You may be surprised to learn that vitamin D is completely different from most other vitamins. It is actually a hormone, a steroid hormone that is produced out of cholesterol when your skin is exposed to the sun. For this reason, vitamin D is often referred to as the "sunshine" vitamin. However, sun exposure is often inadequate these days, making it necessary for people to get more of it from the diet (or supplements). This is important, because vitamin D is absolutely essential for optimal health (1). Unfortunately, only a handful of foods contain significant amounts of this vitamin, and deficiency is extremely common (2, 3). In fact, according to data from 2005-2006, a whopping 41.6% of the US population is deficient in this critical vitamin/hormone (4). This article explains everything you need to know about vitamin D.

What Is Vitamin D? Vitamin D is one of the fat-soluble vitamins (A, D, E and K), meaning that it dissolves in fat/oil and can be stored in the body for a long time. There are actually two main forms found in the diet (5): Vitamin D3 (cholecalciferol): Found in some animal foods, like fatty fish and egg yolks.

Found in some animal foods, like fatty fish and egg yolks. Vitamin D2 (ergocalciferol): Found in some mushrooms. Of the two, D3 (cholecalciferol) is the one we're interested in, because it is almost twice as effective at increasing blood levels of vitamin D as the D2 form (6, 7). Bottom Line: Vitamin D is a fat-soluble vitamin that can be stored in the body for long periods of time. There are two main forms, D2 and D3, of which D3 is much more effective.

What Does Vitamin D Do in The Body? Vitamin D is actually pretty useless at first. It needs to go through two conversion steps to become "active" (8, 9). First, it is converted to calcidiol, or 25(OH)D, in the liver. This is the storage form of the vitamin in the body. Second, it is converted to calcitriol, or 1,25(OH)2D, mostly in the kidneys. This is the active, steroid hormone form of vitamin D. Calcitriol travels around the body, going into the nuclei of cells. There it interacts with a receptor called the vitamin D receptor (VDR), which is found in almost every single cell of the body (10, 11). When the active form of vitamin D binds to this receptor, it turns genes on or off, leading to changes in the cells (12, 13). This is similar to how most other steroid hormones work. It is well known that vitamin D affects various cells related to bone health, for example telling the cells in the gut to absorb calcium and phosphorus (14). But scientists have now found it to be involved in all sorts of other processes, including immune function and protection against cancer (15). So people who are deficient in vitamin D will be deficient in calcitriol (the steroid hormone form), so in effect they are deficient in one of the body's critical hormones. Bottom Line: Vitamin D is turned into calcidiol, the storage form of the vitamin, which is then converted into calcitriol, the active steroid form. Calcitriol binds to the vitamin D receptor inside cells, turning genes on or off.

Sunshine Is The Best Way to Get Vitamin D Vitamin D can be produced out of cholesterol in the skin, when it is exposed to the sun. The ultraviolet B (UVB) rays from the sun provide the energy needed for the reaction to occur (16). If you live where there is abundant sun year round, then you can probably get all the vitamin D you need by just going outside and sunbathing a few times per week. Keep in mind that you need to expose a large part of your body. If you're only exposing your face and hands then you will produce much less vitamin D. Also, if you stay behind glass or use sunscreen, then you will produce less vitamin D, or none at all. This makes the advice to use sunscreen to protect against skin cancer highly questionable. It raises your risk of vitamin D deficiency, which may lead to other diseases instead (17). If you decide to get your vitamin D from the sun, just make sure to never, ever burn. Sunshine is healthy, but sunburns can cause premature aging of the skin and raise your risk of skin cancer (18, 19). If you're staying in the sun for a long time, consider going without sunscreen for the first 10-30 minutes or so (depending on your sensitivity to sun), then apply it before you start burning. Vitamin D gets stored in the body for a long time, weeks or months, so you may only need occasional sun to keep your blood levels