Review of Research Paper

Sunlight: For Better or For Worse? A Review of Positive and Negative Effects of Sun Exposure
Han van der Rhee et al.
Cancer Research Frontiers
May 2016

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Researchers from the Netherlands representing dermatology, public health, and molecular cell biology conducted an extensive review study based on 71 original studies, 21 meta-analyses, 6 systematic reviews and 17 reviews (115 total). The research team, led by Han van der Rhee, reviewed both the positive and negative effects of sun exposure and concluded that both too much and too little sunlight may be harmful to our health.

Overall health benefits of the sun

Multiple meta-analyses have found an inverse association between chronic and occupational sun exposure and melanoma risk – meaning more time in the sun corresponds to a lower risk of developing melanoma, the most deadly form of skin cancer. There was some variation in risk based on latitude.

Similarly, a systematic review of 26 case-control and 19 cohort studies demonstrated a consistent inverse association between chronic sun exposure and the followingcancers: colorectal, breast, prostate, and non-Hodgkin’s lymphoma. Translated – this means that regular, not intermittent, time in the sun is related to a reduced risk of these types of cancer.

A review of epidemiological studies suggest that regular exposure to the sun and a natural exposure to bright light during the day with less light in the evening is inversely associated with the risk of cancers of the colon-, breast-, prostate-, and non-Hodgkin’s Lymphoma, as well as multiple sclerosis, and metabolic syndrome (including hypertension and diabetes mellitus). Again, regular exposure to the sun and natural light patterns is related to a reduced risk of certain cancers.

The biological effects of sunlight

Table 7 of the paper contains a detailed list of biological effects of sunlight such as DNA damage, photo-adaptation, improving immune function, circadian rhythm balancing, and producing vitamin D, nitric oxide, serotonin, endorphins, and folic acid.

How does the sun make your skin stronger?

Human skin adapts to regular, non-burning, sun exposure with increased melanin (pigment in the skin), a thickening of the stratum corneum (outermost layer of epidermis), and a reduction in cyclobutane pyrimidine dimers (CPD) a substance known to cause overall cell damage. Research on these effects have found that regular exposure to non-burning doses of artificial UV for 3 weeks decreases the ultraviolet sensitivity of the skin for erythema (redness) on average by 75%.

Regular exposure to UV, as experienced by outdoor workers and regular sun bathers, leads to an almost complete disappearance of CPDs in the basal and suprabasal layers of the epidermis which suggests less DNA damage in these lower levels of skin where the cells are replicating. In contrast, DNA damage in the top layers of the epidermis is less consequential because the cells are already committed to cell death.

Key Findings – Why there can’t be just one recommendation

The Australian prevention programs of “Slip! Slop! Slap!” (to “slip” on a shirt, “slop” on sunscreen, and “slap” on a hat) and SunSmart established in the 1980s have been adopted by most Western countries. The primary messages of these programs have been avoidance of the mid-day sun, the use of protective clothing, and use of sunscreens with a SPF of 30 or higher that protect against UVA and UVB. Rather than tailor the message to the conditions of individual countries or regions, the same sun exposure advice is used all over the world. While the message is appropriate for persons with sun-sensitive skin living in countries with high ambient UV, such as Australia, it is less applicable in other parts of the world where chronic exposure and outdoor occupations are associated with a relatively low risk of melanoma and basal cell carcinoma and even squamous cell carcinoma.

“Most skin cancers are caused by a mismatch between skin type and geography and/or sun exposure related lifestyle.”

Conclusion

“It is obvious that excessive sun exposure and sunburn should be avoided. During sun-seeking vacations an adequate protection is needed. It is, however, unlikely that continuous protection during daily life contributes to our health, particularly in countries with a temperate climate. Both too much and too little sunlight may be harmful to our health.”

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