Current controversies in carotene nutrition.

Most of the dietary vitamin A is derived from plant foods in the form of pro-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 mechanism of conversion to vitamin A has been controversial. Some of the mechanisms suggested are central cleavage potentially yielding 2 molecules of vitamin A or excentric cleavage producing one molecule of vitamin A from beta-carotene which drastically varied the potency of carotene. A mucosal supernatant from rat intestine was shown to have beta-carotene dioxygenase activity which provided the basis for central cleavage. Many observations on enzyme activity in vitro and efficacy of carotene in vivo did not support the above findings and a re-evaluation of the whole problem was undertaken at the National 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 children 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 curves after a dose of beta-carotene and vitamin A (> 100 micrograms) was developed in rats and later extended to children. In children a dose of 1.5 mg of beta-carotene and vitamin A was used. From these studies intestinal conversion of beta-carotene to vitamin A was found to be an enzymatic reaction involving central cleavage and which needed the presence of oxygen. The substrate was found to bind the enzyme at C-15,15'. The enzyme may be associated with inherent or contaminant enzyme which breaks of other part of the molecule released after central cleavage of carotene. The in vivo conversion of carotene to vitamin A was found to 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. Thus the present results confirm the convertibility of dietary carotenoids to vitamin A and could facilitate further investigations on interactions of different dietary carotenoids on the absorption and cleavage of carotene to vitamin A in children.