Effect of a Ketogenic Diet on Submaximal Exercise Capacity and Efficiency in Runners

David M. Shaw; Fabrice Merien; Andrea Braakhuis; Ed Maunder; Deborah K. Dulson


Med Sci Sports Exerc. 2019;51(10):2135-2146. 

In This Article

Abstract and Introduction


Purpose: We investigated the effect of a 31-d ketogenic diet (KD) on submaximal exercise capacity and efficiency.

Methods: A randomized, repeated-measures, crossover study was conducted in eight trained male endurance athletes (V̇O2max, 59.4 ± 5.2 mL·kg−1·min−1). Participants ingested their habitual diet (HD) (13.1 MJ, 43% [4.6 g·kg−1·d−1] carbohydrate and 38% [1.8 g·kg−1·d−1] fat) or an isoenergetic KD (13.7 MJ, 4% [0.5 g·kg−1·d−1] carbohydrate and 78% [4 g·kg−1·d−1] fat) from days 0 to 31 (P < 0.001). Participants performed a fasted metabolic test on days −2 and 29 (~25 min) and a run-to-exhaustion trial at 70% V̇O2max on days 0 and 31 following the ingestion of a high-carbohydrate meal (2 g·kg−1) or an isoenergetic low-carbohydrate, high-fat meal (<10 g CHO), with carbohydrate (~55 g·h−1) or isoenergetic fat (0 g CHO·h−1) supplementation during exercise.

Results: Training loads were similar between trials and V̇O2max was unchanged (all, P > 0.05). The KD impaired exercise efficiency, particularly at >70% V̇O2max, as evidenced by increased energy expenditure and oxygen uptake that could not be explained by shifts in respiratory exchange ratio (RER) (all, P < 0.05). However, exercise efficiency was maintained on a KD when exercising at <60% V̇O2max (all, P > 0.05). Time-to-exhaustion (TTE) was similar for each dietary adaptation (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). Following keto-adaptation, RER >1.0 vs <1.0 at V̇O2max coincided with the preservation and reduction in TTE, respectively.

Conclusion: A 31-d KD preserved mean submaximal exercise capacity in trained endurance athletes without necessitating acute carbohydrate fuelling strategies. However, there was a greater risk of an endurance decrement at an individual level.


Humans elicit numerous metabolic adaptations in response to shifts in dietary macronutrient intake to reconcile substrate availability with energy expenditure (EE). During continuous submaximal endurance exercise (>3–4 h), exhaustion appears to be associated with depleted endogenous carbohydrate (CHO) stores (i.e., skeletal muscle[1] and hepatic[2] glycogen) and the inability to maintain the CHO oxidation rates exhibited during the early stages of exercise.[1,3] As such, various dietary training strategies have been proposed to spare finite glycogen stores (~700 g) and to optimize competition fuelling.[4] CHO loading and supplementation seem to be the most efficacious strategies for prolonging exercise capacity and improving endurance performance by maximizing glycogen content and preserving CHO oxidation rates throughout exercise.[1,3,5] However, interest has persisted in chronic adaptation to low-CHO diets in an attempt to spare endogenous CHO stores by maximizing fat oxidation rates and increasing hepatic production of fatty acid-derived ketone bodies (KB) as an additional fuel source.[6,7]

Very low-CHO, high-fat, ketogenic diets (KD) contrast typical recommendations for endurance athletes. These are typically characterized by CHO intake <50 g·d−1 and elevated circulating KB (primarily D-β-hydroxybutyrate (D-βHB)) concentrations >0.5 mmol·L−1,[7] although concentrations >0.2 mmol·L−1 may be accepted.[8] The term keto-adaptation has been used to encompass the metabolic adaptations resulting from a KD, which include the following: 1) increased maximal fat oxidation (MFO) to >1 g·min−1;[9–11] 2) reduced blood glucose utilization;[9] and 3) reduced muscle[9,12] and hepatic[12] glycogen utilization during exercise. The importance of KB to EE is uncertain; however, it is postulated as the defining feature differentiating adaptation to ketogenic versus nonketogenic, lower-CHO (~2.5 g CHO·kg−1·d−1 or <25% energy intake (EI)), higher-fat (LCHF) diets (i.e., fat-adaptation). It seems that a minimum of 3–4 wk is required to overcome the initial performance decrement associated with a KD,[9,11] and despite a suggestion that several months is required to optimize keto-adaptation, the only studies having investigated athletes ingesting a KD for this duration did not examine performance[10,12] or failed to rigorously monitor dietary intake and training volume.[13]

Recently, a 3-wk KD negated high-intensity exercise performance in a ~45-min time trial in elite race walkers.[11] However, performance during prolonged, high-intensity events (<2–3 h) demands high rates of CHO, not fat, oxidation.[4,5] As such, keto- (or fat-) adaptation is more likely to benefit submaximal exercise events lasting several hours, as the practically infinite fat stores become the preferred energetic substrate. A single study has investigated the effect of a KD on submaximal exercise capacity (62%–64% of maximal oxygen uptake (V̇O2max).[9] The researchers used a single-arm design, with the pretest acting as the CHO-diet trial and the posttest after 4 wk of ingesting a KD. Of the five trained cyclists, three improved and two impaired their time-to-exhaustion (TTE), resulting in no overall difference between dietary conditions (147 vs 151 min). However, there was the potential of an order, or training, effect, and the results were heavily skewed by the improvement of a single participant from 148 to 232 min. In addition, for the CHO-diet trial, participants commenced exercise after an overnight fast and abstained from CHO during exercise, which is incongruent with recommended performance nutrition strategies.[5,14] Therefore, the study design favored the keto-adapted trial.

In the same study, the researchers stated the efficiency of substrate oxidation improved after keto-adaptation because of a similar oxygen uptake at the same absolute workload.[9] Because the stoichiometry of fat compared with CHO oxidation requires more oxygen for combustion to generate an identical energy yield, it would have been expected that oxygen uptake increased after the KD if exercise efficiency was maintained. This increased oxygen cost of exercise after keto- or fat-adaptation may impair exercise efficiency during prolonged, high-intensity endurance exercise;[11] however, the shift in oxygen uptake at submaximal intensities may simply be a reflection of substrate preference, not exercise efficiency.[15] More appropriate measures of exercise efficiency may be the energy cost of exercise[15] and the discrepancy between measured and predicted oxygen uptake based on shifts in the RER.[16,17] Although differences in these measures tend to be subtle, they may elicit significant effects on submaximal exercise capacity.

To our knowledge, no studies have investigated the effect of keto-adaptation on submaximal exercise capacity in trained endurance athletes using a randomized, repeated-measures, crossover design and acute fuelling strategies to polarize substrate availability and metabolism. In concordance, the aim of the present study was to examine the effect of a 31-d KD on submaximal endurance capacity, substrate utilization, and exercise efficiency.