Effect of caffeine co-ingested with carbohydrate or fat on metabolism and performance in endurance-trained men

We examined the effect of caffeine co-ingested with either carbohydrate or fat on metabolism and performance in eight endurance-trained subjects who p erformed a random order of four experimental trials consisting of 120 min o f steady-state ergometer cycling at 70% of maximal O-2 uptake (SS) followed by a time trial in which subjects completed a set amount of work (7 kJ kg( -1)) as quickly as possible. One hour before SS subjects ingested either 2. 6 g kg(-1) carbohydrate (CHO); 2.6 g kg(-1) CHO + 6 mg kg(-1) caffeine (CHO + CAF); 1.2 g kg(-1) fat with 2000 U I.V. heparin (FAT); or 1.2 g kg(-1) f at with 2000 U I.V. heparin + 6 mg kg(-1) caffeine (FATS CAF). The rate of carbohydrate oxidation was higher (mu mol kg(-1) min(-1): CHO, 243 +/- 39 a nd CHO + CAF, 239 +/- 30 vs. FAT, 196 +/- 48 and FAT + CAF, 191 +/- 55; P < 0.05, values are means +/- S.D.) and the rate of fat oxidation lower (<mu> mol kg(-1) min(-1): CHO, 19 +/- 8 and CHO + CAF, 22 +/- 7 vs. FAT, 35 +/- 1 9 and FAT + CAF, 37 +/- 17; P < 0.05) with carbohydrate than fat ingestion. Yet despite lower carbohydrate use with fat feeding, the time taken to com plete the time trial was less after carbohydrate than after fat ingestion ( min: CHO, 30.37 +/- 7.42 and CHO + CAF, 29.12 +/- 5.62 vs. FAT, 33.02 +/- 8 .50 and FAT + CAF, 32.78 +/- 7.70; P < 0.05). We conclude that (1) caffeine co-ingested with either carbohydrate or fat meals has no additive effect o n substrate utilization or exercise performance and (2) carbohydrate ingest ion before exercise improves subsequent time trial performance compared wit h fat ingestion.