ANOVA with diet, participant and period as fixed effects. From these ANOVAs, the mean values for each study treatment (high-FAT and high-CHO diets) were calculated, including a point estimate and 95% confidence interval (CI) for the mean difference "high-FAT – highCHO", the p-value associated with a test of the null-hypothesis of no difference between treatment means, and the effect size calculated as the ratio of the point estimate of the mean difference divided by the residual standard deviation from the ANOVA. Results: No statistically significant differences were observed between the diets with regard to any of the indirect indices measured [V̇ O₂max, V̇ O₂, V̇ CO₂, RER and V̇ E and carbohydrate oxidation (CHOox) and fat oxidation (FATox] as well as LT. Furthermore, no statistically significant differences were observed with regard to the physiological and perceptual responses (RPE, HR, time to exhaustion, work and absolute and relative power output). Moderate effect sizes were observed for V̇ O₂ at VT1 (d = 0.58) and at VT2 (d = 0.41), and for V̇ O₂max at VT1 (d = 0.61) and VT2 (d = 0.47). Otherwise, moderate effect sizes were observed for speed at VT1 (d = 0.48) and HR at V̇ O₂max (d = 0.41). For fat contribution, moderate effect sizes were observed at both VT1 (d = 0.40) and VT2 (d = 0.43), and a medium effect size at V̇ O₂max (d = 0.56). Conclusion: No statistically significant differences were seen between the effects of the short-term high-CHO and high-FAT diets on any of the respiratory and other indices measured in endurance runners during a GTX to exhaustion. However, some moderate effects sizes observed for some of the indices either favouring high-CHO or high-FAT depending on the individual, suggest that further research is justified, possibly involving longer-term diets.