Objective: Since little information is available on the capacity of the non-obese to adapt to a moderate decrease in energy intake, the effect of a 10-week moderately energy-restricted diet (ER) on energy expenditure and body composition was studied. Design: A controlled intervention study. After a weight-maintaining run-in period of 2 weeks, the ER group received a diet that contained 9.2 MJ/day on average, i.e. 80% of the energy of their habitual diet (estimated by means of a 7-day dietary record) for the next ten weeks while the control group received the weight-maintaining diet. Setting: Subjects continued daily life habits and came to the Institute every evening to have dinner and to receive food for the next 24 h. Subjects: 24 healthy non-obese, middle-aged men participated. Subjects were matched for age and body mass index and randomly assigned to a control group (n = 8) or an ER group (n = 16). Results: Average daily metabolic rate (ADMR, i.e. total energy expenditure), measured with doubly labeled water in eight subjects of the ER group, appeared to be 82.5% of reported energy intake resulting in an actual level of energy restriction in these eight subjects of 33% on average (range 18-42%), rather than 20%. Subjects in the ER group lost 7.4 ± 1.7 kg; 83% of this weight loss was fat mass, 17% was fat-free mass. Subjects in the control group lost some weight too (2.1 ± 1.5 kg). Resting metabolic rate (RMR) (MJ/day) decreased in the ER group (P < 0.001). In this group the thyroid hormone triiodothyronine (T3) decreased (P < 0.001), while reverse T3 (rT3) increased (P < 0.05). ADMR decreased significantly. Conclusion: Under conditions of a controlled moderately energy-restricted diet in daily life a significant weight loss can be induced, similar to that observed after a balanced dietary deficit, providing 5 MJ/day. in addition, moderate energy restriction induces a decrease in fat-free mass and a fall in RMR.