Mathematical model of zinc absorption: effects of dietary calcium, protein and iron on zinc absorption

Abstract

A previously described mathematical model of Zn absorption as a function of total daily dietary Zn and phytate was fitted to data from studies in which dietary Ca, Fe and protein were also measured. An analysis of regression residuals indicated statistically significant positive relationships between the residuals and Ca, Fe and protein, suggesting that the presence of any of these dietary components enhances Zn absorption. Based on the hypotheses that (1) Ca and Fe both promote Zn absorption by binding with phytate and thereby making it unavailable for binding Zn and (2) protein enhances the availability of Zn for transporter binding, the model was modified to incorporate these effects. The new model of Zn absorption as a function of dietary Zn, phytate, Ca, Fe and protein was then fitted to the data. The proportion of variation in absorbed Zn explained by the new model was 0·88, an increase from 0·82 with the original model. A reduced version of the model without Fe produced an equally good fit to the data and an improved value for the model selection criterion, demonstrating that when dietary Ca and protein are controlled for, there is no evidence that dietary Fe influences Zn absorption. Regression residuals and testing with additional data supported the validity of the new model. It was concluded that dietary Ca and protein modestly enhanced Zn absorption and Fe had no statistically discernable effect. Furthermore, the model provides a meaningful foundation for efforts to model nutrient interactions in mineral absorption.

Fig. 1

Model-predicted effect of dietary calcium on zinc absorption relative to dietary zinc intake for calcium intakes of 600 and 1200 mg/d at dietary phytate levels of 600 (—) and 2400 (---) mg/d. Dietary protein was fixed at 60 g/d.

Fig. 2

Model-predicted effect of dietary protein on zinc absorption relative to dietary zinc intake for protein intakes of 40 and 100 g/d at dietary phytate levels of 600 (—) and 2400 (---) mg/d. Dietary calcium was fixed at 1000 mg/d.