Warming yourself by a window on a cold winter day may feel good, but it's not going to help you with your vitamin D production -- UVB rays don't penetrate glass [source: NIH].
How Your Body Converts Sunlight Into Vitamin D
Your body is able to produce its own vitamin D3 when your skin is exposed to the sun's ultraviolet rays, specifically ultraviolet (UVB) radiation. When UVB rays hit your skin, a chemical reaction happens: Your body begins the process of converting a prohormone in the skin into vitamin D. In this process, a form of cholesterol called 7-dehydrocholesterol (7-DHC), naturally found in your skin, absorbs the UVB radiation and gets converted into cholecalciferol. Cholecalciferol is the previtamin form of D3. Next, the previtamin travels through your bloodstream to your liver, where the body begins to metabolize it, turning it into hydroxyvitamin D, which is also known as 25-hydroxyvitamin D or 25(OH)D. The kidneys then convert the 25(OH)D into dihydroxyvitamin D, also called 1,25-dihydroxyvitamin D or 25(OH)2D -- this is the hormone form of vitamin D your body can use [sources: The George Mateljan Foundation, Holick].
It's estimated that we get -- or should get -- more than 90 percent of our vitamin D from casual, daily sun exposure [source: Holick]. Let's figure out what casual, daily sun exposure means. There are variables, which we'll talk about, but some studies have found that between five and 30 minutes of sun exposure to your unprotected face, arms, legs or back between the hours of 10 a.m. and 3 p.m. two to three times every week is enough for your body to produce all the D3 it needs [source: National Institutes of Health - Office of Dietary Supplements].
Under picture-perfect conditions, the human body is able to produce as much as 10,000 IU to 20,000 IU of vitamin D3 in just 30 minutes [source: The George Mateljan Foundation]. Yet vitamin D deficiencies are rising among all age groups in the U.S. It's estimated as many as half of all children, teens and young adults are vitamin D deficient, as are as many as 25 to 57 percent of American adults [source: Lee]. We don't often encounter those perfect conditions for vitamin D production, so let's look at some of the reasons we might not be making enough D3.