A global clinical view on vitamin A and carotenoids.

The clinical importance of vitamin A as an essential nutrient has become increasingly clear. Adequate vitamin A is required for normal organogenesis, immune competence, tissue differentiation, and the visual cycle. Deficiency, which is widespread throughout the developing world, is responsible for a million or more instances of unnecessary death and blindness each year. β-Carotene is an important, but insufficient, source of vitamin A among poor populations, which accounts for the widespread nature of vitamin A deficiency. It has only recently become apparent that the bioconversion of traditional dietary sources of β-carotene to vitamin A is much less efficient than previously supposed. The other major carotenoids, particularly lycopene, lutein, and zeaxanthin, have been found to have important biological properties, including antioxidant and photoprotective activity, and high intake has been linked in observational studies with reduced risk of a number of chronic diseases. But, to date, no clinical trials have proven the clinical value of ingested carotenoids individually or in combination, in either physiologic or pharmacologic doses, with the excepton of the provitamin A activity of carotene. Indeed, several trials have suggested an increased risk of lung cancer among high-risk individuals (smokers and asbestos workers) who were given high doses of β-carotene alone or in combination with other antioxidants. Much more evidence is needed before commonly encountered claims of the value of ingesting high doses of non-provitamin A carotenoids are validated.