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 with 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 7 wk, and thereafter both groups consumed the carbohydrate diet for an eighth week. Training was performed throughout. After 8 wk, during 60 min of exercise (71 ± 1% pretraining maximal oxygen uptake) average leg glucose uptake (1.00 ± 0.07 vs. 1.55 ± 0.21 mmol/min) was lower (P < 0.05) in Fat-CHO than in CHO. The rate of muscle glycogen breakdown was similar (4.4 ± 0.5 vs. 4.2 ± 0.7 mmol · min¹ · kg dry wt¹) despite a significantly higher preexercise glycogen concentration (872 ± 59 vs. 688 ± 43 mmol/kg dry wt) in Fat-CHO than in CHO. 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 with when the carbohydrate diet is consumed throughout training.