Explain the Bio availability of Vitamin A?

By now it is clear that vitamin A is supplied in two forms. One form is retinol, from animal foods such as liver, fatty fish, eggs, and milk, and from fortified foods. Retinol is considered pre-formed vitamins A. The other form is the carotenoids from plant foods ß-carotene, ß-carotene and β-cryptoxanthin). These convert to vitamin A in the body and are called provitamin A carotenoids. Retinol and carotenoids have different vitamin A activity. You would realize that it takes greater amounts of carotenoids to equal the activity of retinol. Different conversion factors have been therefore developed to address this aspect while developing the RDA For vitamin A.  You may come across recommendations for vitamin A expressed as international Unit or as retinol equivalent (RE) or as retinol activity equivalent (RAE). To express the vitamin A activity of carotenoids in diets a common basis, a joint FAO/WHO Expert Group in 1967 introduced the concept or the Retinol equivalent (RE) and established the following relationships among food sources of vitamin A:

More recently, vitamins A recommendations are in mg/day as RAE. The term RAE was introduced to replace the term retinol equivalent (RE) to take into account new research on the vitamin A activity (bio efficacy) of carotenoids. The conversions in terms of RAE, retinol, carotene are presented herewith:

1 mg RAE = 1 pg retinol (vitamin A)

= 12 mg ß-carotene in mixed foods

= 24 mg other provitamin A carolenoids in mixed foods

Hence, with the RAE system, we c:un see that the relative proportion of retinol, ß carotene and other carotenoids is 1:12:24, The RAE system helps to account for the  differences between carotenoids and retinol. It takes about 12 units of ß-carotene and 24 units of other carotenoids to make 1 unit or retinol in the body. In Retinol Equivalents (RE), retinol, beta carotene and other carotenoids proportion is 1:6: 12.

Many food and supplement labels still list vitamin A in International Units (IUs). This measure can be converted to RAEs with some calculations. If all the vitamin A  activity is from retinol, then 3.33 IU vitamin A (retinol) = 1 RAE. Otherwise, we can use these conversions:

1 IU vitamin A activity = 0.3 mg retinol

= 3.6 mg ß-casotene

= 7.2 mg a-carotene or ß-cryptoxanthin

Let us understand this conversion with the help of an example. For example, a dessert prepared from carrots and milk supplies 10,000 IU vitamins A (20% as ß- carotene). The calculation includes:

1) 10,000 IU > (20% = 2000 IU (thus 8000 IU as retinol, 2000 IU as p-carotene)

2) 8000 x 0.3 (or 8000/3.33) = roughly 2400 mg as retinol (= 2400 mg RAE)

3) 2000 x 3.6 = roughly 7200 mg as ß-carotene (7200 mgl12 = 600 mg RAE)

4) 2400 mg RAE + 600 mg RAE = 3000 mg RAE supplied by this supplement