For years, dermatologists have advertised the dogma that one should limit sun exposure to a bare minimum and liberally apply and re-apply sunscreen to prevent skin cancer.
The notion of limiting sun exposure derives its authority from the propensity of ultraviolet light to cause the formation ofharmful thymine dimers. Thymine, one of the four nitrogen bases that compose nucleotides in DNA, forms a bond with itself in the presence of UVB to create dimers. Despite specific DNA repair methods, thymine dimers often cause errors in DNA replication that lead to mutation and thus provide a mechanism for the causative role of UVB in cancer.
However, a growing body of evidence, brought to the forefront by two new articles in the Feb. 2 issue of the Journal of the National Cancer Institute, suggests that this dogma needs revision. These two independent articles from Berwick et al. and Smedby et al. suggest that sunlight exposure is important for decreased, not increased, risk of cancer.
Both groups indicate that vitamin D may play a role in the phenomenon. The majority of vitamin D in the skin is synthesized from reactions which use sunlight as energy in skin cells. Vitamin D has been implicated in DNA repair, in regulation of phosphorous and calcium metabolism and even in certain anti-carcinogenic pathways.
Berwick et al. examined a cohort of 528 case subjects from a population-based study of skin melanoma and found a significant inverse correlation between sun exposure and death from melanoma. Sun exposure was measured from in-person interviews and morphology reviews of lesions with individual cases followed over the course of five years on average. Solar elastosis, a noteworthy change in the connective tissue reducing skin elasticity and strength in sun-exposed areas, was the most sensitive factor used in the analysis. Incidence of severe sunburn, high intermittent sun exposure and solar elastosis were statistically significant pro-survival factors in the univariant analysis.
Using a more realistic, multivariate analysis, solar elastosis remained a statistically significant factor despite competing risk of death from other causes. The conclusion was that the inverse correlation between sun exposure and melanoma could be explained by activation of DNA repair mechanisms or the activity of vitamin D as protective agent.
Smedby et al. analyzed 3,470 Scandinavian patients with incident malignant lymphoma matched with 3,187 control patients and found that high UVB exposure was associated with a reduced risk of non-Hodgkin's lymphoma. The result is contrary to the prevailing hypothesis that sun exposure and lymphoma were positively associated. Sun exposure was assessed by behavioral patterns including sunbathing frequency, sun vacations, use of artificial tanning device, occupational sun exposure and potential and incidence of sunburn. Frequency of sunbathing and sunburns were found to significantly correlate with decreased relative risk of non-Hodgkin's lymphoma.
In terms of immediate impact, the two studies call into question the mantra of avoiding sunlight and seem to indicate that the best strategy is to moderate exposure to sunlight; infrequent sunburns, though painful, will not condemn one to develop life-threatening melanoma.
In addition, the studies indirectly suggest the importance of vitamin D in protective processes, as it has been implicated in reducing the risk of mortality in other cancers, such as breast cancer. This type of cancer is commonly found to be associated with geographical areas with reduced year-round sunlight where vitamin D deficiency is common.
It seems that as one ventures further into scientific discovery, one necessarily finds that what was thought to be simple is actually rife with complexity. In this case, it seemed obvious that ultraviolet light had only a causative role in cancer progression, but as one delves into the details of that dogmatic assertion, one finds it likely that ultraviolet light has important protective consequences as well. One must judiciously consider approaches that examine an issue from a singular perspective, as these approaches will often neglect significant facets to a given problem.
As science becomes increasingly specialized, it is imperative that one is able to maintain a vision of the entire landscape of discovery.
can be reached at mm9kn@virginia.edu.
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