How much vitamin D do you get from the sun?

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.

There are a few things that could affect how well your body converts sunlight into vitamin D3.

First, let's talk about you.

Your skin pigment has a lot to do with how well your skin absorbs UVB. The more melanin in your skin (the darker your skin), the less UVB rays can penetrate. Your weight can also affect your D3 production system. Remember, vitamin D is a fat-soluble vitamin. The more fat cells your body has, the more you may be at risk of a D deficiency, because the vitamin D in your body becomes trapped in those fat cells. When comparing obese and non-obese people, researchers have found those who are obese have 50 percent less vitamin D circulating in their blood than people of normal weight, despite equal sunlight exposure [source: Lee]. And then there's aging. As we get older, it naturally becomes more difficult to convert sunlight into D3. For example, when exposed to the same amount of UVB radiation, a person who is 70 years old will make 75 percent less D3 than a 20-year-old [source: Lee].

And now let's talk about where you live and your habits -- your sunscreen habit, that is.

Where you live in relation to the equator makes a difference regarding how much UVB energy you're exposed to. Live above 37 degrees north of the equator -- in the U.S., that'd put you north of Washington, D.C. and north of the Utah/Arizona border -- or below 37 degrees south of the equator, and you probably aren't getting enough UVB exposure to naturally produce adequate amounts of D3 year-round. For your skin to synthesize vitamin D, it needs UVB sunlight between 290 and 300 nm wavelengths (peaking between 295 and 297 nm wavelengths) -- and UVB wavelengths are only available when the UV index is greater than 3, which is not consistently the case in the areas outside the central equatorial band [sources: The George Mateljan Foundation, Nordqvist].

Let's look at an example. If you're fair skinned, sunburn easily and live in New York City, you would need to spend just four minutes outside on a sunny 4th of July to produce about 1,000 IU of vitamin D, but that turns into 40 minutes on a sunny New Year's Day. Under those same conditions, a person with dark skin would need 16 minutes on July 4 and about 4.5 hours on Jan. 1 to produce the same amount of D3 [source: NIAR]. And on those days with complete cloud (or smog) cover you can only expect about 50 percent of UV to penetrate [source: NIH].

And then there's sunscreen.

If you want your skin to absorb UVB rays, you can't wear sunscreen. Studies have found that sunscreens with sun protection factor (SPF) 8 or higher block our skin's ability to produce vitamin D from sunlight by as much as 95 percent (and SPF 15 or higher by 99 percent) [source: Holick, Ginde]. The bottom line? Be reasonable. Talk to your doctor about the risks and benefits of limited unprotected sun exposure as well as whether or not you might benefit from vitamin D supplements. If you're comfortable with the benefits unprotected sun exposure, two to three times every week might afford you, don't forget to apply sunscreen and cover up once those minutes are up.