diets in a metabolic ward reported skin lesions (Burri et al., 1993). However, this effect was not observed after 60 days of depletion in a subsequent β-carotene depletion study by the same group of investigators (Lin et al., 1998). These studies of carotene-deficient diets were reported to increase various measures of oxidative susceptibility (Dixon et al., 1994, 1998; Lin et al., 1998), but as discussed below, this is of uncertain relevance with regard to clinical outcomes.
Vitamin A equivalency is a possible indicator for establishing requirements for provitamin A carotenoids. However, any such establishment of requirements for carotenoids based on vitamin A activity must be considered in concert with the evaluation of requirements for vitamin A. This information will be presented in a later Dietary Reference Intakes report.
The effect of increasing β-carotene intake on several markers of antioxidant activity has been investigated in a series of studies involving humans. These studies have examined antioxidant marker activity in apparently healthy men and women as well as in subjects who were physiologically challenged (i.e., smokers and patients with coronary disease or cystic fibrosis).
Studies of the effect of β-carotene intake on measures of antioxidant activity are summarized in Table 8-2. The dietary source of β-carotene ranged from modification of diets with normally consumed foods to giving supplements that provided as much as 120 mg/day of a highly bioavailable preparation. In general, subjects in most studies consumed β-carotene in amounts that would be difficult to achieve from foods alone and, as a result, relate to the pharmacological range of intakes.
The findings reported in Table 8-2 indicate that β-carotene supplementation did not alter, or inconsistently alter, markers of antioxidant activity, which were somewhat dependent on β-carotene intake. In studies in which subjects were fed less than 25 mg/day of β-carotene, either from foods or as a supplement, changes in the markers for antioxidant activity were minimal. Exceptions noted