Submitted to: The American Journal of Clinical Nutrition
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/8/2008
Publication Date: 5/1/2008
Citation: Hunt, J.R., Beiseigel, J.M., Johnson, L.K. 2008. Adaptation in human zinc absorption as influenced by dietary zinc and bioavailability. American Journal of Clinical Nutrition. 87(5):1336-1345.
Interpretive Summary: An understanding of the relationship between dietary zinc intake, bioavailability and absorption is necessary for making dietary intake recommendations. Three experiments were conducted to measure zinc absorption by 109 healthy adults from ten controlled diets, that differed in contents of zinc and phytic acid. Phytic acid, a known inhibitor of zinc absorption, is a natural component of whole grains, beans, nuts and seeds. We tested for adaptation by measuring zinc absorption before and after the volunteers consumed the experimental diets for several weeks. Increasing dietary zinc resulted in reductions in the percent of zinc absorbed from both low- and high-phytate diets. After consuming low dietary zinc (less than 11 mg/d) for 4 or 8 weeks, volunteers adapted to increase their zinc absorption to as high as 90%, but only if the diets were low in phytate. The results help validate and refine equations that enable prediction of zinc absorption from dietary zinc and phytate. Although some of the controlled diets contained less than recommended amounts of zinc, this was not possible to detect with several types of blood measurements. The results suggest that humans cannot adapt to increase zinc absorption from diets high in phytate, and such diets are likely to provide inadequate amounts of absorbable zinc without zinc supplementation or reduction of dietary phytate.
Technical Abstract: Background: An understanding of the relationship between dietary zinc (Zn) intake, bioavailability and absorption is necessary for making dietary intake recommendations. Objective: To assess adaptation in human Zn absorption with controlled differences in Zn and phytate intakes and to apply the results to predictive models. Design: In three experiments, radiotracers were used to assess Zn absorption by healthy adults (n=109) from controlled diets, before and after 4 or 8 wk of dietary equilibration. Subjects consumed 4 to 29 mg Zn/d from one of ten diets, five with phytate:Zn molar ratios (P:Zn) from 2 to 7, and five with P:Zn from 15 to 23. Results: Absorptive efficiency was inversely related to dietary Zn from both low- and high-phytate diets. In response to extended low Zn intakes (<11 mg/d), Zn absorption was up-regulated to as high as 90%, but only if the diets were low in phytate. The results help validate and refine a published saturable transport model that predicts Zn absorption from dietary Zn and phytate. Possible biomarkers of impaired Zn status, including erythrocyte osmotic fragility, in vitro erythrocyte 65Zn uptake, and leukocyte expression of the Zn transport proteins Zip1 and ZnT1, were unresponsive to dietary Zn content. Conclusions: The results indicate that humans cannot adaptively increase Zn absorption from high phytate diets, and inadequate absorption is likely with dietary P:Zn >12, unless Zn is supplemented or phytate is reduced. Since sensitive biomarkers of Zn status have not been identified, research on absorptive adaptation may be useful to evaluate Zn dietary requirements.