Journal of sports science & medicine 2018 05 1417(2) 259-268


The purpose of the study was to examine the effects of altering from habitual mixed Western-based (HD) to a very low-carbohydrate high-fat (VLCHF) diet over a 4-week timecourse on performance and physiological responses during high-intensity interval training (HIIT). Eighteen moderately trained males (age 23.8 ± 2.1 years) consuming their HD (48 ± 13% carbohydrate, 17 ± 3% protein, 35 ± 9% fat) were assigned to 2 groups. One group was asked to remain on their HD, while the other was asked to switch to a non-standardized VLCHF diet (8 ± 3% carbohydrate, 29 ± 15% protein, 63 ± 13% fat) for 4 weeks. Participants performed graded exercise tests (GXT) before and after the experiment, and an HIIT session (5x3min, work/rest 2:1, passive recovery, total time 34min) before, and after 2 and 4 weeks. Heart rate (HR), oxygen uptake (O), respiratory exchange ratio (RER), maximal fat oxidation rates (Fat) and blood lactate were measured. Total time to exhaustion (TTE) and maximal O (V̇O) in the GXT increased in both groups, but between-group changes were (ES ± 90% CI: -0.1 ± 0.3) and (0.57 ± 0.5), respectively. Between-group difference in Fat change (VLCHF: 0.8 ± 0.3 to 1.1 ± 0.2 g/min; HD: 0.7 ± 0.2 to 0.8 ± 0.2 g/min) was (1.2±0.9), revealing greater increases in the VLCHF versus HD group. Between-group comparisons of mean changes in V̇O and HR during the HIIT sessions were to , whereas mean RER decreased more in the VLCHF group (-1.5 ± 0.1). Lactate changes between groups were . Adoption of a VLCHF diet over 4 weeks increased Fat and did not adversely affect TTE during the GXT or cardiorespiratory responses to HIIT compared with the HD.