A 24-year study of 1, 500 adults showed that those subjects with a high intake of foods rich in betacarotene were much less likely to die from cancer or heart disease than were those with a low intake of such foods
On the other hand, taking beta-carotene supplements for 12 years had no positive or negative effect on the health of subjects in a separate study of 20, 000 adults
Beta-carotene must be taken for longer than 12 years to have any cancer-preventive effects.