The linkage between diet and health is an inescapable fact of life. However, in some senses this linkage can be a complex and subtle one, and clear causal pathways may be very difficult to establish, whether by time-honoured intuitive techniques or by the sophisticated, systematic methods of modern science. Thus, while this link is widely recognized in human culture, there are seemingly endless variations in the ways in which it is conceptualized and in the ways in which such conceptualizations are translated into actual beliefs and practices. However, as a starting-point it is useful to see conceptualizations of the relationship between diet and health as having two opposed aspects: positive and negative. The positive aspect is based upon the idea that certain food items, combinations of food items or diets can produce beneficial health outcomes. These beneficial outcomes may be viewed, by those who accept such ideas, as generalized and unspecific. That is, certain dietary choices are seen as maintaining, or actually enhancing, an individual’s resistance to disease or as promoting the efficiency or durability of the body. However, such ideas can be much more specific. For example, particular dietary options or particular foodstuffs may be seen as capable of preventing a particular disease. Similarly, certain food items, or a given dietary regime, may be seen as suitable for treating a disease or for managing a disease and relieving its symptoms.
Many of the negative aspects of the linkage between diet and health are self-evident. Most obviously, a grossly inadequate food intake will lead to weight loss and eventually to death (either through starvation or the onset of a related disease). However, nutrient deficiencies which fall short of the absolute deprivation of starvation can result from low food intake, an unbalanced diet or poor assimilation. Thus, dietary protein deficiency in infants after weaning can result in the disease known as kwashiorkor. A deficiency of vitamin D can cause rickets, a disease which mainly affects children and is characterized by the softening of developing bone (resulting in bow legs). The disease scurvy results from a lack of vitamin C, and produces anaemia, spongy gums, and, in infants, is associated with malformations of bones and teeth. Furthermore, inevitably, food intake can act as a channel for the introduction of harmful agents into the body. These may be toxins (whether organic or inorganic, naturally occurring or