Caffeine increases fat oxidation during fed-state aerobic exercise Original paper

In this meta-analysis of randomized controlled trials, supplementation with caffeine before exercise increased fat oxidation during fed-state aerobic exercise.

The effect of supplementation with caffeine before exercise on fat oxidation during fed-state aerobic exercise (exercise within 5 hours of having eaten a meal).

A total of 228 adults (average ages of 19–42; 81% men, 19% women) with no known health conditions.

The participants in the study included aerobically trained athletes (cyclists, runners, recreational athletes, and sports players), physically active men and women, and untrained men and women. The study’s participants also included both habitual caffeine users and nonusers.

A meta-analysis of 18 randomized controlled trials was performed.

The exercise protocol involved continuous aerobic exercise intensity in 12 of the trials and incremental aerobic exercise intensity in the other 6 trials. A cycle ergometer (a type of stationary bike) was used in 14 trials and a treadmill was used in 4 of the trials. In most of the trials, the exercise test was performed 1–4 hours after the last meal of the day. The dose of supplemental caffeine ranged from 3 mg to 9 mg of caffeine per kg of body weight (mg/kg) and was administered 30–120 minutes before the start of the exercise test.

Supplementation with caffeine increased fat oxidation during fed-state aerobic exercise to a moderate degree.

Subgroup analyses:

• Caffeine dose: Supplementation with caffeine increased fat oxidation in the trials that administered less than 6 mg/kg of body weight, but not in the trials that administered 6 mg/kg or more. However, the lack of a statistically significant effect in the latter subgroup may have been due to the inclusion of one outlier trial.^[1]
• Training status: Supplementing with caffeine increased fat oxidation in the trials that recruited physically active participants, but not in the trials that recruited trained athletes/untrained participants. Again, the lack of a statistically significant effect in the trained athletes/untrained participants may have been due to the inclusion of the aforementioned outlier trial.^[1]
• Habitable use of caffeine: Supplementation with caffeine increased fat oxidation in the trials that recruited noncaffeine users, but not in the trials that recruited habitual caffeine users. However, only 3 trials were included in the latter subgroup.

All of the trials included in the meta-analysis were of fair or high methodological quality.