Vitamin D is often called "The Sunshine Vitamin" for good reason: it's the only nutrient other than oxygen that can be obtained completely for free. When the skin is exposed to ultraviolet B radiation from the sun, 7-dehydrocholesterol in the skin is converted into vitamin D3. Studies show that exposing the skin to the sun while wearing a bathing suit in the summer can produce 10,000 to 20,000 units of vitamin D3 in the skin. However, there are several caveats to consider in this process. For one, the sun must be in the right position in the sky to produce the required spectrum of UV-B light. This explains why even though the sun may be out in the northern latitudes during the winter, it's at an angle that is not amenable to producing the required UV-B strength to activate the vitamin D synthesizing process in the skin. Other factors that can interfere with this natural production of vitamin D from sun exposure include having darker skin, as happens in black and Latino people; having large amounts of subcutaneous body fat; and just being older, in which the conversion process is less efficient compared to the young (isn't everything?). Even using a high SPF sun blocker will also block D production in the skin.
Despite the ease at obtaining sufficient vitamin D without even having to eat food or take a supplement, statistics reveal that nearly 90 percent of the world population is deficient in vitamin D. Obviously if you don't expose your skin to the sun, or if the UV-B wavelength isn't optimal, you need to obtain your vitamin D elsewhere. The problem with this is that you are left with only two choices: food sources or supplements. If you rely on food sources, you have an uphill battle to obtain sufficient vitamin D to produce optimal blood levels, which requires an intake of about 2,000 units a day of D. While the optimal blood level of D is still a matter of fierce debate among researchers, most agree that 40 to 50 nanograms per deciliter of blood is optimal for health. The D measured in blood is in the form of 25-hydroxy D, the active form of D. Vitamin D is actually more of a prohormone since it must be converted to the activated or hormonal form of D, 1,25-dihydroxy vitamin D, through enzymatic conversions in the liver and kidneys. The direct precursor to this activated form of D is 25-hydroxyvitamin D, and that is what is measured in blood tests for D. Interestingly, activated vitamin D is considered a steroid hormone, as is testosterone, estrogen, and cortisol. Thus, if you ingest a vitamin D supplement you are actually taking steroids! While this may sound silly at first glance, it's not as silly as it seems, since vitamin D is known to interact . . .