While observational epidemiological studies have consistently shown that beta-carotene is associated with decreased cancer risk, particularly of lung cancer, findings of seven randomised trials testing the effect of beta-carotene supplementation on cancer incidence and mortality generally have not been supportive. Two of these trials even suggested the possibility of harmful effects.
Two large trials of beta-carotene conducted among persons at average risk of cancer found no statistically significant benefit or harm associated with beta-carotene supplementation [16-17].
Two other large trials tested beta-carotene among persons at high risk of cancer [18, 19]. One (The Alpha-Tocopherol, Beta carotene Cancer Prevention Study, 1996) reported a statistically significant 18% increase in lung cancer incidence after 5–8 years of treatment with beta-carotene among male Finnish smokers . Another that used a combination of beta- carotene and retinol, reported a statistically significant 28% increase in lung cancer incidence among United States Smokers, former smokers and asbestos workers. Only one large trial, which tested a combination of beta-carotene, vitamin E and selenium in a poorly nourished Chinese population, found that after 5 years, the treated group experienced a statistically significant 9% reduction in total mortality, primarily as a result of a statistically significant 21%lower stomach cancer mortality rate .
The indirect evidence that beta-carotene may protect from stomach cancer comes from the randomised, controlled double-blinded chemoprevention trial in subjects with gastric dysplasia in an area with a very high gastric cancer risk in Columbia. Gastric biopsy taken at baseline was compared with those taken at 72 months. Treatment with 30 mg beta-carotene resulted in a statistically significant increase in the frequency of regression of preneoplastic lesions of the stomach [relative risk (RR) = 5.1, 95% CI 1.6–14.2]. One small trial of 1805 people with previous non-melanoma skin cancer that tested treatment with beta-carotene (50 mg per day) to reduce the occurrence of new skin cancer did not find any effect of this intervention
It can be concluded that there is evidence at present that beta-carotene supplements have no value as cancer chemoprevention agents and cannot be recommended for use in the general population in this context.