Nutrient databases will need to be revised to give total vitamin A activity in μg RAE. In addition, developers of nutrient databases may choose to provide the amount (μg) of preformed vitamin A and of individual carotenoids. Thus, if the vitamin A activity for provitamin A carotenoids changes in the future, it will be possible to recalculate total vitamin A. In the meantime, it is possible to estimate total vitamin A activity in μg RAE from existing tables in μg RE. For foods, such as liver, containing only vitamin A activity from preformed vitamin A (retinol), no adjustment is necessary. Vitamin A values for foods (e.g., carrots) containing only plant sources (provitamin A carotenoids) of vitamin A can be adjusted by dividing the μg RE by two. For foods that are mixtures containing both plant and animal sources of vitamin A (e.g., a casserole containing meat and vegetables), the adjustment process is more complex. If the recipe for a mixture is known, the new vitamin A value may be calculated after adjusting the vitamin A content of each ingredient, as necessary. Alternatively, if the nutrient database contains values as μg RE for both total vitamin A and carotenoids, then it is possible to calculate a new value for both carotenoids and for total vitamin A. For example, USDA’s Nutrient Database for Individual Surveys contains both these variables. To determine a revised total vitamin A value, the retinol value is calculated as the difference between the original total vitamin A value and the original carotenoid value. The revised total vitamin A content is then calculated as the sum of the retinol value and the adjusted carotenoid value, which is the original carotenoid value in μg RE divided by two. As discussed in the following section, this same procedure may be used to adjust intake data that have been analyzed using other databases.
As shown in Figure 4-2, supplemental β-carotene has a higher bioconversion to vitamin A than does dietary β-carotene. With low doses, the conversion is as high as 2:1, and developers of composition information for dietary supplements should use this higher conversion factor. Little is known about the bioconversion of the forms of β-carotene that are added to foods, so fortification forms of β-carotene should be assumed to have the same bioconversion as food forms, 12:1. Food and supplement labels usually state vitamin A levels in International Units (IU). One IU of retinol is equivalent to 0.3 μg of retinol, or 0.3 μg RAE. One IU of β-carotene in supplements is equivalent to 0.5 IU of retinol or 0.15 μg RAE (0.3 × 0.5). One IU of dietary β-carotene is equivalent to 0.165 IU retinol or