Elsevier

Medical Hypotheses

Volume 97, December 2016, Pages 34-37
Medical Hypotheses

Regular sun exposure benefits health

https://doi.org/10.1016/j.mehy.2016.10.011Get rights and content

Abstract

Since it was discovered that UV radiation was the main environmental cause of skin cancer, primary prevention programs have been started. These programs advise to avoid exposure to sunlight. However, the question arises whether sun-shunning behaviour might have an effect on general health. During the last decades new favourable associations between sunlight and disease have been discovered. There is growing observational and experimental evidence that regular exposure to sunlight contributes to the prevention of colon-, breast-, prostate cancer, non-Hodgkin lymphoma, multiple sclerosis, hypertension and diabetes. Initially, these beneficial effects were ascribed to vitamin D. Recently it became evident that immunomodulation, the formation of nitric oxide, melatonin, serotonin, and the effect of (sun)light on circadian clocks, are involved as well. In Europe (above 50 degrees north latitude), the risk of skin cancer (particularly melanoma) is mainly caused by an intermittent pattern of exposure, while regular exposure confers a relatively low risk. The available data on the negative and positive effects of sun exposure are discussed. Considering these data we hypothesize that regular sun exposure benefits health.

Introduction

In the beginning of the 20th century recreational sunbathing became popular. Decades later an increase in the incidence of skin cancer was noted, starting in northern Australia, with its (sub)tropical climate and its population with a sun-sensitive skin [1]. In 1992 the WHO concluded that solar ultraviolet (UV) radiation is the main environmental cause of skin cancer [2]. Extensive programs for the primary prevention of skin cancer were developed, starting in Australia in the 1980’s [3]. During the last decades new beneficial associations between sunlight and disease were discovered [4]. The question arises whether the present sun-shunning advices benefit our general health; there is no unequivocal scientific proof that they do. In contrast, there is growing evidence that regular exposure to sunlight might benefit health.

Section snippets

The hypothesis

Considering the present evidence of the negative and positive effects of regular sun exposure, we propose the following hypothesis: “Regular sun exposure benefits health.

Change of exposure patterns

The principal theories on variation in pigmentation and UV radiation assume that dark skin protects against sunburn and possibly folate deficiency, whereas light skin allows sufficient photosynthesis of vitamin D and other effects of sunlight in areas with low UV radiation. In the context of human evolution, the variation in pigmentation with latitude is considered a good compromise between the positive and negative effects of sunlight [5]. This is an appropriate statement for our ancestors who

Negative effects of sunlight

The negative effects of sunlight are summarized in table 1. The induction of skin cancer is considered to be the most important one. Skin cancer results from an interaction between genetic susceptibility and environmental exposure, mainly to UV [9]. Increased risks have been shown for those who have red/blonde hair, light eye colour, burn easily, and tan poorly. At present observational studies support the ‘intermittent sun exposure hypothesis’ for melanoma, the most aggressive type of skin

Positive effects of regular sun exposure

Well-established positive effects of sun exposure are prevention and treatment of skin diseases (like psoriasis and eczema), photosynthesis of vitamin D (important for bone and muscle health) and prevention and treatment of seasonal affective disorder [9]. During the last decades new favourable associations between sunlight and disease have been discovered: there is growing observational and experimental evidence that regular exposure to sunlight could contribute to the prevention of

Consequences of the hypothesis and discussion

When it was discovered that UV radiation was the main environmental cause of skin cancer, primary prevention programs were started. These programs advise to avoid the sun in the middle of the day, to use sunlight-protective clothing and to apply more or less continuously sunscreens. These sun advices are similar all over the world. They are meant to prevent skin cancer, but the question was never raised what their effects are on general health. Recently, three prospective studies investigated

Conflict of interest statement

The authors declare that they have no conflict of interest.

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