allows the transport of oxygen to tissues (hemoglobin), transitional storage of oxygen in tissues (myoglobin), and the transport of electrons through the respiratory chain (cytochromes). Biologic functions that depend on Fe include energy metabolism, neurotransmitter synthesis, connective tissue metabolism, immune function, thyroid hormone metabolism, and thermogenesis. Recently, Fe has been found to bind to proteins, forming transcription factors that can affect the expression of other proteins. Thus, impaired Fe status can affect the metabolism of several nutrients.
Fe is present in many natural ingredients. The biologic availability of such Fe has been studied mostly in chickens and rats, and the results might not be completely applicable to nonhuman primates. Generally, the biologic availability of Fe in natural ingredients is about 40-60% (Henry and Miller, 1995). Iron is a substantial contaminant of most sources of dicalcium phosphate, and this makes it difficult to reduce the Fe concentration of natural diets.
Ferrous sulfate is customarily used as the standard in bioavailability studies and is usually assigned an Fe bioavailability of 100%. Ferrous sulfate and ferrous carbonate are the usual sources of Fe added to commercial diets, but various Fe sources are used in purified diets. Fe in ferrous sulfate, ferric chloride, ferric citrate, and ferric ammonium citrate has high biologic availability for several species. Bioavailability of Fe in ferrous carbonate and reduced iron varies with source and possibly particle size. The Fe in ferric oxide, which is occasionally added to feed as a coloring agent, is virtually unavailable (Henry and Miller, 1995). Fe absorption has been studied extensively in humans. Absorption is enhanced by the presence of ascorbic acid in the diet. Meat, fish, and chicken also enhance the absorption of Fe, whereas polyphenols, such as are found in tea and leaves, seem to inhibit absorption (Yip and Dallman, 1996; Zijp et al., 2000). An algorithm has been developed for calculating absorption and bioavailability of Fe in a number of human foods, and concentrations of phytate phosphorus and Fe-binding polyphenols in foods used in human and some nonhuman-primate diets have been published (Hallberg and Hulthé n, 2000).
There are a number of nutritionally significant interactions of Fe with other minerals, although few of these have been studied in nonhuman primates. Dietary concentrations of calcium, copper, manganese, and zinc may influence Fe absorption. Plasma concentrations of chromium and manganese may influence Fe transport. Tissue concentrations of copper and zinc may influence cellular Fe uptake, and tissue concentrations of chromium, copper, and zinc may influence the size and mobility of Fe stores (O’Dell and Sunde, 1997)
Fe absorption from infant formulas has been determined in infant rhesus monkeys and was found to be 20-30% from milk-based and soy-based formulas (Davidson et al., 1990; Lonnerdal et al., 1999). The effect of various dietary factors on Fe absorption has not been studied extensively in primates, but the rhesus monkey has been used as a model to study the effect of lactoferrin, a major Fe-binding protein in the milk of rhesus monkeys (Davidson and Lonnerdal, 1986) and in human milk, on Fe uptake from milk and milk-replacers (Davidson et al., 1990). It appears that a unique receptor-mediated mechanism in the small intestine facilitates the uptake of Fe from lactoferrin (Davidson and Lonnerdal, 1988, 1989). Removal of phytate from soy in a soy-based formula appeared to have little effect on Fe absorption in infant rhesus monkeys (Lonnerdal et al., 1999). Young rhesus monkeys fed a soy protein diet were found to be anemic after 2-7 months, and Fe absorption from this diet was lower than from a casein diet (Fitch et al., 1964). The diet was baked, however, and both the heat treatment and the addition of baking soda (pH) might have affected Fe bioavailability.
Adequate Fe status is needed for normal hematologic characteristics. Age-related changes in hematologic measures have been described in infant rhesus monkeys (Martin et al., 1973). Packed cell volumes were high at birth, declining during the first 2 post-natal weeks. Proportions of neutrophils were high at birth and declined with age, whereas proportions of lymphocytes were low at birth but rose rapidly to adult values. Proportions of eosinophils were low at birth, increasing to adult values during the first post-natal month. Total leukocyte counts were essentially constant from birth to 2 years. The consequences of impaired Fe status on such hematologic measures as hemoglobin, hematocrit, MCV, transferrin saturation, and serum iron have been described in rhesus macaques (Wolcott et al., 1973; Mandell and George, 1991; Bicknese et al., 1993; Sreeramulu et al., 1994; Kriete et al., 1995) and cynomolgus macaques (Giuletti et al., 1991). When 30-70% of blood volume was withdrawn from adult (6.5- to 10-year-old) nonpregnant female rhesus monkeys over a long period (5-10% per week), anemia developed (Mandell and George, 1991).
No firm indices for the identification of anemia or Fe deficiency have been established for nonhuman primates, and indices for human subjects are usually used. Results of a study in which dietary Fe deficiency was induced in rhesus monkeys suggest that serum ferritin is not a good indicator of Fe status in this species (Sreeramulu et al., 1994). The assay used, however, might not have recognized rhesus monkey ferritin, which is necessary if commercially available kits for assay of serum ferritin in humans are to be useful in measuring the response to dietary Fe intake in rhesus infants (Lonnerdal et al., 1996). The effect of transferrin polymorphism on total iron-binding capacity has been examined in rhesus monkeys, and it has been suggested that different types of transferrin (genotypes) affect fertility and growth of offspring (Smith, 1982).