International Units and Retinol Equivalents

Vitamin A activity is expressed as USP units, international units (lU). retinol equivalents (RE), and arotene equivalents. The USP units and lU are equivalent. Each unit ex-pre.sses the activity of 0.3 /ug of all-/rans-retinol. Thus. I mg of all-/rufis-retinol has the activity of 3.333 unit.s. Other equivalents are li.sted in Table 26-3. One RE represents the biological activity of I /7g of all-rnin.t-relinol, 6 /ug of carotene. and 12 /rg of mixed dietary carotenoids. The RE i.s u.sed to convert all dietary sources of vitamin A into a single unit for easy comparison."... 

Table 3 International Units (IU) and Retinol Equivalents (RE) for Expressing Vitamin A Values in Humans...

The vitamin A value of foods has traditionally been expressed in international units (IU). One IU is defined as the amount of vitamin A activity contained in 0.334 fig of all-trans-retinyl acetate, which is equivalent to 0.300 fig of alRran.v-retinol. In 1965, an expert committee decided to abandon the IU for vitamin A, proposing instead that the vitamin A value be designated in terms of retinol equivalents (RE), expressed in micrograms of retinol. The RE is defined as the amount of retinol present plus the equivalent amount of retinol that can be obtained from the provitamins. It is purely a dietary concept and is not an equivalency in the usual chemical sense ... 

The vitamin A content of foods is often given in terms of the international unit (IU). One IU of vitamin A is defined as 0.3 tg of all-trans-retinol. The term retinol equivalent (RE) is used to convert all sources of vitamin A and carotenoids in the diet to a single unit. One RE is by definition 1 pg of all-trans retinol, 12 pg of P-carotene, or 24 pg of other (mixed) provitamin A carotenoids. The recommended dietary allowance for vitamin A ranges from 375 pg RE/day for infants to 1,000 RE/day for adults. 

Labeling Indicate the form of the Vitamin A the presence of any preservative, dispersant, antioxidant, or other added substance and the Vitamin A activity in terms of the equivalent amount of retinol in milligrams per gram and in International Units. 

Vitamin A activity can be expressed either as international units lU (1 lU equaling 0.3 micrograms of aH-trans-retinol or 0.6 micrograms of all-trara-beta-carotene) or, more correctly, in retinol equivalents (RE) where 1 RE equals 1 microgram of all-trara-retinol, 6 micrograms of all-trara-beta-carotene or 12 micrograms of other provitamin A carotenoids. Table 1 gives the activities of different forms of vitamin A in RE and lU. 

MEASUREMENT/ASSAY. The assay of vitamin A is accomplished by two basic methods biological, or chemical. The bioassay procedure is based on a biological response such as growth of rats or chicks deficient in vitamin A. It measures the total vitamin A, including provitamin A, present. But, because of the difficulties and time factor in bioassays, chemical assays are usually used. Until recently, dietary allowances of vitamin A were stated in terms of either International Units (lU) or United States Pharmacopeia (USP) units, which are equal. An International Unit (lU) of vitamin A is defined on the basis of rat studies as equal to 0.344 meg of crystalline retinylacetate (which is equivalent to 0.300 meg of retinol, or to 0.60 meg of beta-carotene). These standards were based on experiments that showed that in rats only about 50% of the beta-carotene is converted to vitamin A. In man, however, beta-carotene is not as available as in the rat, due to poorer absorption in the intestines and other factors, with the result that various factors have been used to compensate for this when vitamin A activity of foods and diets have been expressed in lU. 

In order to quantify vitamin A values for humans within the metric system, therefore, international agencies have now introduced the biological equivalent of 1 microgram (1 meg) of retinol as the standard. In 1967, the FAO/WHO proposed that vitamin A allowances be expressed as the equivalent weight of retinol, and that the use of lU be discontinued. The United Kingdom adopted this change and coined the term Retinol Equivalent (RE). The U.S. Food and Nutrition Board (National Academy of Science, National Research Council) and Canada followed suit. The RE system of measurement takes into account the amount of absorption of the carotenes as well as the degree of conversion to vitamin A hence, it is more precise than the lU system. 

The Food and Nutrition Board of the Institute of Medicine revised the Recommended Dietary Allowance (RDA) of vitamin A in 2001 as 900 retinol activity equivalents (RAF) for men and 700 RAF for women. The RAF was introduced to avoid the ambiguity of international units (lU), which arises because 1 lU of vitamin A (0.3 pg) and 1 lU of the vitamin A precursor (provitamin A) all-tra s-/ -carotene (0.6 pg) do not have the same biological activity. Rather, 6 lU of /3-carotene and 12 lU... 

Prior to 2001, the retinol equivalent (RE) was used and this unit is still found in most food composition tables. While similar in theory to the RAE, the RE is based on older conversion factors for carotenoids in foods. Using RAE, the vitamin A activity of the provitamin A carotenoids in foods is half that using RE. An older unit, the international unit (lU or USP), which should eventually be replaced by these newer units, is still used in food tables and on some supplement labels. One lU is equal to 0.3 pg of all-fra/M-retinol. Finally, another indicator of nutritional value, % daily value (%DV), is a less quantitative but more convenient means for consumers to compare foods and select those with a substantial portion of a given nutrient. The %DV does not require extensive knowledge of nutritional units this value appears on food package labels in the US. Besides its application in food labeling, the %DV is a useful value for quickly comparing the vitamin A contents of various common foods. 

Table 2 Recommended dietary allowances (RDA) for vitamin A in micrograms ( g), retinol activity equivalents (RAE) and international units (lU), and tolerable upper intake levels (UL, pg retinol day" ) for children and adults ...

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