Naturally, we’re disposed to think about vitamin D as vitamin – a substance that we get from our diets, like vitamin C or niacin, and that participates in biological reactions to help the body operate optimally. But despite its name, vitamin D isn’t really a vitamin, you can’t rely on diet to obtain it; you do, however, make it in your skin. Vitamin D is in a class by itself; its far-reaching effects on the body are aligned with how hormones act to influence metabolic pathways, cellular functions, and the expression of myriad genes. Vitamin D’s active metabolic product in the body, in fact, is a molecule called 1,25-dihydroxyvitmain D, which is a secosteroid hormone that directly or indirectly targets more than two thousand genes, or about 6 percent of the human genome.
Generally speaking, vitamins are organic compounds that cannot be made by the body but are necessary for proper functioning. (The term vitamin comes from “vital amine” – a substance that is essential for health but cannot be made by the body). Obtained through the diet or supplementation, vitamins are vital to growth, development, and metabolic reactions. Hormones, on the other hand, are synthesized in the body from simple precursors and go to distant tissues where they have an intended effect and make multiple metabolic improvements. In the case of the manufacture of vitamin D, which requires the help of an outside source to trigger a sequence of events, the precursor of a cholesterol-like molecule found in the skin cell starts the process by absorbing just the ultraviolet B portion of sunlight to create what’s called previtamin D3 quickly rearranges itself with the help of the body’s heat to give birth to vitamin D, which immediately exits the skin cell for the bloodstream. The fact that vitamin D is made in living skin cells explains why it is not possible to wash off vitamin D when you bathe after being exposed to the sun.
Before vitamin D can act as a hormone, however, it must go through two steps of activation – one in your liver and another in your kidneys. The process is the example of how our brilliant bodies operate and self-regulate to ensure optimal health.
If you apply a sunscreen with an SPF of 8 into your skin, it will absorb about 90 percent of UVB radiation and decrease your ability to make vitamin D in your skin by about 90 percent. An SPF of 30 reduces your ability by 99 percent. While it’s true that most people don’t put sunscreen on properly, people are now using sunscreen with an SPF of 45 or above, so even if you put on half or one third of the recommended amount, you’re still getting an SPF of 15 and reducing your ability to make vitamin D in your skin by about 95 percent. Farmers in the Midwest who had a history of nonmelanoma skin cancer were told to always use sun protection, and they did. When they are measured by their blood levels of vitamin D at the end of the summer, most were deficient.
Most humans obtain from sun exposure their vitamin D requirement between the hours of about 10:00 A.M. and 3:00 P.M. and mainly in the late spring, summer, and early fall. Because vitamin D is fat soluble, it’s stored in body fat and released throughout the winter months, allowing you to be vitamin D sufficient throughout the year.
Hormones are more sophisticated, complex molecules than vitamins. They can act in two ways: first, they can simply enter the cell and travel through the sea of cellular cytoplasm until they reach the nucleus – the brain of the cell – and influence its activity; second, they can bind to a receptor on a cell membrane and thereby transmit a signal to the cell, telling it to change what it is doing in any number of ways. Activated vitamin D mainly works by interacting with its receptor within the cell’s nucleus.