Techniques for measuring vitamin A activity from ß-carotene.

ABSTRACT

Dietary ß-carotene is the most important precursor of vitamin A. However, the determination of the efficiency of in vivo conversion of ß-carotene to vitamin A requires sensitive and safe techniques. It presents the following challenges 1) circulating ß-carotene concentration cannot be altered by eating a meal containing ≤6 mg ß-carotene; 2) because retinol concentrations are homeostatically controlled, the conversion of ß-carotene into vitamin A cannot be estimated accurately in well-nourished humans by assessing changes in serum retinol after supplementation with ß-carotene. In the past half-century, techniques using radioisotopes of ß-carotene and vitamin A, depletion-repletion with vitamin A and ß-carotene supplements, measurement of postprandial chylomicron fractions after consumption of a ß-carotene dose, and finally, stable isotopes as tracers to follow the absorption and conversion of ß-carotene in humans have been developed. The reported values for ß-carotene to vitamin A conversion showed a wide variation from 2 µg ß-carotene to 1 µg retinol (for synthetic pure ß-carotene in oil) and 28 µg ß-carotene to 1 µg retinol (for ß-carotene from vegetables). In recent years, a stable isotope reference method (IRM) was developed that used labeled synthetic ß-carotene. The IRM method provided evidence that the conversion of ß-carotene to vitamin A is likely dose dependent. With the development of intrinsically labeled plant foods harvested from a hydroponic system with heavy water, vitamin A activity of stable isotope-labeled biosynthetic ß-carotene from various foods consumed by humans was studied. The efficacy of plant foods rich in ß-carotene, such as natural (spinach, carrots, spirulina), hybrid (high-ß-carotene yellow maize), and bioengineered (Golden Rice) foods, to provide vitamin A has shown promising results. The results from these studies will be of practical importance in recommendations for the use of pure ß-carotene and foods rich in ß-carotene in providing vitamin A and ultimately in preventing either overconsumption or poor intake of vitamin A by humans.