Retinol, international unit

A strikingly early symptom of vitamin A deficiency is night blindness. A variety of other symptoms include dry skin and hair, conjunctivitis of the eyes, retardation of growth, and low resistance to infection. The skin symptoms are particularly noticeable in the internal respiratory passages and alimentary canal lining. About 0.7 mg/day of vitamin A is required by an adult. The content of vitamin A in foods is often expressed in terms of international units 1.0 mg of retinol equals 33331.U. 

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 ... 

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

Vitamin A or a precursor must be provided in the diet. This vitamin occurs in various forms (vitamers) retinol (alcohol), retinal (aldehyde), retinoic acid and vitamin A palmitate (ester). Requirements for vitamin A are usually expressed in international units (IU) per kg of diet. The international standards for... 

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. 

The obsolete International Unit (iu) of vitamin A activity was based on biological assay of the ability of the test compound to support growth in deficient animals (1 iu = 10.47 nmol of retinol = 0.3 //g of free retinol or 0.344 //g of retinyl acetate). 

Vitamin A is a fat-soluble vitamin. The RDA for vitamin A is 1-0 mg of retinol, or its equivalent. The international unit [lU) is used to compare the biological activities of various sources of vitamin A- One iU of vitamin A activity can be supplied by 0.3 pg of all-ffiflMS-relinol, 0.344 pg of all-trans-retinyl acetate, or 0,60 pg of all-frens-p-carotene. 

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. 

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."... 

Vitamin A (international unit) Vitamin A (retinol activity equivalents) Retinol... 

Vitamins The data for vitamins come mainly from tables. The analytical methods were rarely indicated in the sources. Vitamin A vitamin A activity expressed in international units (1 ID = 0.3 pg of retinol) Vitamin D vitamin D2 in plant products and D3 in animal products. Vitamin D activity is expressed in international units (1 ID = 0.025 pg of cholecalciferol) Vitamin E the vitamin activity of tocopherols and tocotrienols is expressed as alpha-tocopherol. 

The biological activity of a pure V. can be expressed in International Units (lU). Thus, 0.3 mg V.A (retinol), 8 mg thiamin hydrochloride, 0.18 mg biotin, 50 mg L-ascorbic acid, 0.025 mg ergocalciferol or 1 mg DL-a-tocopherol acetate each corresponds to 1 lU. The system of lU is retained, even though the structures of all V. are known, because in most cases a V. is a family of closely related compounds all with the same action, but with different activities. 

Plants do not contain vitamin A as such, although many of them include a variety of highly pigmented carotenoids. Some of these, namely the a-, and y-carotenes, can be converted into vitamin A in the body and hence are provitamins, -carotene is the most active of these. It is a symmetrical compound and can be converted into two molecules of vitamin A in the intestinal wall and in the liver, but since /3-carotene is poorly absorbed and only partially converted, its utilization efficiency is only about one-sixth. Unchanged carotene is found in the plasma and in the body fat. Vitamin A and carotene are both fairly stable and not much affected by cooking, although some activity may be lost if fats become rancid. The recommended daily intake for adults is 750 pg (5000 International Units) of retinol, 6 x 750 pg of/3-carotene or 12 x 750 pg of a- and y-carotene. 

Before pure vitamin A was available for chemical analysis, the vitamin A content of foods was determined by biological assays, and the results expressed in standardized international units (iu) 1 iu = 0.3 fig retinol, or 1 fig of retinol = 3.33 iu. Although obsolete, international units are sometimes still used in food labelling. 

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 terms of International Units, E)eta-carotene (by weight) is 1/2 as active, and the other provitamin A carotenoids are 1/4 as active, as retinol. Also, retinol is completely absorlsed by the intestine, but only aixtut 1/3 of the intake of provitamin A carotenoids is ab)sorbed. Of the absorlsed carotenoids, only 1/ 2 of the E)eta-carotene and 1 /4 of the other provitamin A carotenoids are converted to retinol. It follows that beta-carotene is only 1/6 as active, and the other carotenoids 1/12 as active, eis retinol. 

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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