BACKGROUND: The energy content of weight change is assumed to be sex-and age-neutral at 3500 kcal/pound or 32.2 MJ/kg.

OBJECTIVES: As sexual dimorphism in body composition generally exists in mammals, the primary hypothesis advanced and tested was that the energy content of weight change differs between men and women.

DESIGN: The energy content of 129 adult men and 287 women was measured by neutron activation analysis. Cross-sectional energy content prediction models were developed and then evaluated in two longitudinal samples: one that used the same methods in 26 obese women losing weight; and the other a compilation of 18 previously reported weight change–body composition studies.

RESULTS: Multiple regression modeling identified weight, sex, age and height as total energy content predictor variables with significant sex× weight (P< 0.001) and age× weight (P< 0.001) interactions; total model r 2 and see were 0.89 and 107.3 MJ, respectively. The model's predictive value was supported in both longitudinal evaluation samples. Model calculations using characteristics of representative adults gaining or losing weight suggested that the energy content of weight change in women (∼ 30.1–32.2 MJ/kg) is near to the classical value of 32.2 MJ/kg and that in men the value is substantially lower,∼ 21.8–23.8 MJ/kg. The predicted energy content of weight change increases by about 10% in older (age∼ 70 y) vs younger (∼ 35 y) men and women.

CONCLUSIONS: Sexual dimorphism and age-dependency appears to exist in the estimated energy content of weight change and these observations have important clinical and research implications.