We investigated the effect of a 31-day ketogenic (Keto) diet (KD) on submaximal exercise capacity and efficiency.
A repeated-measures, crossover study with pre- and post-intervention outcomes was conducted in eight trained male endurance athletes (VO2max, 59.4 ± 5.2 ml⋅kg⋅min). Participants ingested their habitual diet (HD) (43 ± 8 % carbohydrate and 38 ± 7 % fat) or an iso-energetic KD (4 ± 1 % carbohydrate and 78 ± 4 % fat) from day 0 to 31 (p < 0.001). On days -2 and 29, participants undertook a fasted graded metabolic test (~ 25 min) and on days 0 and 31, participants completed a run-to-exhaustion trial at 70 % of their maximal oxygen uptake (VO2max) (~12.9 km⋅hr) following the ingestion of a high-carbohydrate meal (2 g⋅kg) or an iso-energetic low-carbohydrate, high-fat meal, with carbohydrate (~55 g⋅hr) or iso-energetic fat (coconut oil) supplementation during exercise.
Training load did not differ between trials and there was no effect of diet on VO2max (all p > 0.05). The KD impaired exercise efficiency, particularly at >70 % VO2max, as evident by oxygen uptake that could not be explained by shifts in respiratory exchange ratio and increased energy expenditure (all p < 0.05). However, exercise efficiency was maintained on a KD when exercising at <60 % VO2max (all p > 0.05). There was no effect of diet on time-to-exhaustion (pre-HD, 237 ± 44 vs post-HD, 231 ± 35 min; p = 0.44 and pre-KD, 239 ± 27 vs post-KD, 219 ± 53 min; p = 0.36).
A 31-day KD can preserve submaximal exercise capacity in trained endurance athletes, however, endurance variability increases.
Med Sci Sports Exerc. 2019 Apr 25. doi: 10.1249/MSS.0000000000002008. [Epub ahead of print]