OBJECTIVE: To determine the influence of age on the ability to adjust macronutrient oxidation to changes in diet composition. Our hypothesis was that the ability to adjust macronutrient oxidation to changes in diet composition would be impaired with age. DESIGN: Cross-sectional, randomized to three different isocaloric diets containing a constant percentage protein but varying in percentage fat and percentage carbohydrate: mixed diet (M; 15/30/55); high-fat diet (HF; 15/60/25), and high-carbohydrate (HC; 15/15/70). SUBJECTS: Six young (YM; age = 25 ± 1 y) and five middle-aged and older men (OM; age = 63 ± 3 y). MEASUREMENTS: Each subject underwent 24 h whole-room calorimetry on day 4 of each diet to determine 24 h macronutrient oxidation rates. Macronutrient balance was calculated from the individual macronutrient oxidation rates and the corresponding macronutrient intake. RESULTS: Body mass, percentage fat, and fat-free mass were similar in the two groups. Twenty-four-hour energy expenditure (EE) and energy balance did not differ across diets or between groups; 24 h EE was ∼7% lower (NS) in the OM. Macronutrient oxidation rates were not significantly different in YM vs OM during M. Protein oxidation was similar across diets, but higher (P < 0.05) in OM. Fat oxidation contributed 28.8 ± 7.0% vs 37.8 ± 4.7% to 24 h EE on M (NS) in the OM vs YM, respectively. This increased to 58.4 ± 6.7 vs 51.9 ± 5.3% of 24 h EE (NS) in the OM vs YM, respectively, during HF and decreased to 25.4 ± 9.7 vs 20.2 ± 14.3% (NS) during HC (diet effect, both P < 0.05). Carbohydrate oxidation contributed 54.3 ± 10.5% vs 56.6 ± 2.4% of 24 h EE (NS) on M in the OM vs YM, respectively. This decreased to 19.5 ± 10.6 vs 29.9 ± 12.6% (NS) during HF and increased to 53.6 ± 12.3 vs 64.7 ± 14.3% (NS) in the OM vs YM, respectively during HC (diet effect, P < 0.05). CONCLUSION: Taken together, these results suggest that the ability to adjust macronutrient oxidation to changes in diet composition is maintained in OM and, thus, is unlikely to contribute to the increased susceptibility to weight gain and obesity development that accompanies aging.