LIPOLYTIC SUPPRESSION FOLLOWING CARBOHYDRATE INGESTION LIMITS FAT OXIDATION DURING EXERCISE

This study determined if the suppression of lipolysis after preexercis e carbohydrate ingestion reduces fat oxidation during exercise. Six he althy, active men cycled 60 min at 44 +/- 2% peak oxygen consumption, exactly 1 h after ingesting 0.8 g/kg of glucose (Glc) or fructose (Fru ) or after an overnight fast (Fast). The mean plasma insulin concentra tion during the 50 min before exercise was different among Fast; Fru, and Glc (8 +/- 1, 17 +/- 1, and 38 +/- 5 mu U/ml, respectively; P < 0. 05). After 25 min of exercise, whole body lipolysis was 6.9 +/- 0.2, 4 .3 +/- 0.3, and 3.2 +/- 0.5 mu mol.kg(-1).min(-1) and fat oxidation wa s 6.1 +/- 0.2, 4.2 +/- 0.5, and 3.1 +/- 0.3 mu mol.kg(-1).min(-1) duri ng Fast, Fru, and Glc, respectively (all P < 0.05). During Fast, fat o xidation was less than Lipolysis (P < 0.05), whereas fat oxidation app roximately equaled lipolysis during Fru and Glc. In an additional tria l, the same subjects ingested glucose (0.8 g/kg) 1 h before exercise a nd Lipolysis was simultaneously increased by infusing Intralipid and h eparin throughout the resting and exercise periods (Glc + Lipid). This elevation of Lipolysis during Glc + Lipid increased fat oxidation 30% above Glc (4.0 +/- 0.4 vs. 3.1 +/- 0.3 mu mol.kg(-1).min(-1); P < 0.0 5), confirming that lipolysis limited fat oxidation. In summary, small elevations in plasma insulin before exercise suppressed lipolysis dur ing exercise to the point at which it equaled and appeared to limit fa t-oxidation.