A New Approach to Vitamin A Activity and Dietary Fiber

The Recommended Dietary Allowances (RDAs), established by the Food and Nutrition Board of the Institute of Medicine/The National Academies since 1941, have been applied in many situations—from serving as the basis for nutrition labeling to assessing adequate intakes—some of which were not scientifically appropriate based on how the RDAs were derived.

In addition, the expectation that RDAs were only for nutrients which could be shown to produce deficiency signs in humans limited their use in providing recommendations for many diet and health relationships, particularly where a food component might be shown to be beneficial to health but not essential.

With the growth in the need for multiple values due to these relationships and the availability of functional foods and dietary supplements, FNB has expanded the approach, jointly funded by the United States government agencies and Health Canada, to provide Dietary Reference Intakes (DRIs), which are multiple reference values for a variety of uses. These values include recommended intakes, called RDAs or Adequate Intakes, and tolerable upper intake levels (ULs). The first three DRI reports provided DRIs for many vitamins and minerals; two recently released reports provide recommended intakes for the remaining vitamins and trace elements and propose a definition for dietary fiber for use by U.S. and Canadian regulatory agencies.

Vitamin A. Although a considerable concern in lesser developed countries, overt vitamin A deficiency is rare in the U.S. and Canada. The rationale for development of genetically engineered foods with increased beta-carotene content points to the international importance of improving intake of vitamin A and its dietary precursors.

Of interest to food technologists is that changes are recommended regarding vitamin A bioavailability from fruits and vegetables. Previously, 6 μg of beta-carotene was considered to provide 1 μg of retinol and was equal to 1 mg retinol equivalent (RE). However, new data have shown that the vitamin A activity of provitamin A carotenoids is only half that previously estimated. This means that tabled values, and the relative ability of a specific fruit or vegetable to meet vitamin A requirements, is significantly decreased.

A new term, Retinol Activity Equivalent (RAE), indicates this new relationship: 1 μg RAE = 1 mg retinol = 12 μg betacarotene = 24 μg alpha-carotene or beta-cryptoxanthin (the other two provitamin A carotenoids). The new term enables determination of total vitamin A activity when using food composition tables and databases that list the provitamin A content of foods as μg RE; in this case, the vitamin A contribution in μg RAE of provitamin A carotenoids can be calculated by dividing by 2. If the food is a mixture of plant and animal sources, the vitamin A content of each source needs to be determined and summed. The new RDA is 700 and 900 μg RAE/day for women and men, respectively.

Another issue is the UL for adults, 3,000 mg/day. Risk of birth defects resulting from ingestion of high levels of vitamin A early in pregnancy is increased, as are liver abnormalities at higher intakes. Many Americans consume vitamin A as part of a multivitamin supplement, and perhaps take a vitamin A capsule in addition. It is thus possible to exceed the UL if multiple sources of additional vitamin A are consumed. However, since the UL refers to intake from preformed vitamin A only, carotenoid intake is not included when determining it.

Iron. The RDA for iron is now 8 mg/day for men and postmenopausal women, 18 mg/day for premenopausal women, and 27 mg/day for pregnant women. Because some fortified cereals contain as much as 24 mg/serving, the RDA for pregnant women can easily be met by choosing iron-fortified foods. However, the adult UL of 45 mg/day could be exceeded if multiple servings of iron-fortified foods are consumed daily. The UL for iron is based on gastrointestinal distress; such distress does not occur when consuming high levels of dietary iron naturally found in animal sources, but does occur with certain dietary supplements.

Zinc. The RDA for zinc is 8 and 11 mg/day for women and men, respectively. Zinc is added to a number of breakfast cereals, some containing 100% of the Daily Value (15 mg/day) set by the Food and Drug Administration. Thus, the recommended intake is now lower than the Daily Value. It is also now possible to exceed the adult UL (and much more easily the children’s UL) by consuming highly fortified foods.

Copper and Molybdenum. These nutrients have now been assigned RDAs, which are substantially lower than the high end of the range for previous Estimated Safe and Adequate Daily Dietary Intakes. The RDA for copper and molybdenum is 900 and 45 μg/day, respectively, for both women and men.

Dietary Fiber. Many definitions of dietary fiber, with significant differences, are in use worldwide. At the request of FDA and Health Canada, FNB has proposed the following definitions: Dietary Fiber is nondigestible carbohydrates and lignin that are intrinsic and intact in plants; Added Fiber consists of isolated, nondigestible carbohydrates that have beneficial physiological effects in humans; and Total Fiber is the sum of Dietary Fiber and Added Fiber. The report (available at www.nationalacademies.org/iom/fnb) recommends that the terms Insoluble Fiber and Soluble Fiber be phased out. Comments can be sent to [email protected]. The final definition will be included in FNB’s report on DRIs for macronutrients, expected to be published at the end of the year.

Allison A. Yates is Director of the Food and Nutrition Board, Institute of Medicine/The National Academies, Washington, D.C.; Paula Trumbo is Senior Program Officer with FNB. The DRI reports are available at www.nap.edu .