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Letters to the Editor |
Friedrich Schiller Universitat Institute fur Ernahrung Dornburger Strasse 25 07743 Jena Germany
Dear Sir:
A considerable amount of research over the past 50 y has contributed to our knowledge of iron bioavailability, particularly our understanding of the dietary determinants or variables that influence the iron status of individuals and populations. Studies of iron bioavailability have progressed from studies of single foods to studies of mixed meals and more recently of dietary intake over a given time period (1).The paper by Fleming et al (2), as stated in the accompanying editorial (3), is indeed a valuable contribution to this recent emphasis on the dietary determinants of iron homeostasis over prolonged time periods. Contrary to the findings of most earlier reports, dietary modulators of iron bioavailability influenced the serum ferritin concentration, one of the indexes of functional iron metabolism. The study, I suppose, brings to bear the concept of adaptive responses to iron absorption, metabolism, and balance. The subjects studied were a group of elderly, iron-replete persons whose dietary patterns varied. Iron homeostasis is known to be regulated by absorption from the gastrointestinal tract (4), in which absorption is inversely related to the stores of iron in the body. Consequently, iron bioavailability in humans can be adapted and regulated within a wide range of intakes to establish and maintain healthy iron status, particularly when the diet is varied. As adulthood is attained and in the elderly chosen for study, the physiologic demands for iron are minimal and adequate iron is absorbed to prevent iron deficiency (4) and maintain positive iron balance.
The authors stated 3 reasons that could account for the lack of correlation between dietary modulators and serum ferritin concentrations in previous studies: the need to take into account 1) confounders of serum ferritin values, 2) supplemental iron intake, and 3) small sample sizes. The disparities in the duration of the studies might also account for the differences observed. Although the earlier studies were usually for periods of 4–6 mo (5), the study by Fleming et al (2) lasted 
2 y. The effects of dietary factors might therefore be compensated for initially by adaptive responses in the gastrointestinal tract. After a period of adjustment, sustained variables in dietary composition could then exert influences on serum ferritin concentrations. It might be useful for future studies to evaluate the time course of effects of dietary factors on serum ferritin.
The authors also reported that the study corroborated on a population basis the well-known superiority of meat in iron nutrition (6). Although this finding is unquestionable, it might be worth considering at this stage of nutrition research the issue of meat intake in the general nutrition of adults and the elderly. Public health recommendations now focus on not only reducing fat intake but also reducing red meat consumption (a superior source of highly available iron) and increasing consumption of whole-grain products and cereals (which contain factors that inhibit iron absorption). These public health recommendations are consistent with those for the prevention of other diet-related chronic diseases, such as cancer, diabetes, coronary artery diseases, and obesity. That is, the consumption of a variety of plants foods with greater emphasis on fruit and vegetables is advocated. Paradoxically, however, these nonheme iron sources contain ascorbic acid, an enhancer of iron bioavailability comparable in magnitude to the effect of meat. Ascorbic acid, however, does not seem to maintain this positive influence for long periods. One of the reasons for this, as suggested by Fleming et al (2), could be the compounding influences of supplemental ascorbic acid.
Perhaps the study by Fleming et al (2) confirms further the basis of these recommendations, especially for adults and the elderly. Although heme iron was correlated with high serum ferritin values and nonheme plant sources were associated with low serum ferritin values, the average figures fell within the normal reference range for both groups. Because the subjects were iron replete, the different dietary variables resulted in the maintenance of normal serum ferritin values. Although the specifications and recommendations for groups most vulnerable to iron deficiency would obviously be very different, it is hoped that dietary patterns and the nutrition issues considered in policy formulations would be amalgamated to promote the overall well-being and healthy status of adults and the elderly.
REFERENCES
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