Beta-carotene was cast in a more favorable light by a study published 2 months later by Valacchi et al. The researchers, acknowledging the apparent conflict between some studies suggesting that beta-carotene exhibits skin-protecting qualities as an antioxidant and other studies indicating pro-oxidant characteristics, examined the potential effects of oral beta-carotene supplement consumption on oxidative stress on the skin caused by ozone. They measured markers of oxidative stress in hairless mice after 1 month of a diet supplement with 0.5% beta-carotene and subsequent exposure to ozone (0.8 ppm; 6 hr/day; 7 days). The researchers found that beta-carotene downregulated the induction of tumor necrosis factor-alpha, macrophage inflammatory protein 2, inducible nitric oxide synthase, and HO-1 caused by ozone exposure. They concluded that beta-carotene does indeed confer protection against ozone-induced cutaneous oxidative stress in vivo (Toxicol. Ind. Health 2009;25:241-7).
Beta-carotene and lung cancer risk link. It is clear that dietary beta-carotene affords at least some cutaneous protection, though not against skin cancer, but the limits have not been fully elucidated as yet. It is important to note that beta-carotene supplementation has been shown to contribute to heightening the risk of lung cancer development in smokers and individuals exposed to asbestos (Evid. Rep. Technol. Assess. 2006;139:1-117).
In 2008, Tanyetyanon and Bepler conducted a meta-analysis of the literature on large randomized trials to assess the risk of lung cancer posed by beta-carotene consumption by smokers or former smokers. In their Medline search, the investigators identified four studies totaling 109,394 subjects. While they noted no increased risk of lung cancer among former smokers, they found a significant link between beta-carotene supplementation and increased lung cancer risk among current smokers (Cancer 2008;113:150-7).
In addition to this disturbing risk, there are some minor disadvantages associated with consuming excessive amounts of beta-carotene and other provitamin A substances. Specifically, the consumption of excessive amounts of beta-carotene through the diet or in supplement form can lead to skin yellowing. Because beta-carotene is inefficiently converted into retinol, supplementation with beta-carotene poses less risk of rendering the skin yellow than does vitamin A supplementation. It is best that patients and consumers derive the benefits of beta-carotene through the diet, but it can be a beneficial supplement to people living in warm climates where frequent sun exposure is more likely and whose diets do not include enough of this carotenoid.
Conclusions. As a prominent member of the carotenoid family, beta-carotene is a compelling subject for the investigation of its potential health benefits. While much more research is necessary to determine the range of effects (broadly, antioxidant to pro-oxidant) that this provitamin A substance can deliver to overall health as well as cutaneous health, in addition to its risks, there appears to be enough evidence to warrant confidence in suggesting a healthy dose of fruits and vegetables rich in beta-carotene to individuals who are not smokers.
I suspect that the strong pigmentary component of beta-carotene will continue to pose significant challenges to manufacturers who hope to harness its antioxidant potential in topical formulations. That said, it will be interesting to keep an eye on continuing investigations regarding the cutaneous effects of dietary consumption and oral supplementation with beta-carotene.