U.S. physicians reported no significant effect of 12 years of supplementation of β-carotene (50 mg every other day) on cancer or total mortality (Hennekens et al., 1996).

Summary. Higher consumption of carotenoid-containing fruits and vegetables and higher plasma concentrations of several carotenoids, including β-carotene, are associated with a lower risk of many different cancers, especially lung, oral cavity, pharyngeal, laryngeal, and cervical cancers. These prospective blood concentration studies show that β-carotene concentrations in the range of 0.28 µmol/L (15 µg/dL) or less are associated with higher risk of many cancers (Table 8-3), whereas concentrations greater than 0.28 to 0.37 µmol/L (15 to 20 µg/dL) are associated with reduced risk of many cancers. This approximate threshold for cancer risk reduction is concordant with that for the prevention of all-cause mortality, given above. Furthermore, these studies show that increased consumption of foods containing these carotenoids, including carotenoids lacking vitamin A activity, is associated with risk reduction. However, in three large randomized clinical trials using high-dose β-carotene supplements (20 or 30 mg/day or 50 mg given every other day) for 4 to 12 years, no protection was reported with respect to lung cancer, or any other cancer.

Epidemiological studies, including descriptive, cohort, and case-control studies, suggest that carotenoid- and β-carotene-rich diets are associated with a reduced risk of cardiovascular disease (Gaziano and Hennekens, 1993; Kohlmeier and Hastings, 1995; Manson et al., 1993). Beginning with biochemical epidemiological studies of plasma carotenoids, Gey et al. (1993a) reported data from the Vitamin Substudy of the World Health Organization's Monitoring Cardiovascular (WHO/MONICA) Project, in which plasma was obtained from approximately 100 apparently healthy men from each of 16 study sites within Europe. A comparison between median plasma β-carotene concentrations and ischemic heart disease mortality revealed no association when all 16 study sites were considered (r² = 0.04). However, a reasonably strong inverse association was evident (r² = 0.50) when three study sites, all apparent outliers (and all Finnish sites), were excluded from the analysis.

Men in the Basel Prospective Study, who had low blood concentrations of β-carotene and vitamin C initially and who were followed for 12 years, had a significantly higher risk of subsequent ischemic