The sun emits electromagnetic radiation, which encompasses a large spectrum of wavelengths. Of these, only infrared, visible light, and ultraviolet (UV) radiation are able to reach the earth’s surface and hence our skin.1 So far, most of the research regarding the effects of the sun on human health has focused on UV radiation,1, 2 which is divided into UVC, UVB and UVA.3 As depicted in figure 1, virtually no UVC radiation reaches the earth’s surface, except at extremely high altitudes, since it is efficiently absorbed by the stratospheric ozone layer.1-3 Ozone also absorbs most of the UVB and UVA radiation.2 Indeed, only about 0.1% of UVB radiation and 5% of UVA radiation reach the surface of our planet.2 In contrast, 39% of visible radiation and 56% of infrared radiation can reach the earth’s surface.2
Nevertheless, even these small levels of UVA and UVB radiation have a significant impact on human physiology, as I will try to show you.
Figure 1. Forms of solar UV radiation and the degrees to which they reach our skin.
UV radiation and Vitamin D
When we expose ourselves to sunlight, the UVB radiation is absorbed in the epidermis (which is the outer layer of our skin) by a molecule called 7-dehydrocholesterol causing it to be converted into previtamin D3. Previtamin D3 is easily transformed into Vitamin D3, which exits the skin and diffuses into circulation, reaching the liver. In the liver, it is converted by one or more enzymes into 25-hydroxyvitamin D3 [25(OH)D3].2, 4 25OHD3 can then reach the kidneys, where it will be further metabolized into the so-called active form of vitamin D: 1α,25-dihydroxyvitamin D3 [1α,25(OH)2D3].2, 4 Once formed 1α,25(OH)2D3 travels to the cells of virtually all tissues and organs in the body, where it will bind specific receptors. The Vitamin D receptor (VDR) being the most studied.2 Moreover, there is good evidence that various cells in other tissues besides the kidney can convert 25OHD3 into 1α,25(OH)2D3. This locally-synthesized 1α,25(OH)2D3 can then bind the VDR in those cells or in nearby cells (figure 2).
Figure 2. Formation of the active form of vitamin D3 and ist influence on nearly 2000 genes within cells throughout the body.
The binding of 1α,25(OH)2D3 (either from the kidneys or derived from local production) to the VDR will affect the expression of approximately 2000 genes and regulate many cellular processes.2, 4 This explains why Vitamin D insufficiency is associated with all-cause mortality and many health problems, such as musculoskeletal disorders (rickets, osteomalacia, osteoporosis, muscle pain, and sarcopenia), various types of cancer, hypertension, type II diabetes, cardiovascular disease, infectious and autoimmune diseases (multiple sclerosis, inflammatory bowel disease, rheumatoid arthritis, systemic lupus erythematosus, type 1 diabetes, and psoriasis among many others,) pregnancy complications, neurodegenerative diseases, depression, and even schizophrenia.4-8
Other beneficial effects of Sunlight
UV radiation and Immune-mediated diseases
Various observational studies have found that sun exposure and living at low latitude are associated with a reduced risk of various autoimmune diseases.5 The most cited explanation is vitamin D, because of its known regulatory effects on the immune system.6 Other mechanisms may also be involved however. Indeed, increasing evidence from epidemiological and animal studies suggest that UVA and UVB radiation exposure might prevent or even suppress the development of immune-mediated diseases, such as asthma, multiple sclerosis, and type 1 diabetes through various vitamin D-independent mechanisms.9-12 Moreover, clinical trials with UV radiation have shown positive effects in some immune-derived skin disorders, such as psoriasis, vitiligo, localized scleroderma, and atopic dermatitis.13 This suggests that UV radiation affects our immune system not only because it induces vitamin D synthesis, but through a variety of other pathways as well.
Neuropsychiatric effects of Sunlight
When humans are exposed to UVA and especially UVB radiation they increase the production of an opioid called ß-endorphin in their skin. ß-endorphin can be released into the blood and reach various tissues and organs, such as the brain, thereby improving mood and relaxation and decreasing pain.14-17 This could be one explanation for why patients who have undergone spinal surgery and been exposed to more sunlight during their hospital recovery period took less analgesic drugs and experienced less stress.18 It could also be one of the mechanism explaining the decrease in pain reported by fibromyalgia patients following UV radiation.13
ß-endorphin (and other chemicals of the hypothalamic-pituitary-adrenal axis produced in the skin that affect stress-related homeostasis15) may also be involved in the seasonal depression that affects some people during the winter time.2 Moreover, virtually all cells in our body, including the ones in the skin, contain genes that control our circadian rhythm.19, 20 The expression of those genes is increased when human skin is exposed to UVB radiation.21
Of course, a simpler explanation might lie in the direct effect of light on our central nervous system.20 Indeed, bright-light therapy has been shown to be effective for this disorder2 and also for jet-lag symptoms.22
UV radiation and cardiovascular risk factors
Epidemiological studies suggest that sun exposure decreases cardiovascular disease (CVD) risk.2, 5 Again Vitamin D stands out as the most plausible explanation.23 Nevertheless, the sun has many other effects beyond vitamin D that could reduce the risk. For instance, exposure to UVB radiation might reduce cholesterol production.24 However blood cholesterol’s connection to CVD is controversial, is not completely resolved,25 and is beyond the scope of this article.
Blood pressure may be a more important factor. In that sense, there is evidence that exposure to UV radiation enhances blood flow and reduces blood pressure.26-28 Possible mechanisms are: 1) the production of vasodilation substances by the skin after exposure to UVA and UVB radiation, such as nitric oxide, substance P and calcitonin gene-related peptide;2 2) release of skin stores of the powerful vasodilator nitric oxide (as well as its precursors, namely nitrite and S-nitrosothiols) following UVA exposure;27-29 and 3) release by hemoglobin of another vasodilator called carbon monoxide after UVB radiation.2
Given the above evidence coupled with the results of a recent study that observed higher mortality rates among Swedish women who avoided sun exposure,30, 31 it can be assumed that sunlight regulates human physiology in many ways. It also suggests that replacing sun exposure with Vitamin D3 (either through supplementation or diet) is a simplistic approach.
This fits with the evolutionary template, which states that some degree of sun exposure is necessary for optimal health. A sun-rich environment supposedly characterized the ecological niches of most of our hunter-gatherer ancestors32 until about 40 to 60 thousand-years-ago when a significant number of humans migrated to higher latitudes33 (although extreme latitudes appear to have only been systematically occupied more recently34-36).
So, how much sun exposure do we need? And what about potential risks, such as skin cancer? All of these questions will be answered in my next post.
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 Jussila A, Huotari-Orava R, Ylianttila L, Partonen T, Snellman E. Narrow-band ultraviolet B radiation induces the expression of β-endorphin in human skin in vivo. J Photochem Photobiol B. 2016 Feb;155:104-8.
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 Kawara S, Mydlarski R, Mamelak AJ, et al. Low-dose ultraviolet B rays alter the mRNA expression of the circadian clock genes in cultured human keratinocytes. J Invest Dermatol. 2002 Dec;119(6):1220-3.
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