An animal study has shown that high doses of beta carotene supplements may increase the risk of precancerous changes in lung tissue. The changes were even more pronounced when the animals were exposed to cigarette smoke.
For six months, researchers at Tufts University in Boston fed ferrets either a normal diet or a diet supplemented with high doses (equivalent to 30 mg per day in an adult) of beta carotene. Some of the animals were exposed to high doses of cigarette smoke.
Examination showed that all of the ferrets receiving high dose supplements had precancerous lesions on lung tissue. This response seemed to be enhanced by exposure to tobacco smoke. Ferrets in the high dose group also had low levels of retinoic acid in their lungs, a form of vitamin A thought to protect against lung cancer.