Beta-Carotene: The Controversy Continues

Alternative Medicine Review, Dec, 2000 by Lyn Patrick

They also compared the absorption of the labeled cis [Beta]-carotene by giving a single dose of unlabeled cis [Beta]-carotene eight months later.[52] As a result of the comparison data, they produced three provocative points. First, this was the first definitive proof that cis [Beta]-carotene is isomerized in the intestinal lining to the trans isomer before absorption. Second, the authors suggest that greater amounts of all-trans [Beta]-carotene are absorbed and secreted into chylomicrons after cis [Beta]-carotene dosing than occurred after an equivalent dose of all-trans [Beta]-carotene, alluding to the fact that (unlike other researchers' findings) the cis isomer may actually be better absorbed. Third, most of the radioactively-labeled retinol produced by the study subjects was made from the labeled trans-isomer rather than being produced directly from the labeled cis-isomeric form of [Beta]-carotene, pointing out a more efficient mechanism for retinol production by transforms of [Beta]-carotene. They concluded the isomerization process could serve as a control mechanism for the production of 9-cis retinol. Because this study used a very small dose of [Beta]-carotene (1666 IU), absorption mechanisms may not be reproducible with higher doses.

The results of this study point out the gaps in our knowledge about cis [Beta]-carotene and its relationship to [Beta]-carotenoid absorption and metabolism. They also provide an explanation for the previous dosing studies in which 9-cis [Beta]-carotene blood levels were always lower than dosing levels, a result of the isomerization of the cis isomer to the trans isomer in the intestine.

Although 9-cis [Beta]-carotene (along with the 13-cis and 15-cis isomers found in food and naturally-occurring substances) may serve an important function in human physiology that cannot be replaced by synthetic [Beta]-carotene, the consequences of using all-trans synthetic [Beta]-carotenes are at this point unknown. There is evidence that 13-cis [Beta]-carotene can be produced from all-trans [Beta]-carotene in significant quantities: as much as 10 percent of total [Beta]-carotene intake.[59] Certainly, using the [Beta]-carotene carotenoid exclusively may be problematic as there appears to be a possible competitive inhibition between [Beta]-carotene and lutein. However, as discussed previously, long-term [Beta]-carotene studies have not found such competition. Whether or not the lower lutein levels seen in the Finnish Smokers Study are relevant is unknown.[1]

Beta-Carotene Trials

Intervention studies with [Beta]-carotene have not shown the efficacy predicted by observational and animal studies. The well-publicized intervention trials with [Beta]-carotene, the Alpha-Tocopherol Beta-Carotene Cancer Prevention Study (ATBC)[60] and the Beta-Carotene Retinol Efficacy Trial (CARET),[61] were designed to assess the effects of tocopherol, [Beta]-carotene, and retinol in populations at high risk for lung cancer -- former and present smokers and asbestos-exposed workers.