4 Clinical aspects of vitamin and trace element metabolism (original) (raw)

1988, Baillière's Clinical Gastroenterology

It is customary in reviews of this type to group all the micronutrients together, whether organic (vitamins) or inorganic (essential trace elements), and to keep them quite separate from discussions of the major nutrients of amino acids, fats and carbohydrates. Although this brings benefits of simplicity of presentation, in that many micronutrients can be covered relatively briefly, the classification is arbitrary and potentially misleading. Individual micronutrients have more relevance when considered together with the effects which they have on the metabolism of the major nutrients, such as the role of thiamine in carbohydrate metabolism, or of zinc in protein metabolism. Moreover, in the context of clinical nutrition of hospital patients, deficiencies of individual micronutrients are unlikely to occur in isolation, being more commonly found in association with deficiency of many components of the diet, including protein energy malnutrition as well as other vitamins or trace elements. Interactions between the micronutrients themselves may be especially important, and a wide range are now recognized, such as zinc and vitamin A, selenium and vitamin E, and iron and vitamin C (Levander and Cheng, 1980). However, for clarity, the metabolism of each element and the effects of deficiency are usually described as though they exist independently. GENERAL CONCEPTS The usual definition of an essential trace element is that if it is removed from the diet, it will cause reproducible pathological changes (either structural or biochemical) which can be uniquely reversed by addition of the element to the otherwise complete diet. A similar definition holds for the vitamins, although with some slight variations to account for those vitamins which can be produced in the body under certain conditions, such as vitamin D or vitamin K. An important feature of micronutrient metabolism is the progressive nature of the abnormalities. Thus, although they are characterized by the end result of the 'full blown' deficiency picture, this usually represents the end result of prolonged depletion of the nutrient. As Brubacher (1982)