One of the candidate factors for determining the increase of fat oxidation after a switch from a reduced-fat diet to a high-fat diet is the size of the glycogen storage. Therefore, we studied the effect of low glycogen stores on fat oxidation after a switch from a reduced-fat diet to a high-fat diet. Twelve healthy, nonobese males and females (age: 22 +/- 1 yr, body mass index: 21.0 +/- 0.7, maximal power output: 254 +/- 11 W) consumed a reduced-fat (RF) diet (30, 55, and 15% of energy from fat, carbohydrate, and protein, respectively) three times a day at home for 3 days (days 1-3). On two occasions subjects came to the laboratory on day 3 at 1500 to perform an exhaustive glycogen-lowering exercise (EX), after which they went into the respiration chamber for a 36-h stay. On one occasion, subjects directly entered the respiration chamber at 1800 for a 36-h stay. In the respiration chamber they were given either a high-fat (HF) diet (60, 25, and 15% of energy from fat, carbohydrate, and protein, respectively) or a RF diet. In both cases they were fed at energy balance. All diets were consumed as breakfast, lunch, dinner, and two or more snacks per day. On the HF treatment, fat oxidation was below fat intake, indicating the slow change of oxidation to intake on a HF diet. After the HF+EX treatment, however, fat oxidation matched fat intake. In conclusion, lean subjects are capable of rapidly adjusting fat oxidation to fat intake when glycogen stores are lowered by exhaustive exercise.