continued on the same diet, and their behavior was compared with that of animals fed a diet that was similar but contained Mn at about 40 mg·kg-1. Mothers fed the deficient diet had normal pregnancies. The infants had normal birth weights and grew normally on the low-Mn diet. Behavioral development was evaluated with a series of tests. Infants fed the low-Mn diet had abnormally strong clasping and clinging responses, but their righting responses, which required release from clasping, were inadequate (Riopelle and Hubbard, 1977).
Signs of Mn deficiency other than the changes in behavioral development have not been described in nonhuman primates. Typical commercial diets, which appear to be adequate, contain Mn at 70-100 mg·kg-1 (Knapka et. al, 1995). Those concentrations are probably far in excess of the minimal requirement, which is 10 mg·kg-1 for rats and 2-20 mg·kg-1 for swine in various stages of their life cycle (National Research Council 1995, 1998). The level of Mn in the control diet used by Riopelle and Hubbard (1977) would provide about 44 mg·kg-1 of dietary DMand probably exceeds primate available Mn requirements, as well.
Zinc (Zn) is the most abundant of intracellular trace elements and is involved in structural, catalytic, and regulatory roles. Loss of Zn from biomembranes, as a consequence of Zn deficiency, can result in increased susceptibility to oxidative damage, structural strains, and alterations in specific receptor sites and transport systems (King and Keen, 1999). Over 200 Zn enzymes with diverse functions have been found, and Zn is involved in the metabolism of carbohydrate, protein, lipids, and nucleic acids (DNA and RNA polymerase and thymidine kinase). Extracellular superoxide dismutase activity in primates is affected by dietary Zn intake (Olin et al, 1995a). Zn also serves as a structural part of several important cellular constituents, such as transcription factors. In so-called zinc-finger structures, Zn is involved in gene expression at a very fundamental level. Growth, reproduction (pregnancy outcome), bone formation, immune function, skin integrity, morbidity, appetite, cognitive function, and behavior have been shown to be impaired in Zn deficiency in nonhuman primates and in humans. Zn deficiency affects embryogenesis, resulting in malformations, stillbirths, abortions, and smaller than normal offspring (King and Keen, 1999). Although Zn deficiency affects organisms in many ways, some might be due to the effects of Zn deficiency on cytokine synthesis and metabolism, particularly Tumor Necrosis Factor-a (TNF-a) and interleukin-2. Cell cycle events and apoptosis (programmed cell death) are affected by Zn nutriture.
Zn is usually added to commercial diets as zinc sulfate, ZnSO4; zinc oxide, ZnO; or zinc carbonate, ZnCO3. The Zn in ZnSO4 and ZnCO3 has high biologic availability in livestock. In some earlier studies, Zn in ZnO was demonstrated to have high biologic availability, but more recent reports indicated a biologic availability of about 50% (McDowell, 1992; Baker and Ammerman, 1995b). A number of factors can affect Zn availability. Diets high in wheat bran lowered Zn concentrations in the serum and bone of male (but not female) baboons, despite a low phytate:Zn molar ratio and high Zn intake (Kriek et al., 1982). Dietary phytate, which can be present in significant amounts in oilseeds and cereal grains, markedly decreases the absorption of Zn in chicks, rats, and swine. High dietary calcium exacerbates the effect. The effect can be overcome by feeding higher concentrations of Zn (hence a high Zn requirement will be observed) or by the concurrent feeding of some, but not all, chelating agents (Baker and Ammerman, 1995b). Proprietary products containing Zn chelates, or other organic complexes containing Zn, are sometimes used in diets to ensure good absorption.
Diets based on soy protein have been used in studies of experimental Zn deficiency in primates because soy-protein sources usually contain enough phytate to inhibit Zn absorption (Lonnerdal et al., 1988). When phytate was removed or reduced in the soy-protein diet, Zn absorption by infant rhesus monkeys increased significantly (rising from 27% to 45%) (Lonnerdal et al., 1988, 1999), to a point similar to that of Zn absorption from milk-based formulas (46%). Zn absorption from monkey milk has been shown to be about 54% (Lonnerdal et al., 1988). Absorption of Zn from a formula based on casein hydrolysate was lower than that from a regular milk formula, but the presence of a soy-protein source reduced Zn absorption further (Rudloff and Lonnerdal, 1992).
It has been suggested that iron can interfere with the absorption of Zn (Solomons and Jacob, 1981). The interaction has been demonstrated in humans when high amounts of iron were given with Zn at a ratio of 25:1 in a water solution but not when iron and Zn were given in this ratio in a meal (Sandstrom et al., 1985). Studies in pregnant and lactating rhesus monkeys showed no negative effect of iron supplementation (iron at 4 mg·BWkg-1·d-1) on Zn absorption when the diet contained Zn at 4 or 100 mg·kg-1 (Lonnerdal et al., 1990b). Similarly, when infant rhesus monkeys were given infant formulas with a high iron:Zn ratio (iron at 12 mg·L-1 and Zn at 1 mg·L-1), there was no difference in Zn absorption or retention as compared with those in infants fed formula with a lower iron:Zn ratio (1:1) (Polberger et al., 1996).
Rhesus monkeys (Sandstead et al., 1978) and bonnet monkeys (Swenerton and Hurley, 1980) have been used as animal models for human Zn deficiency. In both, the Zn-deficient diets contained Zn at less than 1 mg·kg-1. Signs included anorexia, apathy, weight loss, dermatitis, reproductive failure, and lowered plasma and tissue Zn