Provitamin conversion factors

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. 

Old food composition tables report the provitamin A carotenoid content of food as international units (lU). The following conversion factors can be used to obtain values in micrograms ... 

Because many factors affect the assimilation of carotenoids from foods (Figure 2), conversion factors need to be considered. This is especially important when most sources of vitamin A are from provitamin A carotenoids in the population. Bioavailability of preformed vitamin A, i.e., retinol and retinyl esters, is not a major concern because 80-95% of them are absorbed. However, foods that are high in preformed retinol (liver, eggs, and fortified milk) are not necessarily consumed by everybody. When discussing carotenoids from food, four terms need to be defined (see Table 1) ... 

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. 

Another complicating factor in the intestinal mucosal cell is the partial conversion of provitamin A carotenoids (/3- and a-carotenes and cryptoxanthin) to vitamin A (primarily to retinyl esters). Therefore, in absorption studies these metabolic reactions must be accounted for in measuring intestinal transport. Nonprovitamin A carotenoids such as lycopene, lutein, and zeaxanthin are incorporated intact, although some cleavage can occur. Earlier studies on rats indicated that lycopene and /3-carotene are absorbed by passive diffusion. However, recent evidence from the kinetics of /3-carotene transport through Caco-2 cell... 

---