be of practical importance as a factor that limits absorption and maintenance of zinc balance. While high-fiber-containing foods tend also to be phytate-rich, fiber alone may not have a major effect on zinc absorption.
A metabolite of tryptophan metabolism, picolinic acid has a high metal binding affinity. Picolinate complexes of zinc and chromium are not formed in nature in appreciable amounts, but are sold commercially as dietary supplements. Zinc picolinate as a zinc source for humans has not received extensive investigation. In an animal model, picolinic acid supplementation promoted negative zinc balance (Seal and Heaton, 1985), presumably by promoting urinary excretion.
To date, a useful algorithm for establishing dietary zinc requirements based on the presence of other nutrients and food components has not been established, and much information is still needed to develop one that can predict zinc bioavailability (Hunt, 1996). Algorithms for estimating dietary zinc bioavailability will need to include the dietary content of phytic acid, protein, zinc, and possibly calcium, iron, and copper. The World Health Organization (WHO, 1996) developed zinc requirements from low, medium, and high bioavailability diets on the basis of estimates of fractional absorption on single test meals with varying zinc and phytate content. The results of single test meals for measuring zinc absorption, however, may be different from the long-term response of zinc absorption, as has been shown to be the case for iron (see Chapter 9).
No functional criteria of zinc status have been demonstrated that reflect response to dietary intake in infants. Thus, recommended intakes of zinc are based on an Adequate Intake (AI) that reflects the observed mean zinc intake of infants exclusively fed human milk.