Intervention studies have shown that the adaptation of fat oxidation to fat intake, when the dietary fat content is changed, is not abrupt. This study was conducted to measure the time course of adaptation of oxidation rates to increases in the fat content of the diet while subjects were fed at energy balance. Twelve healthy, nonobese males and females [age: 26 +/- 2 y, body mass index (in kg/m2): 21.4 +/- 0.5; and habitual fat intake: 29 +/- 1% of energy] consumed a low-fat diet for 6 d (days 1-6) followed by a high-fat diet for 7 d (days 7-13). Days 5-9 and 13 were spent in a respiration chamber. After adjustment for energy intake to 24-h energy expenditure on day 5, subjects were in energy balance (range: -0.15 to 0.23 kJ/d) on days 6-9 and 13. Fat balance was zero on day 6 but became positive after subjects changed to the high-fat diet (1.06 +/- 0.15, 0.75 +/- 0.15, and 0.55 +/- 0.14 MJ/d for days 7, 8, and 9, respectively, P < 0.05), reaching a new balance on day 13, 7 d afterward. In conclusion, when in energy balance, lean subjects are capable of adjusting fat oxidation to fat intake within 7 d of when dietary fat content is increased.