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

Abstract

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 (V̇O2), respiratory exchange ratio (RER), maximal fat oxidation rates (Fatmax) and blood lactate were measured. Total time to exhaustion (TTE) and maximal V̇O2 (V̇O2max) 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 V̇O2 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 Fatmax and did not adversely affect TTE during the GXT or cardiorespiratory responses to HIIT compared with the HD.

Keywords: High-intensity exercise; graded exercise test; high-fat diet; ketosis; low-carbohydrate diet.

Figures

Figure 1.

Dietary characteristics across the 5-day recordings before the intervention started (black bars) and during the 4-week experimental period (white bars) in the very low-carbohydrate high-fat diet (VLCHF) and high carbohydrate (HD) groups. Values are expressed as mean ± standard deviation.

Figure 2.

Mean (and standard deviation) V̇O2, HR, RER and lactate during the high-intensity interval training (HIIT) session before the experimental period (PRE), after 2 weeks (MID) and at 4 weeks (POST). The differences over time between the very low-carbohydrate high-fat diet (VLCHF) and habitual mixed Western diet (HD) groups expressed as magnitude-based inferences and their likelihood are shown (*possible, **likely, ***very likely, ****most likely. No asterisk means that 90% CL overlapped the thresholds for substantiveness - unclear difference) (see also Methods).

Figure 3.

Respiratory exchange ratio (RER) and lactate concentrations during HIIT in the very low-carbohydrate high-fat diet (VLCHF) and habitual mixed Western diet (HD) groups. x – before exercise, xx- after warm-up. The differences over time in lactate between the VLCHF and HD groups expressed as ES ± 90% CI and their likelihood are shown (*possible, **likely, ***very likely, ****most likely. No asterisk means that 90% CL overlapped the thresholds for substantiveness - unclear difference) (see also Methods). Lactate values are expressed as mean ± standard deviation.

Figure 4.

Fat oxidation during the graded exercise test in the very low-carbohydrate high-fat diet (VLCHF) and habitual mixed Western diet (HD) groups. The differences over time between the VLCHF and HD groups expressed as ES ± 90% CI and its likelihood is shown (***very likely) (see also Methods). Values are expressed as mean ± standard deviation.