Daily urinary nitrogen (N) excretion and body weight data of 42 healthy young adults, who received constant, adequate diets for up to 90 days, and maintained constant level of physical activity, were examined retrospectively. From the N excretion data, it was determined that, when corrected for long-term trends, daily variability was random and much larger than variability due to the long-term trends. Thus, N balance data based on commonly used diet periods of about 2 wk adequately estimates population variability. Body weight data for most subjects showed significant changes and many subjects experienced persistent linear changes throughout the entire experimental period. Thus, for most healthy young adults, neither dietary-induced thermogenesis nor changes in the efficiency of energy utilization appears to play a quantitatively important role in the maintenance of body weight (and body composition) in response to relatively modest discrepancies between the level of food intake and energy intake needed to balance initial total energy expenditure. The key importance of food intake regulation and/or physical activity in weight maintenance is emphasized by these findings.