Biological actions of carotenoids ¹

Abstract

Of 600 carotenoids from natural sources that have been characterized, fewer than 10% serve as precursors of vitamin A. Many dietary carotenoids, both with and without provitamin A activity, are found in the blood and tissues of humans. beta-Carotene, the most nutritionally active carotenoid, comprises 15-30% of total serum carotenoids. Vitamin A is formed primarily by the oxygen-dependent central cleavage of beta-carotene and other provitamin A carotenoids. Several carotenoids show enhancement of the immune response, inhibition of mutagenesis, reduction of induced nuclear damage, and protection from various neoplastic events in cells, tissues, and whole animals. Carotenoids also protect against photo-induced tissue damage. Some carotenoids, including beta-carotene, quench highly reactive singlet oxygen under certain conditions and can block free radical-mediated reactions. In epidemiological studies, the intake of carotenoid-rich fruits and vegetables has been correlated with protection from some forms of cancer, particularly lung cancer. Similarly, serum beta-carotene levels have been associated with a decreased chance of developing lung cancer. It must be stressed, however, that these epidemiological associations do not show cause and effect. In this regard, long-term intervention trials with beta-carotene supplements are in progress. Whatever the results of these trials, carotenoids clearly show biological actions in animals distinct from their function as precursors of vitamin A.

Plasma carotenoid levels in Dutch men and women, and the relation with vegetable and fruit consumption

“…Plasma concentrations are probably maintained from deposits in adipose tissues. It is estimated that more than 90% of the carotenoids in the body is found in tissues and o10% in plasma (Bendich & Olsen, 1989). The concentration of b-carotene in tissues is assumed proportional to that in plasma (Peng et al, 1995).…”

Retinol, α-tocopherol and carotenoids in diabetes

“…Although the precise mechanism of many of the complications of diabetes is unknown, it is increasingly believed that oxidative tissue damage may play a role in their pathogenesis. Beta-carotene has been reported to be a potent free radical quencher, singlet oxygen scavenger and lipid antioxidant (Burton & Ingold, 1984;Krinsky, 1993); it is possible that the reduced serum concentrations of b-carotene in diabetic patients may be due to its depletion as a sacri®ced antioxidant (Bendich & Olson, 1989). …”

A Study on Serum Carotenoid Levels in Breast Cancer Patients of Indian Women in Chennai (Madras), India

“…The results obtained in this study suggest that low consumption of these foods relate to lower serum levels of * -carotene and calcium among breast cancer cases, especially in postmenopausal women, compared to the healthy controls. The carotenoids such as * -carotene and zeaxanthin play a role as antioxidant 43,44). Serum lipid peroxides, thiobarbituric acid-reactive substances (TBARS), are a biomarker of lipid peroxidation 45).…”