We tested the hypothesis that a shift to carbohydrate diet after prolonged adaptation to fat diet would lead to decreased glucose uptake and impaired muscle glycogen breakdown during exercise compared to ingestion of a carbohydrate diet all along. We studied 13 untrained men, 7 consumed a high-fat (FAT-CHO; 62% fat, 21% carbohydrate) and 6 a high-carbohydrate diet (CHO; 20% fat, 65% carbohydrate) for 7wks; thereafter both groups consumed the carbohydrate diet for an 8th week. Training was performed throughout. After 8wks, during 60min exercise (71±1% pre-training VO₂max) average leg glucose uptake (1.00±0.07 vs 1.55±0.21 mmol min-1) was lower (p<0.05) in FAT-CHO than CHO, respectively. The rate of muscle glycogen breakdown was similar (4.4±0.5 vs 4.2±0.7 mmol min^-1kg^-1d.w.) despite a significantly higher pre-exercise glycogen concentration (872±59 vs 688±43 mmol kg^-1d.w.) in FAT-CHO than CHO, respectively. In conclusion, shift to carbohydrate diet after prolonged adaptation to fat diet and training causes increased resting muscle glycogen levels but impaired leg glucose uptake and similar muscle glycogen breakdown, despite higher resting levels, compared to when the carbohydrate diet is consumed throughout training.