Caffeine Shifts Substrate Utilization Toward Fat During Outdoor Running at Fatmax: A Randomized Field-Based Crossover Study.

Purpose

Although caffeine-induced increases in fat oxidation are well established under laboratory conditions, evidence during field-based exercise is limited. Therefore, this study aimed to examine the acute effects of 3 mg·kg-1 of caffeine on fat oxidation during a 1-hour field-based outdoor run performed at the individual intensity eliciting maximal fat oxidation (Fatmax).

Methods

Fourteen healthy, endurance-trained runners (11 men, 3 women; V˙O2max = 58.6 [6.3] mL·kg-1·min-1) participated in a randomized, double-blind, placebo-controlled, crossover experiment. Participants completed 1 preexperimental trial to determine V˙O2max and Fatmax, followed by 2 experimental trials. During the experimental trials, participants ingested either caffeine (3 mg·kg-1) or a placebo 60 minutes before a 1-hour run at their individual Fatmax intensity covered on a 1.97-km outdoor running course. Substrate oxidation rates and energy expenditure were measured via indirect calorimetry using a portable gas analyzer and calculated through stoichiometric equations while heart rate was continuously monitored and ratings of perceived exertion were recorded using the Borg scale.

Results

Caffeine significantly increased total fat oxidation during the 1-hour run (33.62 [9.23] vs 40.46 [11.32] g; P = .018; d = 0.76) and decreased carbohydrate oxidation (93.16 [37.84] vs 77.32 [30.35] g; P = .036; d = 0.66) compared with placebo, with no effect on total energy expenditure (P = .936). Caffeine also reduced the rating of perceived exertion (9 [2] vs 8 [2]; P = .024; d = 0.75) with no effect on heart rate (P = .140).

Conclusion

Acute ingestion of 3 mg·kg-1 of caffeine enhances fat oxidation during 1 hour of running at Fatmax under field-based conditions, suggesting that caffeine may be an effective nutritional strategy to optimize fat utilization during real-world submaximal endurance exercise.