velocity is a primary clinical feature of mild zinc deficiency and can be corrected with zinc supplementation (Hambidge et al., 1979b; Walravens et al., 1989). Other functions that respond to zinc supplementation include pregnancy outcome (Goldenberg et al., 1995) and immune function (Bogden et al., 1987). Evidence of the efficacy of zinc lozenges in reducing the duration of common colds is still unclear (Jackson et al., 2000).

Severe zinc deficiency has been documented in patients fed intravenously without the addition of adequate zinc to the infusates (Chen et al., 1991) and in cases of the autosomal recessively inherited disease acrodermatitis enteropathica (Walling et al., 1989). Because of the ubiquity of zinc and the involvement of this micronutrient in so many core areas of metabolism, it is not surprising that the features of zinc deficiency are frequently quite basic and nonspecific, including growth retardation, alopecia, diarrhea, delayed sexual maturation and impotence, eye and skin lesions, and impaired appetite. Clinical features and laboratory criteria are not always consistent. This inconsistency poses a major difficulty in the quest to validate reliable, sensitive clinical or functional indicators of zinc status that apply to a range of otherwise potentially useful laboratory indicators such as alkaline phosphatase activity.

A further major conundrum is posed by the impressive, yet apparently imperfect, homeostatic mechanisms that maintain a narrow range of zinc concentrations within the body in spite of widely diverse dietary intakes of this metal and in spite of differences in bioavailability. This situation applies, for example, to circulating zinc in the plasma, which consequently provides only an insensitive index of zinc status (King, 1990). Therefore, it has become increasingly apparent that homeostatic mechanisms fall short of perfection and that clinically important features of zinc deficiency can occur with only modest degrees of dietary zinc restriction while circulating zinc concentrations are indistinguishable from normal.

The selection of zinc absorption (more specifically, the minimal quantity of absorbed zinc necessary to match total daily excretion of endogenous zinc) as the principal indicator for adult Estimated Average Requirements (EAR) has been based on the evaluation of a factorial approach to determining zinc requirements. Details of this