Vitamin D and Bone Health. As stated by the National Cancer Institute:
Vitamin D is essential for the formation, growth, and repair of bones and for normal calcium absorption and immune function. It is obtained primarily through exposure of the skin to ultraviolet radiation in sunlight, but it can also be obtained from some foods and dietary supplements. [Emphasis added]
According to a study published in the U.S. Government journal Environmental Health Perspectives:
Unlike other essential vitamins, which must be obtained from food, vitamin D can be synthesized in the skin through a photosynthetic reaction triggered by exposure to UVB radiation. The efficiency of production depends on the number of UVB photons that penetrate the skin, a process that can be curtailed by clothing, excess body fat, sunscreen,
Unlike other essential vitamins, which must be obtained from food, vitamin D can be synthesized in the skin through a photosynthetic reaction triggered by exposure to UVB radiation. The efficiency of production depends on the number of UVB photons that penetrate the skin, a process that can be curtailed by clothing, excess body fat, sunscreen, and the skin pigment melanin. For most white people, a half-hour in the summer sun in a bathing suit can initiate the release of 50,000 IU (1.25 mg) vitamin D into the circulation within 24 hours of exposure; this same amount of exposure yields 20,000–30,000 IU in tanned individuals and 8,000–10,000 IU in dark-skinned people.
The initial photosynthesis produces vitamin D3, most of which undergoes additional transformations, starting with the production of 25-hydroxyvitamin D (25[OH]D), the major form of vitamin D circulating in the bloodstream and the form that is routinely measured to determine a person’s vitamin D status. Although various cell types within the skin can carry out this transformation locally, the conversion takes place primarily in the liver. Another set of transformations occurs in the kidney and other tissues, forming 1,25(OH)D. This form of the vitamin is actually a hormone, chemically akin to the steroid hormones.
1,25(OH)D accumulates in cell nuclei of the intestine, where it enhances calcium and phosphorus absorption, controlling the flow of calcium into and out of bones to regulate bone-calcium metabolism. Michael Holick, a medical professor and director of the Bone Health Care Clinic at Boston University Medical Center, says, “The primary physiologic function of vitamin D is to maintain serum calcium and phosphorous levels within the normal physiologic range to support most metabolic functions, neuromuscular transmission, and bone mineralization.”
Without sufficient vitamin D, bones will not form properly. In children, this causes rickets, a disease characterized by growth retardation and various skeletal deformities, including the hallmark bowed legs. More recently, there has been a growing appreciation for vitamin D’s impact on bone health in adults. In August 2007, the Agency for Health Care Policy and Research published Effectiveness and Safety of Vitamin D in Relation to Bone Health, a systematic review of 167 studies that found “fair evidence” of an association between circulating 25(OH)D concentrations and either increased bone-mineral density or reduced falls in older people (a result of strengthened muscles as well as strengthened bones). “Low vitamin D levels will precipitate and exacerbate osteoporosis in both men and women and cause the painful bone disease osteomalacia,” says Holick.
A 2009 study published in Clinical Medicine: Women’s Health reported that:
Our results show healthy women with regular UVB exposure via tanning beds have significantly greater vitamin D status and lower serum osteocalcin concentrations than those without and that there is a significant inverse relationship between serum serum 25(OH)D and osteocalcin concentrations which appears to be PTH-dependent.
Other Potential UV-mediated effects.
As summarized in the Envirionmental Health Perspectives article, “Benefits of Sunlight: A Bright Spot for Human Health“:
Direct immune suppression. Exposure to both UVA and UVB radiation can have direct immunosuppressive effects through upregulation of cytokines (TNF-α and IL-10) and increased activity of T regulatory cells that remove self-reactive T cells. These mechanisms may help prevent autoimmune diseases.
Alpha melanocyte-stimulating hormone (α-MSH). Upon exposure to sunshine, melanocytes and keratinocytes in the skin release α-MSH, which has been implicated in immunologic tolerance and suppression of contact hypersensitivity. α-MSH also helps limit oxidative DNA damage resulting from UVR and increases gene repair, thus reducing melanoma risk, as reported 15 May 2005 in Cancer Research.
Calcitonin gene-related peptide (CGRP). Released in response to both UVA and UVB exposure, this potent neuropeptide modulates a number of cytokines and is linked with impaired induction of immunity and the development of immunologic tolerance. According to a report in the September 2007 issue of Photochemistry and Photobiology, mast cells (which mediate hypersensitivity reactions) play a critical role in CGRP-mediated immune suppression. This could help explain sunlight’s efficacy in treating skindisorders such as psoriasis.
Neuropeptide substance P. Along with CGRP, this neuropeptide is released from sensory nerve fibers in the skin following UVR exposure. This results in increased lymphocyte proliferation and chemotaxis (chemically mediated movement) but may also produce local immune suppression.
Endorphins. UVR increases blood levels of natural opiates called endorphins. Melanocytes in human skin express a fully functioning endorphin receptor system, according to the June 2003 Journal of Investigative Dermatology, and a study published 24 November 2005 in Molecular and Cellular Endocrinology suggests that the cutaneous pigmentary system is an important stress-response element of the skin.