Zinc: requirements, bioavailabilities and recommended dietary allowances

This review has approached zinc requirements, bioavailabilities and recommended dietary allowances from a historical view. For example, a requirement for zinc was first demonstrated for the microorganism Aspergillus niger more than a century ago, although zinc has been recognized as a dietary nutrient for humans only within the last decade. Dietary requirement was defined as that quantity of zinc which must be provided daily in order to meet the metabolic requirement. The degree of bioavailability determines the total dietary zinc which must be consumed by humans to remain in metabolic equilibrium. In regard to recommended dietary allowances (RDA), they are recommended average daily intakes of nutrients that population groups should consume. RDA are not synonymous with requirements. Metabolic requirements (the quantity that must be provided to the circulating blood daily) have been determined previously using radio-isotope studies and have indicated a calculated need of approximately 6 mg for an adult per day. More recently a similar amount (4-6 mg) has been found to be required daily (intravenously) in order to maintain plasma zinc concentrations and daily urinary zinc excretion within normal range in hospitalized subjects. Average zinc intakes of large segments of the U.S. population are receiving levels approaching one-half or less the RDA with no apparent deleterious effects. These subjects include pregnant and lactating women. In addition, the zinc content of breast milk is lower than previous studies indicated. Thus, it appears that the majority of breast-fed infants are receiving no more than 70% of the recommended intake, with a recent study indicating less than 50%. Factors reported to affect bioavailability of zinc from foods are discussed. These include fiber and phytate. It was concluded that no definite conclusion can be reached regarding the overall effect of food fiber on zinc balance. Many of the studies were of short duration, with the longest being 32 days. It is probable that different sources of food fiber may have different effects on zinc balance. The apparent discrepancy in the literature regarding the effect of soy protein on zinc requirement was cited. Lastly, the phytate:zinc molar ratio concept was designed to test it as a predictor of zinc bioavailability to humans. The concept must be expanded to recognize the relevance of the total daily dietary zinc intake. Specifically, impairment of absorption of zinc by phytate would be more critical if the total dietary intake was low since the metabolic requirements might not be met.(ABSTRACT TRUNCATED AT 400 WORDS)