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

Abstract

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 (V˙O2max, 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% V˙O2max 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 V˙O2max was unchanged (all, P > 0.05). The KD impaired exercise efficiency, particularly at >70% V˙O2max, 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% V˙O2max (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 V˙O2max 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.

Effect of Low-and High-carbohydrate Diets on Swimming Economy: A Crossover Study

“…Results from the present study are in contrast with previous research reporting an effect of diet on movement economy in cyclists, runners, and race walkers (15)(16)(17). It is possible that exercise intensities used in our study may have been lower than in the previous studies and, therefore, more reliant on fat oxidation.…”

“…Differences in training status may also have affected our results, as higher-caliber athletes generally have better running economy (2) and cycling e ciency (24). The VO 2max values of our participants averaged 42.4 ml kg − 1 min − 1 whereas other studies showing an effect of diet on economy used participants with a mean VO 2max of 56-66 ml kg − 1 min − 1 (15)(16)(17). However, swimmers tend to have lower VO 2max values measured while swimming compared to running (25,26) and triathletes record higher VO 2max values when cycling or running compared to swimming (26,27), so we likely underestimated the true VO 2max of our subjects.…”

“…It is possible that exercise intensities used in our study may have been lower than in the previous studies and, therefore, more reliant on fat oxidation. In support of this, Shaw et al (16) reported that running economy was impaired at intensities over 70% VO 2max , but preserved at intensities lower than 60% VO 2max , following 31 days of a lowcarbohydrate diet in trained runners. Additionally, four weeks of a LCHF diet had no impact on cycling economy in endurance-trained athletes cycling at ~ 63% VO 2max (22).…”

“…An increase in cycling e ciency (i.e., lower rate of energy expenditure at the same power) has been reported following three days of a high-carbohydrate diet (15). Movement economy has also decreased (i.e., higher rate of energy expenditure at the same velocity) in runners and race walkers following 3-4 weeks of low-carbohydrate dietary interventions (16,17).…”

A Short-Term Ketogenic Diet Impairs Markers of Bone Health in Response to Exercise

“…Unlike the previous investigations, we identified clear and consistent effects on bone metabolism at rest and in response to exercise following 3.5-weeks of a ketogenic LCHF diet (Figures 2-4), with increases in a marker of bone resorption (CTX) and decreases in markers of bone formation (P1NP) and metabolism (OC). Although some might argue that a complete adaptation to a LCHF diet requires much longer than the 3.5week period utilized in the current study, it should be noted that adaptations in substrate metabolism and exercise economy have been reported across this (19,25), and much shorter (26), time periods. Nevertheless, the current study is reflective of a shorterterm adaptation to a LCHF diet and our findings warrant further investigation across longer time periods.…”