The second factor that is critical to determining the EAR is dietary iron bioavailability. Although much is known about the factors that enhance and inhibit iron absorption, the application of specific algorithms based on these factors to complex diets remains imprecise. Based on the general properties of the major dietary enhancers, the FAO/WHO (1988) identified three levels of bioavailability and the associated compositional characteristics of such diets. The typical diversified U.S. and Canadian diets containing generous quantities of flesh foods and ascorbic acid were judged to be 15 percent bioavailable. Constrained vegetarian diets, consisting mainly of cereals and vegetable foods with only small quantities of meat, fish, and ascorbic acid, were judged to be 10 percent bioavailable; very restricted vegetarian diets were judged to be 5 percent bioavailable. These levels of absorption were predicted for individuals who were not anemic, but had no storage iron. A mixed American or Canadian diet would therefore be predicted to allow the absorption of about 15 percent of the dietary iron in an individual whose iron status was selected as a basis for calculating the EAR (serum ferritin concentration of 15 μg/L).

Hallberg and Rossander-Hulten (1991) suggested that the bioavailability of iron in the U.S. diet may be somewhat higher than 15 percent: approximately 17 percent. Some support for this contention was provided by the observation of Cook and coworkers (1991) who measured nonheme iron absorption over a 2-week period in free-living American volunteers eating their customary diets. After correcting nonheme iron values (to a serum ferritin concentration of 15 μg/L), the bioavailability of nonheme iron in self-selected diets was 16.8 percent ([34 μg/L ÷ 15 μg/L] × 7.4 percent). Heme constitutes 10 to 15 percent of iron in the adult diet (Raper et al., 1984) and the diet of children (see Appendix Table I-2) and is always well absorbed. Based on a conservative estimation for overall heme absorption of 25 percent (Hallberg and Rossander-Hulten, 1991) and again a conservative estimate for the proportion of dietary iron that is in the form of heme (10 percent), estimated overall iron bioavailability in the mixed American or Canadian diet is approximately 18 percent:

For these reasons, 18 percent bioavailability is used to estimate