Provitamin retinol equivalency

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 ... 

For food labeling purposes, which require the actual amounts of vitamin A in the food rather than the nutritional value, data obtained by physicochemical assay are expressed on a weight basis. In plant-derived foods, the appropriate units are /3-carotene equivalents expressed in micrograms of /3-carotene. By definition, 1 /3-carotene equivalent is equal to 1 fig of all-trans-/3-carotene or 2 fig of other, largely animal-derived foods, the units are either micrograms of retinol or retinol equivalents (32). 

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. 

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. 

The best documented and established function of some carotenoids is their provitamin A activity, especially of P-carotene. One mole of P-carotene can theoretically be converted, by cleavage of C 15 = C 15 double bond, to yield two moles of retinal (Reaction 9.1). However, the physiological efficiency of this process appears to be only 50%. The observed average efficiency of intestinal P-carotene absorption is only two thirds of the total content. Thus, a factor of 1/6 is used to calculate the retinol equivalent (RE) from P-carotene, but only 1/12 from the other provitamin A carotenoids in food (Combs, 1992). In fruits and vegetables P-carotene content is used as a measure of the provitamin A content. 

Concerning the fat-soluble vitamins, the range of RDA value of vitamin E is 5-50 mg of a-tocopherol equivalent per day, the median 10 mg/day. The RDA value of vitamin A is 360-1650 gg/day of retinol equivalent (RE) with the median of 800 RE/day. As mentioned before, seaweed does not contain intrinsic vitamin A, but its provitamins, with p-carotene as the most abimdant. The relevant dose of p-carotene intake is about 15 mg/day (Krinsky, 1998). The vitamin A activity (gg RE/100 g) is recalculated from determined content of p-carotene. 

As retinol activity equivalents (RAEs). 1 RAE = 1 pg retinol, 12 pg p-carotene, 24 pg a-carotene, or 24 pg p-cryptoxanthin. The RAE for dietary provitamin A carotenoids is twofold greater than retinol equivalents (RE), whereas the RAE for preformed vitamin A is the same as RE As cholecalciferol. 1 pg cholecalciferol = 40 lU vitamin D Under the assumption of minimal sunlight... 

Lycopene is one of the most widely consumed carotenoids however, it lacks provitamin A activity. Through cyclisation at one or both end groups of lycopene, carotenes are formed, containing one (8 and 7 carotenes) or two a and 3 carotenes) ionone rings. Vitamin A (all-tmns-retinol) consists of a (3-ionone ring with a side chain of three isoprenoid units. Hence a and 3 carotenes provide two molecules of retinol in the human body whereas 8 and 7 carotenes provide only one. In reality, carotenoids are incompletely absorbed and 6 jLg of p carotene is equivalent to 1 pg retinol equivalent (RE). Carotenoids with hydroxylated ionone rings (e.g. lutein) provide no vitamin A activity (Eitenmiller and Landen 1999). 

Retinol equivalents = xg retinol + xg p-carotene x 0.167 + xg other provitamin carotenoids X 0.083. For comparison with values in the older literature, the equivalent lU values of retinol can be calculated by multiplying the RE values by 3.33. For examples of the application of these formulas to the evaluation of diets, the reader is referred to the 9th edition of the Recommended Dietary Allowances (Food and Nutrition Board, 1980). 

Adults— The FNB-NRC recommended daily allowance of vitamin A for adult men is set at 1,000 meg RE (3,333 lU). The allowance is based on the assumption that the average American diet provides half the total vitamin A activity as retinol and half as provitamin A carotenoids. In terms of Retinol Equivalents (RE), this is 750 meg retinol (1 RE = 3.33 lU retinol) and 250 Retinol Equivalents as beta-carotene (1 RE = 10 lU beta-carotene) for a total of 1,000 RE. 

The provitamin A equivalency of a food could be calculated using the conversion factors proposed by the FAO/WHO on the basis of the REs or following the recommendation of the Food and Nutrition Board of the lOM with the RAF. On the first case, 1 RE is equal to 1 p,g of retinol and corresponds to 14 p,g of (3-carotene or to 28 pg of other provitamin A carotenoids. The second standard suggests factors of 1 12 for 3-carotene and 1 24 for other provitamin A carotenoids. 

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. 

Ref. 2. Vitamin A is reported as retinol [68-26-8] equivalents/L. RE = 1 /ig of all trans-i.etSio 6 )lg of all /ra j -(3-carotene, and 12 )lg of other provitamin A cartenoids, with older definitions giving 3.33 lU vitamin A from retinol and 10 lU vitamin A activity from -carotene. 

Values of RDI and RDA for vitamin A are given as retinol activity equivalents (RAE) where 1 microgram of retinol equals 1 RAE, 12 micrograms of p-carotene equals 1 RAE, and so forth for all provitamins A. 

Vitamin A is available either from (i) preformed retinol (present in animal foods as retinyl esters), or (ii) metabolised from provitamin A precursors. The recommended dietary allowance for preformed vitamin A is 0.9 mg/day for men and 0.7 mg/day for women. Provitamin A somces are graded according to their retinol activity equivalence (RAE), e.g. since 12mg of 3-carotene in food 5delds Img retinol, its RAE is 12. 

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. 

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... 

The efficiency of absorption of -carotene is considerably lower (9-22%) and more variable than that of retinol. In fact, in controlled studies some subjects have absorbed little, if any, of a test dose of -carotene. In individuals who do absorb dietary carotenoids, the efficiency of absorption tends to fall as intake increases. The type of carotenoid and its physical form in the ingested foodstuff also affect the efficiency of carotene absorption. Pure /3-carotene in an oily solution or supplements is absorbed more efficiently than an equivalent amount of /3-carotene in foods. Much of the carotenoid present in foods is boimd within a matrix of polysaccharides, fibers, and phenoHc compounds that is incompletely digested. Although the absorption of provitamin A carotenoids from fruits is generally better than from fibrous vegetables, it is still low as compared to yS-carotene in oil (see section on Units). 

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