mg/day (Hess et al., 1977). Therefore, the calculated total loss of endogenous zinc for women via routes other than the intestine is 1.0 mg/day (0.44 + 0.46 + 0.10).

Step 2: Relationship Between Excretion of Endogenous Zinc via the Intestine and Quantity of Zinc Absorbed. In contrast to other endogenous zinc losses, the quantity of endogenous zinc excreted via the intestine is positively correlated with the quantity of zinc absorbed over a wide range. This correlation is shown in Figure 12-1. This figure is based on 10 sets of balance data from seven studies (Hunt JR et al., 1992; Jackson et al., 1984; Lee et al., 1993; Taylor et al., 1991; Turnlund et al., 1984, 1986; Wada et al., 1985) of healthy young men, which also included isotopic tracer measurements of fractional zinc absorption. This correlation, in turn, allows for the quantification of daily zinc absorption and intestinal excretion of endogenous zinc. Importantly, this linear relationship, which indicates that for each milligram of zinc absorbed the intestine excretes approximately 0.6 mg/day of endogenous zinc, has been demonstrated only for zinc absorption ranging from 0.8 to 5.5 mg/day. It is also noted that most of these data were relatively short-term, and these variables were not examined while the participants were consuming habitual diets. However, the studies did extend as long as 6 months, a duration that suggests the observed relationship between absorption and endogenous losses via the intestine is a long-term phenomenon. Therefore, in contrast to other endogenous losses of zinc, losses from the intestine cannot be treated as a constant.

To achieve balance, absorption must match the sum of nonintestinal and intestinal endogenous zinc losses. The minimum amount of zinc that must be absorbed before absorption matches the losses is determined in step 3 below.

Corresponding data for women are both limited and divergent (Hunt JR et al., 1992, 1998; Sian et al., 1996; Turnlund et al., 1991). It has therefore been assumed that there are no significant gender differences for this relationship between absorbed zinc and intestinal excretion of endogenous zinc.

Step 3: Determination of Minimal Zinc Absorption Required to Replace Total Endogenous Zinc Excretion. The sum of nonintestinal endogenous zinc losses (1.27 mg/day for men and 1.0 mg/day for women) is added to the linear regression line for excretion of endogenous zinc in the feces versus absorbed zinc (Figure 12-1). These “adjusted” lines depict the quantitative relationship between absorbed zinc and total endogenous zinc losses for men and women.