Zinc retention from an experimental diet based on the U.S.F.D.A. total diet study

Fractional Zinc Absorption for Men, Women, and Adolescents Is Overestimated in the Current Dietary Reference Intakes

BACKGROUND

The fractional zinc absorption values used in the current Dietary Reference Intakes (DRIs) for zinc were based on data from published studies. However, the inhibitory effect of phytate was underestimated because of the low phytate content of the diets in the studies used.

OBJECTIVE

The objective of this study was to estimate the fractional absorption of dietary zinc from the US diet by using 2 published algorithms.

METHODS

Nutrient intake data were obtained from the NHANES 2009-2010 and the corresponding Food Patterns Equivalents Database. Data were analyzed with the use of R software by taking into account the complex survey design. The International Zinc Nutrition Consultative Group (IZiNCG; Brown et al. Food Nutr Bull 2004;25:S99-203) and Miller et al. (Br J Nutr 2013;109:695-700) models were used to estimate zinc absorption.

RESULTS

Geometric means (95% CIs) of zinc absorption for all subjects were 30.1% (29.9%, 30.2%) or 31.3% (30.9%, 31.6%) with the use of the IZiNCG model and Miller et al. model, respectively. For men, women, and adolescents, absorption values obtained in this study with the use of the 2 models were 27.2%, 31.4%, and 30.1%, respectively, for the IZiNCG model and 28.0%, 33.0%, and 31.6%, respectively, for the Miller et al. model, compared with the 41%, 48%, and 40%, respectively, used in the current DRIs. For preadolescents, estimated absorption values (31.1% and 32.8% for the IZiNCG model and Miller et al. model, respectively) compare well with the conservative estimate of 30% used in the DRIs. When the new estimates of zinc absorption were applied to the current DRI values for men and women, the results suggest that the Estimated Average Requirement (EAR) and RDA for these groups need to be increased by nearly one-half of the current values in order to meet their requirements for absorbed zinc.

CONCLUSIONS

These data suggest that zinc absorption is overestimated for men, women, and adolescents in the current DRI. Upward adjustments of the DRI for these groups are recommended.

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

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.

The use of dysprosium to measure endogenous zinc excretion in feces eliminates the necessity of complete fecal collections

The secretion of endogenous zinc (Zn) into the gut and subsequent excretion in the feces is understood to play a major role in maintaining Zn homeostasis in humans. Therefore, the measurement of endogenous Zn losses in the feces (EFZ) can be an important aspect of the study of Zn metabolism and homeostasis. The methods currently used to measure EFZ have the disadvantage of requiring complete fecal collections over multiple days. We have investigated the use of dysprosium (Dy), a nonabsorbable rare earth metal, in a method of determining EFZ that does not require complete fecal collections and permits the measurement of EFZ from several fecal samples. The method was evaluated using data from a study of free-living adult females in which Dy was administered 3-4 times/d over a period of 5 or 6 d to monitor completeness of fecal collections. The results did not differ from those obtained using an established isotope dilution method. We found that the measurement of the sample Dy:Zn ratio was useful for selecting samples for measurement. We conclude that the Dy method of determining EFZ is a valid and less burdensome alternative to current techniques.

Dietary indicators for assessing the adequacy of population zinc intakes

The assessment of dietary zinc intakes is an important component of evaluating the risk of zinc deficiency in populations, and for designing appropriate food-based interventions, including fortification, to improve zinc intakes. The prevalence of inadequate zinc intakes can describe the relative magnitude of the risk of zinc deficiency in the population and identify subpopulations at elevated risk. As a cornerstone to evaluating the adequacy of population zinc intakes globally, a set of internationally appropriate dietary reference intakes must be defined. The World Health Organization/Food and Agriculture Organization/International Atomic Energy Agency (WHO/FAO/IAEA) and the Food and Nutrition Board/US Institute of Medicine (FNB/IOM) have presented estimated average requirements (EAR) for dietary zinc intake, and, more recently, the International Zinc Nutrition Consultative Group (IZiNCG) presented a revised set of recommendations for international use. A prevalence of inadequate zinc intakes greater than 25% is considered to represent an elevated risk of population zinc deficiency. As the requirement estimates are derived from smaller, clinical studies and, for children, most components of the estimates are extrapolated from data for adults, it was desirable to evaluate their internal validity. The estimated physiological requirements for adult men and women appear to adequately predict zinc status as determined by biochemical indicators of status and/or zinc balance. With the use of data from available studies, the reported prevalence of low serum zinc concentration and the estimated prevalence of inadequate zinc intakes predict similar levels of risk of zinc deficiency, particularly among pregnant and nonpregnant women. Conformity between these two indicators is less consistent for children, suggesting that further data and/or direct studies of zinc requirements among children are needed.

A mathematical model of zinc absorption in humans as a function of dietary zinc and phytate

The quantities of zinc and phytate in the diet are the primary factors determining zinc absorption. A mathematical model of zinc absorption as a function of dietary zinc and phytate can be used to predict dietary zinc requirements and, potentially, enhance our understanding of zinc absorption. Our goal was to develop a model of practical and informative value based on fundamental knowledge of the zinc absorption process and then fit the model to selected published data to assess its validity and estimate parameter values. A model of moderate mathematical complexity relating total zinc absorption to total dietary zinc and total dietary phytate was derived and fit to 21 mean data from whole day absorption studies using nonlinear regression analysis. Model validity, goodness of fit, satisfaction of regression assumptions, and quality of the parameter estimates were evaluated using standard statistical criteria. The fit had an R(2) of 0.82. The residuals were found to exhibit a normal distribution, constant variance, and independence. The parameters of the model, A(MAX), K(R), and K(P), were estimated to have values of 0.13, 0.10, and 1.2 mmol/d, respectively. Several of these estimates had wide CI attributable in part to the small number and the scatter of the data. The model was judged to be valid and of immediate value for studying and predicting absorption. A version of the model incorporating a passive absorption mechanism was not supported by the available data.

Dietary boron intakes of selected populations in the United States

Knowledge of daily boron (B) intakes will assist researchers in establishing B requirements and elucidating the metabolic role of B in humans. B concentrations in commonly consumed foods were utilized to approximate the B intake of selected US populations. Triplicate food samples were digested at low temperatures with 16 M HNO3 and 30% H2O2, and analyzed by inductively coupled plasma spectroscopy (ICP). Milk and dairy products, juices, and beverages were the largest contributors to dietary B. Total B content of average daily diets ranged from 548 microg for toddlers (2 yr old) to 883 microg for mature males (60-65 yr old), excluding tap water contributions. Toddlers consumed 3.7 times more B than mature males when adjusted for body weight (body wt) and consumed the diet with the highest boron density (1.8 microg/kJ or 0.43 microg/kcal). Adolescent females consumed a diet with the lowest B density (1.12 microg/kJ or 0.26 microg/kcal). Food B concentrations applied to diet records (1020 microg/d) and ICP analysis of the corresponding food composites (1170 microg/d) were comparable (p < 0.05). Current estimations of B in US diets are consistent with reports in the literature stating that normal adult daily B intakes are approx 1 mg.