Carotene, a substance from plants that the body can convert into vitamin A, also acts as an antioxidant and immune system booster. Despite the overlap between natural and synthetic forms, natural beta-carotene may possibly have activity that is distinct from the synthetic form. For example, the natural form has antioxidant activity that the synthetic form has been reported to lack in both animals and in people. Also, in one trial, precancerous changes in people reverted to normal tissue with natural beta-carotene supplements, but not with synthetic supplements. Increasingly, nutritionally oriented doctors recommend that people supplement only natural beta-carotene.