The application of nutritional science to the problems of hunger and malnutrition has passed through a phase of great confidence and hope, followed by one of increasing uncertainty and doubt. Analysis of the nutritional problems within a society may therefore have implications for agriculture at many different levels. At one level, it may consider impact of agricultural change on the ability of people to earn an entitlement to food. To produce more, but at a cost many people cannot afford, may be self-defeating. Lack of progress in attempts to define human energy and nutrient needs illustrates one reason why it is useful to think in terms of the level of nutrition needed to maintain the functional capacity of the body. Work intensity may impose limitations bearing in mind that the physical work required for certain tasks may be much greater than average and may only be sustainable by individuals with high level of physical fitness.