At baseline, the participants ate a high-carbohydrate diet and completed a 14-kilometer tempo hill-training session. This same training session was completed weekly during the study.

The participants ate either a high-carbohydrate diet or a ketogenic diet for 4 to 5 days before starting the 3-week study. The participants chose which group they wanted to be in, as opposed to random assignment. This was done to control for the potential influence of an individual’s beliefs. For example, the ketogenic diet could impair an individual’s performance just because they believed it would.

High-carbohydrate diet: 8 grams of carbohydrate per kg of body weight per day (63%–65% of calorie intake), 2 grams of protein per kg of body weight, and the remaining calories allotted to fat. The participants consumed 1 to 1.5 grams of carbohydrate per kg of body weight 60 to 90 minutes before the training session and 40 grams of carbohydrate from a sports drink during the training session.
Ketogenic diet: 0.5 grams of carbohydrate per kg of body weight per day (<50 grams or < 5% of calorie intake), 78% of calories from fat, and 2.2 grams of protein per kg of body weight per day. The participants consumed a low-carbohydrate high-fat snack before the training session and an electrolyte drink during the training session.

On weeks 2 and 3, the participants chewed a caffeine-containing gum that provided approximately 3 mg of caffeine per kg of body weight or a placebo gum 20 minutes before the training session.

At baseline, the participants who would eventually be assigned to the ketogenic diet group completed the hill session faster than the participants who would eventually be assigned to the high-carbohydrate diet group.

However, during the study, the completion time did not differ between groups on weeks 1 and 3, and the completion time was faster in the high-carbohydrate diet group on week 2.

Looking at changes within each group, the high-carbohydrate diet group improved their performance from baseline in weeks 2 and 3 (medium to large effect size). In contrast, the ketogenic diet group’s performance was worse than baseline in weeks 1 and 2 (large effect size).

Hill training session completion time relative to baseline

Supplementing with caffeine led to 2.4% faster completion times in both groups (a small effect size).

Carbohydrates and caffeine during high-intensity exercise Original paper