Effects of a 4-Week Very Low-Carbohydrate Diet on High-Intensity Interval Training Responses

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 (<(V)over dot> O-2), respiratory exchange ratio (RER), maximal fat oxidation rates (Fat(max)) and blood lactate were measured. Total time to exhaustion (TTE) and maximal <(V)over dot> O-2 (<(V)over dot>O-2max) in the GXT increased in both groups, but between-group changes were trivial (ES +/- 90% CI: -0.1 +/- 0.3) and small (0.57 +/- 0.5), respectively. Between-group difference in Fatmax 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 large (1.2 +/- 0.9), revealing greater increases in the VLCHF versus HD group. Between-group comparisons of mean changes in <(V)over dot>O-2 and HR during the HIIT sessions were trivial to small, whereas mean RER decreased more in the VLCHF group (-1.5 +/- 0.1). Lactate changes between groups were unclear. Adoption of a VLCHF diet over 4 weeks increased Fat(max) and did not adversely affect TTE during the GXT or cardiorespiratory responses to HIIT compared with the HD.