Physicians have long understood the role of vitamin D in bone growth (1). Recently, though, new attention has focused on the role of vitamin D in many other body processes, including the functioning of immune and neural cells. Vitamin D deficiency has been associated with an increased susceptibility to influenza and tuberculosis infections; cardiovascular, autoimmune, and neurodegenerative diseases; and multiple cancers, including colon, breast, ovary, and prostate. Given the importance of this vitamin to overall health and the role of ultraviolet sunlight in producing it, strategies to raise vitamin D levels – without harmful sun exposure – has been the subject of recent controversy.
Vitamin D is a steroid hormone that is produced in humans in a series of steps. There are several forms of vitamin D found in nature, labeled D1–D5. The human body generates the D3 form of the vitamin; some plants and other organisms produce alternative forms of the vitamin (D1–D2, D4–D5). In humans, the first step in vitamin D3 production occurs in the outermost layer of skin (the epidermis) in response to exposure to ultraviolet light (UVB, specifically). This results in the formation of an inactive vitamin D3 precursor, which must travel through the bloodstream to the liver, where the next step occurs. After a second chemical reaction, an additional precursor is formed that travels through the bloodstream to the kidneys, where the final conversion step takes place. The end result is the production of the biologically active form of the hormone called 1,25-dihydroxycholecalciferol. Thus, the pathway to generation of the active hormone is a complex interaction between 3 organ systems (the skin, the liver, and the kidneys) and exposure to ultraviolet light.
The intensity and type of ultraviolet exposure (ie, the appropriate wavelengths) are key factors that influence the first step involving the skin. The intensity of ultraviolet light is related to the distance from the equator and the time of year of the exposure. Additionally, the amount of natural melanin (pigment) in the skin determines how much ultraviolet light can penetrate. Individuals with darker skin require a longer exposure to the sun’s ultraviolet rays to produce the same amount of vitamin D3 precursors that are produced in individuals with lighter skin. Along with melanin, one of the body’s natural filters for ultraviolet light, sunscreens and sun blocks can also influence the amount of vitamin D3 precursor that is produced. Maintaining adequate levels of the biologically active form of the vitamin also requires normal functioning of the liver and kidneys, as they are also critical sites in the production of the vitamin.
In addition to sun exposure, increasing vitamin D levels can also be achieved through dietary sources – the richest include fatty fish, such as herring, catfish, mackerel, and tuna. Additionally, many foods are now fortified with vitamin D, including milk, yogurt, cereals, and bread. Because the human body can use both D3 and D2 forms of the vitamin, taking an over-the-counter daily multivitamin tablet that contains vitamin D2 is an easy way to increase the body’s vitamin D level. Given the risks of long-term sun exposure, the American Academy of Dermatology (AAD) recommends that we raise levels of vitamin D through use of dietary sources and supplements rather than increasing time in the sun or refraining from sunscreen use.