The effects of a three-week restricted carbohydrate diet on exercise metabolism and performance of three cyclists

Introduction. Restricted carbohydrate diets (RCDs) have become popular amongst endurance athletes as a means of increasing fat oxidation and improving health outcomes. However, it is unclear if these adaptations improve exercise performance. Aim of Study. This case study investigated the effect of a three-week RCD on exercise metabolism and performance, alongside evaluating the subjective experiences, of three recreational cyclists. Material and Methods. Participants were randomly assigned to the normal diet (ND) (~50% CHO, ~30% fat, ~20% protein) or RCD (10% carbohydrate, 70% fat and 20% protein) and switched diets after three weeks. The participant’s performed a weekly laboratory assessment consisting of an incremental, sub-maximal cycling step-test and a 20-minute time trial (TT). Body fat (skinfold) measures were repeated after each diet and the participants recorded their food/drink intake using a smartphone app throughout the study. Results. Whole body peak fat oxidation, measured during sub-maximal cycling, markedly increased during the RCD (ND 0.61 ± 0.1 vs RCD 1.45 ± 0.3 g/min). There was no improvement in average power output during the TT for any participant following the RCD (243 ± 5 W) versus the ND (253 ± 5 W) condition. Two participants experienced a reduction (–8% and –10%) in (Σ7) skinfolds following the RCD. The participants’ subjective reports indicated an improvement in general dietary habits, but there were reports of increased perceived exertion to exercise during the RCD. Conclusions. The RCD increased whole-body fat oxidation, promoted positive subjective dietary habits and decreased body fat amongst amateur cyclists. However, these outcomes did not translate into improved exercise performance.