Protein-energy deficiency occurs at all ages but its incidence is greatest in the weaning and immediate post-weaning periods; deprived of a high quality protein food, the child is not yet old enough to fend for himself in the family circle and is particularly subject to dietary taboos and prejudices. Milder forms of dietary deficiency, however, continue to occur among children and adolescents in low-income groups in developing countries. The growth rates of children in developing countries deviate sharply from the norm at the time of weaning and continue at a low level throughout the entire period of growth, resulting in stunted adult stature. Apart from the effect on growth, mild or moderate protein deficiency renders infants and young children particularly susceptible to respiratory and gastrointestinal infections. The incidence of such diseases is much higher in malnourished than in well-nourished children; and mortality in the age group 1 to 4 years is 20 to 50 times higher in the developing than in the developed countries; it is probable that this difference is due in large part to malnutrition. Besides this, among people with low incomes in developing countries there is a high prevalence of weaning diarrhoea, because of the combined effects of poor hygiene and protein deficiency.
Protein malnutrition in young children is the major nutritional problem of the world, and if both its direct and indirect effects are considered, is a major cause of ill health. Frank protein deficiency is common in the less developed countries, and latent or subclinical protein deficiency is probably even more prevalent. Although protein deficiency may predominate, often calorie deficiency contributes important effects and the simultaneous insufficiency of other nutrients complicates the picture in varying degrees.