The objective of this paper is to develop a Model that integrates the biologically determined human need for food energy and the economic activity (most) people Bust engage in to be able to eat. The optimal work effort and the optimal body size of individuals in nutritionally constrained populations are derived. The model suggests that such individuals are economically better off keeping their work activity and body size down. The nutritional requirement of the individual is derived endogenously in the model and contrasted to the exogenously determined nutrition norms used by the FAO/WHO and other international organizations in order to assess the food situation in the poor countries. In the household version of the model, the optimal intra-family distribution of work activity and of food consumption, as well as the optimal male/female body weight ratio, are derived. The model suggests that in the economic optimum, the woman works more intensively then the man in relation to the food she consumes and that her optimal body weight (for height) is higher than the man's. The paper is part of a larger study that has as its main aims to assess the nutrition situation in Sub-Saharan Africa and to explain the reasons for the undernutrition that exists. According to the international organizations, the world's food problems of today are concentrated in Sub-Saharan Africa. The FAO claims that the food 'available' in the region in the aid 1980s is only 80 percent of what is required even if distributed equally, which it is not. The World Bank has estimated that almost half the population in the region is undernourished and one-quarter severely so. In the larger study, the main corollaries following the analysis in this theoretical paper are tested on data from a large set of countries in Sub-Saharan Africa. The theoretical finding that the type of 'exogenous' nutrition norms used by the FAO/WHO and the World Bank induces a substantial upward bias in the estimated prevalence of undernutrition, is vindicated by the tests. The empirical analysis also corroborates the theoretical argument why women in this region have a higher body weight (for height) than men and, by implication, works harder relative to what they eat.