A 20-year follow-up study based on the Melanoma in Southern Sweden study concluded that all-cause mortality was inversely related to sun-exposure habits—but why do the authors not consider vitamin D supplementation, which is proven to enrich the body while not causing skin cancers?
Lindqvist PG, Epstein E, Landin-Olsson M, et al. Avoidance of sun exposure is a risk factor for all-cause mortality: results from the Melanoma in Southern Sweden cohort. J Intern Med. 2014;276(1):77-86. Epub 2014 Apr 23.
A prospective 20-year follow-up of a cohort of 29,518 Swedish women in the Melanoma in Southern Sweden (MISS) study
Women with no history of malignancy (N=29,518) aged 25 to 64 years at the start of the study were recruited between 1990 and 1992.
Study Parameters Assessed
Detailed information at baseline was collected in a survey on sun-exposure habits and potential confounders including marital status, educational level, family income, smoking habits, alcohol consumption, number of pregnancies, body mass index, and physical exercise.
MISS researchers asked the participants 4 questions regarding sun exposure (answer choices provided by the researchers):
- Do you sunbathe during the summer? (never, 1-14 times, 15-30 times, or >30 times)
- Do you sunbathe during the winter, such as during holidays to the mountains? (no, 1-3 days, 4-10 days, or >10 days)
- Do you use tanning beds? (never, 1-3 times per year, 4-10 times per year, or >10 times per year)
- Do you go abroad on holiday to swim and sunbathe? (never, once per 1-2 years, once per year, ≥2 times per year).
Primary Outcome Measures
Effect of sun exposure on all-cause mortality, risk of melanoma, and melanoma fatality
There were 2,545 deaths overall. All-cause mortality was inversely related to sun-exposure habits. The mortality rate among avoiders of sun exposure was approximately twofold higher (hazard ratio [HR]: 2.0, 95% confidence interval [CI]: 1.6-2.5) compared with the highest sun exposure group. Those with moderate exposure had a 40% increased risk (HR:1.4, 95% CI: 1.1-1.7) of mortality. Women with “normal” sun-exposure habits were not at significantly increased risk of melanoma or melanoma-related death.
Not surprisingly, the authors found that avoiding sun exposure is a risk factor for all-cause mortality. This is in keeping with the mounting evidence that vitamin D deficiency is associated with increased risk for death from cardiovascular disease1 and cancers.2 The authors concluded that restricting sun exposure in countries with low solar intensity (eg, Sweden) might be harmful to women’s health, supposedly because it would decrease endogenous production of vitamin D.
Not restricting sun exposure is one way to increase vitamin D, but it is not the most reliable way. A better extension of the authors’ conclusion, in my opinion, would have been a recommendation for taking vitamin D dietary supplements to ensure vitamin D levels that are associated with lower risk of cardiovascular disease, cancer, and melanoma3 in particular. Ultraviolet (UV) protection is desirable for many reasons, including reducing the risk of nonmelanoma skin cancer and skin aging, and it is best achieved by avoiding sunlight exposure midday, applying (and reapplying if necessary) a full-spectrum physical sunblock such as (non-nanoparticle) zinc or titanium, and wearing lightweight UV protective apparel such as hats, long sleeves, and long pants with an ultraviolet protection factor (UPF) of 15 or more.
Not restricting sun exposure is one way to increase vitamin D, but it is not the most reliable way.
Assuming that UV radiation is the major cause of melanoma and that low vitamin D levels are associated with thicker, more aggressive melanomas with shorter survival times,3 the authors posit that opposing mechanisms may act with regard to melanoma risk and avoidance of UV exposure. The active metabolite of vitamin D—1,25(OH)2D— has been shown to have anticancer effects such as cell-cycle arrest, apoptosis induction, and inhibition of proliferation.4
Additionally, there are several flaws in the study design. For example, the study team categorized regular exercise into 3 groups of none, at least once per week, or strenuous exercise—weak definitions with serious limitations. The 4 questions asked of participants in this study regarding sun exposure are perhaps the most serious weakness in its design. Defining sun exposure as sunbathing and tanning beds is a very limited and questionable measure. Many people are exposed to the sun through outdoor physical activities, such as walking, hiking, biking, or camping, or even through outdoor occupations. The authors also assumed that sun-exposure habits did not change over the course of the 20-year study. The finding that women with “normal” sun-exposure habits (undefined by the authors) were not at significantly increased risk of melanoma or melanoma-related death also suggests that the method for measuring sun-exposure habits in this study was flawed, as the evidence basis is very strong for increased melanoma risk from repeated overexposure to UV radiation.5
The authors of this study should be credited for acknowledging that their “findings are of association and not necessarily causal” and that “additional evidence and possibly other study designs are needed to draw causal conclusions.” They even acknowledge the lack of any data in the study on vitamin D supplementation or vitamin D levels, stating only that “sun exposure is the most important determinant of vitamin D status.” This assertion may have applied to the population recruited 20 years ago for their study, but it is probably no longer accurate today as large numbers of people are now taking vitamin D supplements in doses that may far exceed those produced endogenously.