Dietary reference intakes for zinc may require adjustment for phytate intake based upon model predictions

Abstract

The quantity of total dietary zinc (Zn) and phytate are the principal determinants of the quantity of absorbed Zn. Recent estimates of Dietary Reference Intakes (DRI) for Zn by the Institute of Medicine (IOM) were based on data from low-phytate or phytate-free diets. The objective of this project was to estimate the effects of increasing quantities of dietary phytate on these DRI. We used a trivariate model of the quantity of Zn absorbed as a function of dietary Zn and phytate with updated parameters to estimate the phytate effect on the Estimated Average Requirement (EAR) and Recommended Daily Allowance for Zn for both men and women. The EAR predicted from the model at 0 phytate was very close to the EAR of the IOM. The addition of 1000 mg phytate doubled the EAR and adding 2000 mg phytate tripled the EAR. The model also predicted that the EAR for men and women could not be attained with phytate:Zn molar ratios > 11:1 and 15:1, respectively. The phytate effect on upper limits (UL) was predicted by first estimating the quantity of absorbed Zn corresponding to the UL of 40 mg for phytate-free diets, which is 6.4 mg Zn/d. Extrapolation of the model suggested, for example, that with 900 mg/d phytate, 100 mg dietary Zn is required to attain 6.4 mg absorbed Zn/d. Experimental studies with higher Zn intakes are required to test these predictions.

FIGURE 1

Three-dimensional graph of the trivariate model of absorbed Zn (mg/d) as a function of dietary Zn and dietary phytate. Also shown are the 32 mean data from 15 total diet absorption studies to which the model was fit using nonlinear regression analysis. Stems on data symbols show residuals, i.e. deviation of data from fitted model. Graph positioned with origin (0, 0, 0) at right rear to provide clearer perspective.

FIGURE 2

Model predicted effect of increasing total dietary phytate on the total dietary Zn required to achieve the absorption of the physiologic requirements of 3.8 and 3.2 mg Zn/d for adult men and women, respectively. These values are derived from the IOM (1) with minor adjustment for adult women. The dotted part of the lines showing the predicted relationships between dietary Zn and phytate indicate extrapolation beyond the current experimental data. The y-intercepts of these lines indicate the EAR at 0 dietary phytate. Similarly, the y-intercepts of the dashed lines show the predicted EAR for dietary phytate levels of 1000, 2000, and 3000 mg/d for both genders.

FIGURE 3

Model predictions of the relationship between dietary phytate:Zn molar ratios and the potential for achieving the physiological requirements determined by the IOM (1), with minor correction for physiological requirements for women. Selected phytate:Zn molar ratios indicated with dotted lines.

FIGURE 4

Three-dimensional graph of model surface and data with dietary Zn extrapolated to 100 mg Zn/d. The upper end of the range of Zn absorption shown, 6.4 mg/d, corresponds to the absorption from the UL for adults. Thus, any dietary Zn and phytate combination in the ranges covered by the model surface is predicted to result in absorption less than the predicted safe limit of 6.4 mg/d.