I am a big believer in the therapeutic properties of sunlight, at least partly because of the sun's ability to stimulate vitamin D production in the skin.
Sunlight exposure and heightened vitamin D levels have been associated with a range of health benefits, including a reduced risk of several forms of cancer and relative protection from multiple sclerosis. I was therefore interested to read over the weekend an editorial in the British Medical Journal that highlights the importance of sunlight and vitamin D in health.
Written by professor Michael Holick (professor of medicine, physiology, and biophysics at Boston University Medical Center), the editorial details some of the evidence for the benefits of sunlight exposure and vitamin D, including their role in bone and muscular health. (Vitamin D deficiency can lead to bone disease and muscular fatigue and pain.) Not surprisingly, Michael Holick's editorial mentions cancer too.
When the words sunlight and cancer are heard together, it's natural for our thoughts to turn to skin cancer. There are three main forms of skin cancer—squamous cell carcinoma, basal cell carcinoma, and malignant melanoma.
The first two tend to develop on the most sun-exposed parts of the body (for example, the top of the ear) and are generally very treatable. Malignant melanoma, on the other hand, is generally much less treatable, is quite often deadly, and is usually the major reason cited why we should protect ourselves from the sun.
However, is the relationship between sunlight exposure and risk of malignant melanoma, as clear-cut as we are led to believe it to be? Michael Holick's editorial contains information that might cause us to question traditional wisdom on this.
He writes: "Notably, non-melanoma skin cancers occur on the most sun exposed areas, such as the face and hands, whereas most melanomas occur on the areas least exposed to the sun [2]. Intermittent and occupational sun exposure has been found to reduce the risk of malignant melanoma [2–5]."
In short, Professor Holick appears to be asking: "If sunlight exposure causes malignant melanoma, why does it tend to develop on parts of the body that are not typically very sun-exposed, and why is there evidence linking sun exposure with reduced risk of this condition?"
Professor Holick appears to cast considerable doubt on the notion that excessive exposure to sunlight is a major risk-factor for malignant melanoma.
Even if it is a risk-factor, I still think it's worth bearing in mind all the benefits that come from sunlight that may offset any downside.
References:
1. Holick M. Deficiency of sunlight and vitamin D. British Medical Journal 2008; 336:1318–1319
2. Kennedy C, et al. The influence of painful sunburns and lifetime of sun exposure on the risk of actinic keratoses, seborrheic warts, melanocytic nevi, atypical nevi, and skin cancer. Journal of Investigative Dermatology 2003; 120:1087–93
3. Holick MF. Vitamin D deficiency. New England Journal of Medicine 2007; 357:266–81
4. Grant WB, et al. The association of solar ultraviolet B (UVB) with reducing risk of cancer: multifactorial ecologic analysis of geographic variation in age-adjusted cancer mortality rates. Anticancer Research 2006; 26:2687–99
5. Moan J, et al. Addressing the health benefits and risks, involving vitamin D or skin cancer, of increased sun exposure. Proceedings of the National Academy of Sciences 2008; 105:668–73
Dr. John Briffa is a London-based doctor and author with an interest in nutrition and natural medicine.
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