The effect of moderate physical activity (90 min in fasting and 90 min in feeding at 45-50% of VO₂-max) upon 24h macronutrient utilization was studied in 8 healthy men, after a diet-exercise-adjustment period of 6 days when the subjects were fed 2.5g protein·kg^-1·d^-1. 24% of the energy intake came from protein, 32% from fat and 44% from carbohydrate. Energy turnover (ET) was determined by indirect and direct (suit) calorimetry. Protein (PROT) metabolism was assessed by a 24h primed, continuous iv infusion of [1-¹³C]leu and [¹⁵N¹⁵N]urea. Total ET was 14.9±2.0 MJ·d^-1 (Mean±SD), resulting in a slightly positive energy balance of 0.6±1.2 MJ· d^-1. PROT contributed to 22%, fat to 45% and CHO to 33% of total 24h ET. During physical activity, the contribution from PROT decreased from 15 to 3% during fasting and from 36 to 9% during feeding while CHO contributed to 48% (fasting), 62% (feeding). Fat contributed to 75% of ET after exercise during fasting. During feeding the percent energy from PROT and CHO increased while that from fat decreased. Thus a high protein intake increased fat oxidation despite a lower fat intake when compared to a normal protein intake (1g protein·kg^-1·d^-1) under the same energy turnover conditions. The data will be discussed in reference to effect on body composition.