Current controversies in carotene nutrition

Most of the dietary vitamin A is derived from plant foods in the form of pr o-vitamin A, the carotenoids. Though in 1930 it was first demonstrated that beta-carotene is the precursor for vitamin A and it is well accepted that 1 mole of beta-carotene is equivalent to one mole of vitamin A, the mechani sm of conversion to vitamin A has been controversial. Some of the mechanism s suggested are central cleavage potentially yielding 2 molecules of vitami n A or excentric cleavage producing one molecule of vitamin A from S-carote ne which drastically varied the potency of carotene. A mucosal supernatant from rat intestine was shown to have beta-carotene dioxygenase activity whi ch provided the basis for central cleavage. Many observations on enzyme act ivity in vitro and efficacy of carotene in vivo did not support the above f indings and a re-evaluation of the whole problem was undertaken at the Nati onal Institute of Nutrition (NIN), Hyderabad. Intestinal conversion of beta -carotene to vitamin A both in vitro and in vivo in rats and in vivo in chi ldren was evaluated. A novel method of obtaining the in vivo conversion of carotene to vitamin A using the ratio of area under plasma vitamin A time c urves after a dose of beta-carotene and vitamin A (>100 mu g) was developed in rats and later extended to children. In children a dose of 1.5 mg of be ta-carotene and vitamin A was used. From these studies intestinal conversio n of beta-carotene to vitamin A was found to be an enzymatic reaction invol ving central cleavage and which needed the presence of oxygen. The substrat e was found to bind the enzyme at C-15,15'. The enzyme may be associated wi th inherent or contaminant enzyme which breaks the other part of the molecu le released after central cleavage of carotene. The in vivo conversion of c arotene to vitamin A was found tb vary from 20 to 80 per cent depending on the nutritional status. Vitamin A deficiency was found to enhance both the in vitro and in vivo conversion and protein deficiency to decrease both. Th us the present results confirm the convertibility of dietary carotenoids to vitamin A and could facilitate further investigations on interactions of d ifferent dietary carotenoids on the absorption and cleavage of carotene to vitamin A in children.