In adults, body mass (BM) and its components fat-free mass (FFM) and fat mass (FM) are normally regulated at a constant level. Changes in FM and FFM are dependent on energy intake (EI) and energy expenditure (EE). The body defends itself against an imbalance between EI and EE by adjusting, within limits, the one to the other. When, at a given EI or EE, energy balance cannot be reached, FM and FFM will change, eventually resulting in an energy balance at a new value. A model is described which simulates changes in FM and FFM using EI and physical activity (PA) as input variables. EI can be set at a chosen value or calculated from dietary intake with a database on the net energy of foods. PA can be set at a chosen multiple of basal metabolic rate (BMR) or calculated from the activity budget with a database on the energy cost of activities in multiples of BMR. BMR is calculated from FFM and FM and, ifnecessary, FFM is calculated from BM, height, sex and age, using empirical equations. The model uses existing knowledge on the adaptation of energy expenditure (EE) to an imbalance between EI and EE, and to resulting changes in FM and FFM. Mobilization and storage of energy as FM and FFM are functions of the relative size of the deficit (EI/EE) and of the body composition. The model was validated with three recent studies measuring EE at a fixed EI during an interval with energy restriction, overfeeding and exercise training respectively. Discrepancies between observed and simulated changes in energy stores were within the measurement precision of EI, EE and body composition. Thus the consequences of a change in dietary intake or a change in physical activity on body weight and body composition can be simulated.Energy intake : Physical activity : Simulation model Body mass (BM) in adult man is regulated at a constant level, an everyday experience backed by surprisingly little data in the literature. One of the few studies providing information on the constancy of BM is the Framingham Study, a long-term sampling of 5209 adults, 30-59 years of age, living in the town of Framingham at the start of the study from 1948 to 1949. They underwent, every 2 years, a standard medcal examination, including the measurement of BM, for at least 20 years if not prevented by illness or death. Most subjects lost or gained no more than 5-10 kg over some part of the 20-year period as calculated by James (1985). This demonstrates a nearly perfect system for preserving energy balance as the total energy turnover of an average adult subject over 20 years is 73000 MJ. A discrepancy of 1 % between energy intake (EI) and energy expenditure (EE) would add up to 730 MJ over 20 years, equivalent to about 24 kg BM as fat tissue with an energy density of 30 MJ/kg. Thus, in the long term EI matches EE within 1 YO.There are situations where subjects are brought into a positive or negative energy balance by an intervention and where one wants to know the resulting consequences for BM. Examples are overeating or undereating and a restriction of physical...