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ing requirements is kinetic modeling of body pools, using steady-state compartmental analyses. This approach is unlikely to supplant depletion-repletion studies because it suffers from a number of drawbacks. Several assumptions that cannot be tested experimentally are often needed, and the numbers obtained for body pool sizes are inherently imprecise. Even if accurate assessments of body pools were possible and were obtained, such information would be useful in setting a requirement only if one could establish the body pool size at which functional deficiency occurs. The amount needed for restoration of biochemical status indicators to baseline values is not necessarily equivalent to the requirement for the nutrient.

For many of the nutrients under review, useful data are seriously lacking for setting requirements for infants, children, adolescents, pregnant and lactating women, and the elderly. Studies should use graded levels of nutrient intake and a combination of response indexes, and they should consider other points raised above. For some of these nutrients, studies should examine whether the requirement varies substantially by trimester of pregnancy. Data are lacking about gender issues with respect to metabolism and requirements of these nutrients.

More information is needed on the vitamin A activity of carotenoids from plant foods and mixed meals, including meat. Field trials, studying the vitamin A efficacy of plant foods, are needed in which preformed vitamin A (positive control) is used at a supplementation level equivalent to plant food interventions. Assessment of the bioconversion and retinol molar equivalency ratio of carotenoids has mostly been conducted on single foods, rather than on a mixture of fruits and vegetables. Newer methods, such as stable isotopic methods, to evaluate the bioconversion of provitamin A carotenoids to vitamin A are encouraged. With such data, more information can be obtained about the relative contribution of dietary provitamin A carotenoids and dietary preformed vitamin A to vitamin A nutrition.

Further research is needed to evaluate the impact of nonnutritional factors on nutrient indicators. Evidence from national survey data provided in this report suggests that body mass index and plasma glucose concentration are positively correlated with indicators of iron status. Such non-nutritional factors may markedly affect the interpretation of the survey data for certain subpopulations where the prevalence of non-nutritional factors is high.

There is increasing evidence to suggest that the interaction between many of these nutrients and other food components affect nutrient absorption and metabolic utilization (bioavailability), but



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