Physiological requirements for zinc

Abstract

The estimates of zinc physiological requirements by the International Zinc Nutrition Consultative Group (IZiNCG) in 2004 were conspicuously low in comparison with those estimated by the Institute of Medicine (IOM) in 2001. The objective of this review is to explore the reasons for this gap and to reflect on lessons learned. All estimates of inevitable losses of endogenous zinc, especially intestinal excretion of endogenous zinc, were reviewed. An error in zinc menstrual losses, as well as a minor error in the linear regression of endogenous fecal zinc (EFZ) vs. total daily zinc absorption (TAZ) by IOM, were corrected. The review revealed an error by IZiNCG in selecting two data points for the linear regression of EFZ on TAZ. A second major reason for the "gap" is attributable to weighting of the data in the regression analysis by number of subjects per study by IZiNCG. Adjusting for these factors, together with use of the same reference data for body weights, resulted in satisfactory agreement between the two estimates of physiological requirements. The lessons to be learned from this review are discussed together with suggestions for future action by IOM as well as a constructive role for IZiNCG.

Figure 1

Data and regression lines of intestinal losses of endogenous zinc on absorbed zinc used by the FNB/IOM (black square symbols and solid line) and the IZiNCG (gray circle symbols and dashed line). This graph is similar to Figure 1.3 in the IZiNCG document [2], but shows the actual data used by the IOM which differ slightly from those reported by the IZiNCG.

Figure 2

Revision of the intestinal endogenous zinc loss versus absorbed zinc data used by the IZiNCG to replace two erroneous data (x symbols) with more accurate data (open circles) from the same publication. The original IZiNCG regression line is also shown.

Figure 3

Comparison of estimates of zinc physiological requirements. Bars at the extreme left depict the original IZiNCG estimates of requirements for adult men (3a) and adult women (3b). Corrected original IOM estimates by IOM are depicted by extreme right hand bars. This figure shows the extent to which the gap between these estimates is closed by correction of the IZiNCG data (Figure 2) and the elimination of weighting by number of subjects per data point in regression of endogeous fecal zinc versus total absorbed zinc per day. In addition, Figure 3b shows the extent to which the gap for females is further narrowed by correction of an error by FNB/IOM in the estimate of zinc losses in menses.

Figure 4

Relation of measured sample variance to sample size for 16 of the data used by the IZiNCG for which sufficient information to calculate the variances was reported. The “2” next to a symbol indicates that the symbol represents two data having the same values. These are cases where a pooled standard deviation was reported for two measurements. Counter to expectation, there is a significant positive correlation between variance and sample size (r = 0.52, P=0.041). When the outlying points are removed, the remaining data are uncorrelated (r = −0.075, P=0.81).

Figure 5

Regressions of IOM (black square symbols and solid line) and IZiNCG (gray circle symbols and dashed line) data after correction of IZiNCG data and elimination of weighted regression.

Figure 6

The further reduction in the difference between FNB/IOM and IZiNCG estimates of physiological zinc requirements for adult men (6a) and women (6b) if, in addition to the corrections depicted in Figure 3, the same body weight standards used by FNB/IOM are applied (not as a correction factor) to IZiNCG estimates. The percentages of the gap unaccounted for are 4 % and 2.5 % for men and women, respectively, of the IOM estimates.