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 prevention² was not duplicated in studies using synthetic pills.³ In smokers, synthetic beta-carotene has apparently caused an increased risk of lung cancer^{4, 5, 6} and disease of the blood vessels⁷ 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.⁸

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.⁹ Also, absorption of 9-cis beta-carotene appears to be poor,¹⁰ though some researchers question this finding.¹¹

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 animals¹² and humans¹³ 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.¹⁴ Israeli researchers have investigated whether the special anti-oxidant effects of natural beta-carotene might help people suffering from asthma attacks triggered by exercise.¹⁵ 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.”