Children in developing countries pass stools that are very different from those passed by children in Europe. These stools reflect a diet of unrefined carbohydrate with low-energy density, and which due to the large volume results in an energy deficit in the child. This energy deficit is now considered to be the major cause of the almost universal undernutrition. Much of the improved health in European children during the last century has probably arisen through better nutrition due to more-refined carbohydrates and to more fat in the diet. Over the same period as children have become healthier, diseases of civilisation have appeared. One factor in such diseases is clearly that of diet, and these diseases may partly be caused by the high-energy density and the low-dietary fibre content. Changes in diet offer the greatest hope for a rapid improvement in health. In the developing world we need to find a means for making foods with a high-energy density more easily available to overcome the undernutrition in childhood. In industrialised countries older children need to become accustomed to a diet of lower-energy than at present, containing more unrefined carbohydrate and less fat.