A low-carbohydrate, high-fat (LCHF) or “ketogenic” diet has grown in popularity due to its ability to increase the rate of fat burning during exercise. For elite athletes this comes at the expense of athletic performance. The LCHF diet also increases concentration of ketones (substances produced by the liver that the muscles and brain can use as fuel) in the blood. Previous research suggests ketones are a more efficient fuel compared to carbohydrates or fats and that supplementing with ketones may improve performance. However, the interaction between LCHF diets and ketone supplementation on performance remains unexplored.
In this study, elite race walkers consuming a LCHF diet for five days increased their fat oxidation rates by about 300% but also increased oxygen consumption, making them less efficient during exercise. Taking a ketone supplement before exercise increased the concentration of circulating ketones in the blood, but did not change oxygen consumption when taken alone or in combination with the LCHF diet. Athletes consuming the LCHF diet and ketone supplements were 6% slower during a 10,000m race compared to athletes who consumed a high carbohydrate diet. These results suggest that ketone supplements do not alter exercise efficiency, and that a short-term LCHF diet does not increase the ability of the body to use this fuel during exercise.