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Although a large body of observational epidemiological evidence suggests that higher blood concentrations of (5-carotene and other carotenoids obtained from foods are associated with a lower risk of several chronic diseases (see Dietary Reference Intakes for Vitamin C, Vitamin E, Selenium, and Carotenoids [IOM, 20001), no evidence pointed to the need for a certain percentage of dietary vitamin A to come from provitamin A carotenoids to meet the vitamin A requirement. In view of the health benefits associated with consumption of fruits and vegetables, existing recommendations for increased consumption of carotenoid-rich fruits and vegetables are strongly supported (see IOM, 2000). Consumption of five servings of fruits and vegetables per day could provide 5.2 to 6 mg/day of provitamin A carotenoids (Lachance, 1997), which would contribute approximately 50 to 65 percent of the men's RDA for vitamin A.
Excessive alcohol consumption results in a depletion of liver vitamin A stores (Leo and Lieber, 1985). Depletion is partly due to the reduced consumption of foods. Furthermore, mobilization of vitamin A out of the liver may be increased with excessive alcohol consumption (Lieber and Leo, 1986). Because alcohol intake has been shown to enhance the toxicity of vitamin A (Leo and Lieber, 1999) (see "Tolerable Upper Intake Levels"), individuals who consume alcohol may be distinctly susceptible to the adverse effects of vitamin A and any increased intake to meet one's needs should be in the context of maintaining health.
A number of factors can influence the requirement for vitamin A, including iron status, the presence and severity of infection and parasites, the level of dietary fat, protein energy malnutrition, and the available sources for preformed vitamin A and provitamin A carotenoids.
Parasites and Infection. Malabsorption of vitamin A can occur with diarrhea and intestinal infestations (Jalal et al., 1998; Sivakumar and Reddy, 1972). Furthermore, the urinary excretion of vitamin A
is increased with infection, and especially with fever (Alvarez, ct al., 1995; Stephenson ct al., 1994). For these reasons, with parasitic infestation and during infection, the requirement for vitamin A may be greater than the requirements set in this report, which arc based on generally healthy individuals.
Protein Energy Malnutrition. Protein synthesis generally, and specifically rctinol binding protein synthesis, is reduced with severe protein energy malnutrition (PFM) (marasmus and kwashiorkor), and therefore release of rctinol from the liver (assuming stores arc present) is also reduced (Large ct al., 1980). With successful dietary treatment of PFM, growth and tissue weight gain will be stimulated, and the relative requirement of vitamin A will increase during the recovery period.
Vegetarianism. Preformed vitamin A is found only in animal-derived food products. A clinical sign of vitamin A deficiency, night blindness, is prevalent in developing countries where animal and vitamin A-fortificd products arc not commonly available. Although caro-tcnoids such as p-carotcnc arc abundant in green leafy vegetables and certain fruits, because it takes 12 yig of dietary p-carotcnc to provide 1 rctinol activity equivalent (RAF) (as compared to previous recommendations where 1 jig of rctinol was thought to be provided by 6 yig of p-carotcnc [NRC, 1989 and Table 4-3]), a greater amount of fruits and vegetables than previously recommended arc required to meet the daily vitamin A requirement for vegetarians and those whose primary source of vitamin A is green leafy vegetables.
Analyzing intakes of vitamin A and p-c.arotcnc and using an RAF of 12 yig for dietary p-carotcnc indicate that the RDA for vitamin A can be met by those consuming a strict vegetarian diet containing the deeply colored fruits and vegetables (1,262 yig RAF) that arc major sources of p-c.arotcnc in the United States (Chug-Ahuja ct al., 1993) (Table 4-6). The United States has several vitamin A-fortificd foods, including milk, cereals, and infant formula. Furthermore, certain food products, such as sugar, arc being fortified with vitamin A in some developing countries. If menus arc restricted in the amounts of provitamin A carotcnoids consumed and such fortified products arc not part of routine diets, then vitamin A supplements may be required.
Populations Where Consumption of Vitamin A-Rich Foods is Limited. Three major intervention trials have been conducted in developing countries to evaluate the efficacy of provitamin A carotcnoids in
1 20 DTETARY REFERENCE INTAKES
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