Most beta-carotene in supplements is synthetic, consisting of only one molecule called all trans beta-carotene. Natural beta-carotene, found in food, is made of two molecules--all trans beta-carotene and 9-cis beta-carotene.
Researchers originally saw no meaningful difference between natural and synthetic beta-carotene. This view was questioned when the link between beta-carotene-containing foods (all natural) and lung cancer prevention115 was not duplicated in studies using synthetic pills.116 In smokers, synthetic beta-carotene has apparently caused an increased risk of lung cancer117, 118, 119 and disease of the blood vessels120 in double-blind research. Animal research has begun to identify the ways in which synthetic beta-carotene might cause damage to lungs, particularly when animals are exposed to cigarette smoke.121
Much of natural beta-carotene is in the all trans molecule form--the same as synthetic beta-carotene. Moreover, much of the 9-cis molecule found only in natural beta-carotene is converted to the synthetic molecule before it reaches the bloodstream.122 Also, absorption of 9-cis beta-carotene appears to be poor,123 though some researchers question this finding.124
Despite the overlap between natural and synthetic forms, natural beta-carotene may possibly have activity that is distinct from the synthetic form. For example, studies in both animals125 and humans126 have shown that the natural form has antioxidant activity that the synthetic form lacks. Also, in one trial, pre-cancerous changes in people reverted to normal tissue with natural beta-carotene supplements, but not with synthetic supplements.127 Israeli researchers have investigated whether the special antioxidant effects of natural beta-carotene might help people suffering from asthma attacks triggered by exercise.128 People with asthma triggered by exercise were given 64 mg per day of natural beta-carotene for one week. In that report, 20 of 38 patients receiving natural beta-carotene were protected against exercise-induced asthma. However, because synthetic beta-carotene was not tested, the difference between the activity of the two supplements cannot be deduced from this report.
Increasingly, doctors are recommending that people supplement only with natural beta-carotene. However, no studies have explored whether the adverse effect of synthetic beta-carotene in cigarette smokers would also occur with natural beta-carotene supplementation. Until more is known, smokers should avoid all beta-carotene supplements and others should avoid synthetic beta-carotene.
In supplements, the natural form can be identified by the phrases "from D. salina,""from an algal source,""from a palm source," or as "natural beta-carotene" on the label. The synthetic form is identified as "beta-carotene."