Functional consequences of iron deficiency in human populations.

The widespread occurrence of iron deficiency in developing countries is now well documented. Its high prevalence in these countries is attributable not only to the poor availability of iron in diets because of high fiber and phytate content, but also to chronic blood losses due to hookworm, schisto somiasis, and malaria (1). A high prevalence of iron deficiency anemia obviously means an equal or greater prevalence of iron deficiency not severe enough to cause anemia. If it should be demon strated that subclinical iron deficiency has func tional consequences, the significance of iron de ficiency for these countries is even greater than has been recognized. There is growing evidence, as reviewed in this paper, that this is indeed the case. In fact, some of the first evidence for this came from a study of preschool children in Cambridge, Massachusetts that failed to find anemia in this population. However, it demonstrated that iron deficiency per se, as diagnosed by levels of serum iron, ferritin, and transferrin saturation, was suf ficient to affect this performance on some cognitive tests (2). This has led to a broader search for the functional consequences of iron deficiency to in clude subclinical as well as clinical iron deficiency. While neither iron deficiency nor iron deficiency anemia is limited to developing countries, pre valence rates are highest in them. Prevalence rates in the industrialized countries for iron deficiency ane mia are variously reported as from 10 to 20% in the U.S., Europe, and Japan (3, 4). Rates are highest for women of childbearing age, but are by no means limited to this group. There has been a great deal of recent research on the availability of dietary iron in various kinds of diets, on practical ways of fortifying staple food and diets, and on the prevalence of iron deficiency anemia in various populations. To complement such studies, the United Nations University has established a network of institutions in developing countries to investigate the functional consequences of iron deficiency. Extensive work is under way in Egypt, Thailand, Chile, and Indonesia, and ad ditional studies are planned. While the results of these studies are not yet available, there is already evidence that iron deficiency impairs resistance to infection, cognitive performance, physical capacity, work output, and possibly even maintenance of body temperature. This paper will review and eval uate the currently available evidence. The total amount of iron in the body of a 70 kilo man is about 3.5g, of which 70% is contained in hemoglobin. Other iron compounds constitute a very small percentage of the total, and are located primarily in solid tissues. Myoglobin accounts for about 4% of the total, and most of the additional iron is present in a large number of oxidative enzymes, including mitochondria, cytochromes, and flavoproteins. While these latter account for only a minute fraction of the iron in the body, they fulfill