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.