Beyond Vitamin D

Sunlight has been an integral part of human evolution and the development of life on this planet. Sunlight provides energy to grow plants, providing us with food, shelter and various tools. It is also essential to regulating the seasons. Thus, it has become a necessary component of life as we know it. From a human physiological perspective, sunlight is a well known source of vitamin D. There have been many suggestions that if you are not exposed to sunlight, then take your vitamin D supplements. However, sunlight has a number of other physiological mechanisms associated with it other than simply providing vitamin D. This paper will address some of these mechanisms, as well as sunlight's positive and negative effects, and some sun-safe strategies.

Effects of Vitamin D

Vitamin D3, also known as calcitriol is the most active form of vitamin D. It acts as a potent steroid hormone that has a number of functions in the body, such as immunomodulatory effects (i.e. balancing inflammatory responses) ^[1], regulating growth, and influencing the differentiation of epithelial cells ^[2]. This may have a number of benefits, such as inhibiting cancer ^[2] and decreasing inflammation in Crohn's disease ^[1]. It may also have certain negative impacts, such as depressing defences against pathogens through some of its immunosuppressive qualities ^[1].

Sunlight's Effects on Physiology

Although vitamin D is often used synonymously with sunlight, sunlight is also likely to affect other mechanisms. One study found that sunlight exposure and not vitamin D improved depression and fatigue in individuals with multiple sclerosis ^[3]. It has also been found that the same ultraviolet radiation found in sunlight will inhibit demylenation and inflammation of the spinal cord in animal models with experimental autoimmune encephalomyelitis ^[4]. This occurs independently of vitamin D by altering other factors such as spinal cord chemokine CCL5 mRNA and protein levels, as well as suppressing IL10 in the skin and spleen ^[4]. Sunlight may also have the potential to suppress obesity and metabolic syndrome, as was found in studies with mice. However, vitamin D supplementation did not elicit the same results. This study found that the ultraviolet radiation induced other mechanisms such as nitric oxide which lead to the suppression of obesity and metabolic syndrome ^[5]. It is interesting to note, that if sunlight does directly effect nitric oxide, there are a number of other potential benefits that may occur through this mechanism. Nitric oxide is also responsible for other actions in the body, such as in platelet function, inflammation, vasodilation and pain perception ^[6]. A large prospective cohort study of woman in the Agricultural Health Study, found that increased sunlight exposure was associated with a decreased risk of breast cancer. This study also indicated that exposure over a longer period of time may have more protective effects ^[7]. It was also found that vitamin D including supplementation did not appear to play a role in reduced breast cancer risk ^[7].

Sunlight has the potential to damage the skin in various ways. However, sunlight also promotes a number of adaptive responses through both vitamin D ^[8] and other mechanisms ^[9], that may help to mitigate this damage. Responses associated with vitamin D may help to reduce sunlight induced DNA damage ^[8]. Other mechanisms, such as melanin, which is produced by melanocytes in response to ultraviolet radiation damage from sunlight, enhances skin pigmentation. This is a natural protective response ^[9]. However, some individuals have a tendency to burn and not tan, such as those with a fair skinned complexion, freckling and red/blonde hair ^[9], and therefore, the protective responses are not likely to be as effective for everyone. Exposure to sunlight may produce a number of other local responses as well. In studies using isolated cultured skin cells, it has been demonstrated that Ultraviolet B can stimulate a number of local neuroendocrine factors, such as corticotropin-releasing hormone, proopiomelanocortin, ACTH, Beta endorphin, the glucocorticolidogenic pathway, and glucocorticoid receptors ^[10]. Wavelength dependent changes in almost all aspects of the skins hypothalamic-pituitary axis functions have been observed, indicating that ultraviolet radiation can coordinate homeostatic mechanisms in the skin via stimulation through various neuroendocrine stress responses ^[10]. Similarly, sunlight has the ability to regulate various rhythms in the body. The human body will synchronize with light, by entraining to its timing, duration, intensity, wavelength and pattern of exposure ^[11]. This entrainment works best when it follows natural light patterns (i.e. includes a twilight period, as opposed to abrupt on/off cycles) ^[12]. The signals are received via peripheral cells and retinal photoreceptors, sending hormonal signals to the master clock in the brain, the suprachiasmatic nucleus ^[12]. This entrainment has been found to influence cortisol levels, which is important, because cortisol is an important factor in regulating homeostasis ^[13].

Health Concerns of Sunlight Exposure

Excessive sunlight exposure is not recommended because it can have damaging effects. It is considered the main cause of skin cancer, especially when it is associated with sunburns ^[14], and when exposure to ultraviolet radiation is intermittent and intense ^[9]. The ultraviolet light produced by sunlight has also been linked to a number of other physical changes, such as inflammation, impaired wound healing and DNA damage ^[9]. If someone has had more than 5 sunburns in their life, their risk of Melanoma doubles ^[9], and increased sunburns in childhood are associated with an increased risk of Melanoma as a young adult ^[9]. Despite the promotion of protective clothing, sunscreens, etc., the prevalence of sunburns has not declined ^[9]. This indicates that these protective strategies are not being adequately/properly utilized. Dutch data suggests that local inhabitants are well adapted to the Dutch climate, but when this pattern is changed, there is a steady increase in skin cancer. The shift is generally from a decrease in daily regular exposure to intermittent exposure ^[15].

Safety of Tanning Beds

In certain climates access to sunlight in winter months is limited as a result of decreased skin exposure, angle of solar radiation and decreasing hours of sunlight ^[16], therefore, sunlight exposure may not be adequate. Thus, tanning beds are often used, especially in winter months in response to seasonal decreases in sunlight. Tanning beds have shown some benefits, such as being able to increase vitamin D levels as well as improving other functions such as systolic blood pressure ^[17]. Also, because tanning beds utilize ultraviolet radiation, many of the other benefits discussed above may also be present. The ultraviolet light emitted by the sun is divided into three categories: Ultraviolet A (UVA), Ultraviolet B (UVB), and Ultraviolet C (UVC) ^[18]. The ultraviolet light given off by tanning beds is predominantly UVA (95-99%) and UVB (1-5%) ^[18]. Although tanning beds may have some benefit, both UVA and UVB have demonstrated carcinogenic effects ^[18], similar to that in sunlight. It appears that tanning beds may be associated with the development of Squamous Cell Carcinoma ^[18], and an increased risk of Melanoma ^{[9, 19]}. The risk of developing Melanoma was highest with more than 10 sessions of tanning bed use ^[19]. There was also no difference in association before or after the year 2000, which suggests that newer tanning beds may not be any safer than the older models ^[19].

Sun Safe Strategies

Various strategies have been promoted to offer protection from the damaging effects of ultraviolet light, such as the Slip-Slop-Slap campaign (i.e. slip on a shirt, slop on some sun-screen, and slap on a hat) from the early 1980's ^[17]. Protection from excess ultraviolet rays is important to prevent sunburns, which as mentioned above are linked to an increased risk of skin cancer. Avoiding excess use of tanning beds is also advisable as they have been linked with an increased risk of cancer ^[19]. As well, regular screening for early detection of skin cancer is an important strategy ^[14]. Regular exposure to sunlight is likely to be beneficial, although as discussed above, irregular intense exposures that result in burning can be potentially harmful ^[15]. Therefore, protection is important. This may include wearing protective clothing and sunscreen. Regular sunscreen use appears to reduce the risk of skin cancer ^[20], as well, studies indicate that using sunscreen does not appear to negate some of the health benefits of sunlight, such as the reduced risk of breast cancer ^[7]. It appears that an adequate amount of ultraviolet light is able to penetrate sunscreen to produce vitamin D ^[20]. However, more research may be needed to assess whether the other physiological mechanisms promoted by sunlight are affected by sunscreen use. There may also be some adverse effects associated with sunscreen use. Animal studies have indicated that certain compounds in sunscreens may promote developmental/reproductive toxicity as well as disturb the hypothalamic-pituitary-thyroid axis ^[21]. Therefore, more research may be required in this area to ensure that the sunscreens used are safe. A number of natural and synthethic products are available that may help mitigate ultraviolet induced damage by enhancing antioxidant enzymes. Antioxidants may include alpha-tocopherol, ferulic acid, flavangenol, phloretin, lipoic acid and uric acid, lipid soluble carotenoids, vitamin C and a number of flavonoids derived from plants ^[9]. Thus, products and sunscreens containing these and other related substances may be of benefit in protection against ultraviolet induced damage.

Supplementation

As discussed above, vitamin D is an important aspect of sunlight exposure and has a number of physiological benefits. Supplementation may be relevant especially for individuals in northern latitudes who are at an increased risk of deficiencies associated with decreased sun exposure. However, because vitamin D may also have negative effects, supplementation should be done under the guidance of a health professional. Despite vitamin D often being referred to as a sunlight substitute, there appears to be a number of other mechanisms involved in sunlight exposure.

Conclusion

Overall, it appears that our understanding of sunlight’s benefits are limited. It is also apparent that not everyone has the same tolerance to sunlight, and thus may be more prone to its ill effects ^[9]. Speaking with your health care providers can help you to develop sun safe strategies to maximize its benefits and minimize it's dangers.