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.