More Sunlight May Be the Cure to Your Ailments
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. 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. Indeed, only about 0.1% of UVB radiation and 5% of UVA radiation reach the surface of our planet. 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.
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 It can then reach the kidneys, where it will be further metabolized into the “active” form of vitamin D: 1α,25-dihydroxyvitamin D3 [1α,25(OH)2D3].2, 4 Once formed, the D3 travels to the cells of virtually all tissues and organs in the body, where it will bind specific receptors. 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 its 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 2,000 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 (e.g., rickets, osteomalacia, osteoporosis, muscle pain, sarcopenia), various types of cancer, hypertension, cardiovascular disease, infectious and autoimmune diseases (e.g., multiple sclerosis, inflammatory bowel disease, rheumatoid arthritis, systemic lupus erythematosus, diabetes, psoriasis) pregnancy complications, neurodegenerative diseases, depression, and even schizophrenia.4-8
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 recently.)34-36
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