An important limitation for some food composition tables is the method of expressing the activity of the nutrient. The estimation of nutrient activity is a large and expanding field of research and includes studies of both the absorption of the nutrient and its bioavailability for metabolic processes. For example, iron bioavailability has been debated extensively, and many algorithms for calculations have been proposed. Virtually all these require separating iron that is found as heme-iron in animal products from nonheme sources of iron. If these two variables are not carried on the composition table, it will not be possible to calculate iron bioavailability for specific intakes.
Vitamin A also illustrates the complexity of properly expressing the physiologically meaningful form of a nutrient. Until 1967, the vitamin A value of foods was expressed in international units (IUs), which was equivalent to 0.3 mg of retinol and 0.6 mg of ^-carotene. This form of expression is still used in many composition tables and also on nutrition labels for both foods and dietary supplements. A more relevant unit of activity, micrograms of retinol equivalents (REs), was adopted in 1967 and has been used to set recommended nutrient intake levels. A lower relative pro-vitamin A activity of carotenoids was assumed, and thus it is not possible to directly convert IUs into REs, unless both the retinol and the carotenoid levels of a food are given. Recently, the estimated pro-vitamin activity of carotenoids has been further reduced and a newer unit proposed: micrograms of retinol activity equivalents (RAEs). Again, it is not possible to convert between REs and RAEs (or between IUs and RAEs), unless the retinol and carotenoid components of a food are available. Increasingly, food composition tables are carrying separate variables for the specific forms of nutrients such as vitamin A and iron, but this is not the case for many of the older tables. Such disaggrega-tion is an obvious advantage because it allows for recalculation of nutrient activity when there is a scientific consensus that new availability factors are needed. Tables that cannot be easily updated to reflect new information will lag behind the current knowledge and thus will have more limited usefulness.
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