the cut-point method would lead to an estimated prevalence of inadequacy of 7.3 percent, which differs considerably from the estimate of 16.5 percent obtained by using the full probability approach. The reason for the discrepancy is that one of the conditions needed for the cut-point approach (a symmetrical requirement distribution) is not true for iron requirements of menstruating women.
Comparison of Assessments Using the Probability Approach to Biochemical Assessment. If requirement estimates are correct and both the dietary data and biochemical measures are reliable estimates of true usual intake and true blood concentrations in the same population, then the prevalence of apparently inadequate dietary intakes and biochemical deficiency should be similar, as discussed in Chapter 9. In the example above, one would expect to observe a prevalence of low serum ferritin concentrations (< 15 μg/L) that approximates the prevalence of inadequate intakes, or about 16.5 percent. The individuals with low serum ferritin concentrations are not necessarily the same as the individuals with low intake values, so the probability approach is not appropriate for identifying specific individuals with low serum ferritin values.
Special situations in which iron requirements may vary are summarized in Table 14-2 along with suggestions on how to adjust estimates of requirements.
Bioavailability of zinc is known to vary greatly, depending on the intakes of other dietary components, most notably phytate, that inhibit absorption. The World Health Organization (WHO, 1996) suggested that bioavailability of zinc might range from 15 percent in a diet with low bioavailability to a high of 50 percent in diets with high bioavailability. Characteristics associated with diets varying in bioavailability are summarized in Table 14-3. Gibson and Ferguson (1998) have reviewed the use of the phytate:zinc ratio for assessing dietary zinc intake. Table 14-3 indicates that diets of most North Americans would have “medium” bioavailability, approximating the fractional absorption rate of 38 percent that was used in estimating the EAR for adults. It also indicates that diets of some strict vegetarians may have low bioavailability, with the result that their dietary requirements for zinc would be increased. A quantitative estimate