Menstrual Losses. Additional iron is lost from the body as a result of menstruation in fertile women. Menstrual iron losses have been estimated in a number of studies (Beaton, 1974) (see review by Hefnawi and Yacout, 1978) and in three large community surveys conducted in Sweden (Hallberg et al., 1966b), England (Cole et al., 1971), and Egypt (Hefnawi et al., 1980). There was a reasonable degree of consistency between the different studies. The median blood volume lost per period reported in the three largest studies was 20.3 mL (Egypt), 26.5 mL (England), and 30.0 mL (Sweden). Losses greater than 80 mL were reported in less than 10 percent of women.
Accretion. The requirement for pregnancy and for growth in children and adolescents can also be estimated from known changes in blood volume, fetal and placental iron concentration, and the increase in total body erythrocyte mass.
Chemical balance is the classical method for measuring nutrient requirements through the estimation of daily intake and losses. While this direct approach is conceptually appealing, its use in measuring iron requirements presents several major technical obstacles (Hegsted, 1975). For instance, it is difficult to achieve a steady state with nutrients such as iron that are highly conserved in the body. Because the fraction of the dietary intake that is absorbed (and excreted) is very limited, even small errors in the recovery of unabsorbed food iron in the feces invalidate the results.
Thirteen adult balance studies were evaluated (Table 9-4). All of these studies yielded values that exceed the daily iron loss calculated on the basis of the disappearance of a long-lived iron radio-isotope after uniform labeling of body iron (Green et al., 1968). One might therefore conclude that all of the subjects were in positive balance during the period of observation. Moreover, the magnitude of estimated positive balance in most cases predicted the relatively rapid accumulation of body iron. Neither of these conclusions is compatible with numerous other experimental observations. Therefore, balance studies were not considered in estimating an average requirement.