Benefits and liabilities of vitamin A and carotenoids.

Vitamin A and carotenoids are often cited as members of a family of antioxidant vitamins that can show protective effects against oxidative stress and some chronic diseases. The great advantages of aerobic metabolism are offset in part by adverse physiological effects that can be caused by highly reactive forms of oxygen and its reduction products. Nature has developed a variety of sophisticated defenses against these undesirable side effects of oxygen. Vitamin A and carotenoids can serve chemically both as electron acceptors and donors (i.e., as oxidants and reductants), although in nature vitamin A seems to be protected from oxidation by other physiological antioxidants rather than serving a protective role itself. Furthermore, large doses of vitamin A are toxic. Carotenoids may serve a broader protective role in humans as they do in plants and microorganisms, but the evidence supporting such a role in humans is mixed. The widely used term antioxidant vitamins is too narrow in scope for the properties attributed to them. A much better term is physiological modulators, which broadens both the set of compounds that are included and the possible nature of their actions, whether beneficial or adverse.