In developing countries, lack of food and poor usage of the available food can result in protein-energy malnutrition (PEM); 50 million pre-school African children have PEM. In developed countries, excess food is available and the most common nutritional problem is obesity.

Diet and disease are interrelated in many ways. Excess energy intake, particularly when high in animal (saturated) fat content, is thought to contribute to a number of diseases, including ischaemic heart disease and diabetes. A relationship between food intake and cancer has been found in many epidemiological studies; an excess of energy-rich foods (i.e. fat and sugar containing), often with physical inactivity, plays a role in the development of certain cancers, while diets high in vegetables and fruits reduce the risk of most epithelial cancers. Numerous carcinogens, either intentionally added to food (e.g. nitrates for preserving foods) or accidental contaminants (e.g. moulds producing aflatoxin and fungi), may also be involved in the development of cancer.

The proportion of processed foods eaten may affect the development of disease. A number of processed convenience foods have a high sugar and fat content and therefore predispose to dental caries and obesity respectively. They also have a low fibre content, and dietary fibre is possibly necessary in the prevention of a number of diseases. Some epidemiological data suggest that there are long-term effects of undernutrition; low growth rates in utero being associated with high death rates from cardiovascular disease in adult life.

In 1991 the Department of Health published the dietary reference values for food and energy and nutrients for the United Kingdom. Values were based on all of the available information including data from the 1985 Food and Agriculture Organization (FAO-WHO), United Nations University (UNU) expert committee, so that there is broad agreement on the reference values given. In the UK recommended daily amounts (RDAs) are no longer used, but have been replaced by the reference nutrient intake (RNI) to provide more help in interpreting dietary surveys.

The RNI is roughly equivalent to the previous RDA, and is sufficient or more than sufficient to meet the nutritional needs of 97.5% of healthy people in a population. Most people's daily requirements are less than this, and so an estimated average requirement (EAR) is also given, which will certainly be adequate for most. A lower reference nutrient intake (LRNI) which fails to meet the requirement of 97.5% of the population is also given. The RNI figures quoted in this chapter are for the age group 19-50 years. These represent values for healthy subjects and are not always appropriate for patients with disease.