Q. Is vitamin D stored in the body and if so, for how long? If I get 3-5 hours of direct sunlight per day during the summer, will I have enough vitamin D in my body to last me the rest of the year, or will I still need to get some from diet and/or supplementation?
A. Your body has only a limited capacity to store vitamin D and the effects of sun exposure on vitamin D levels only last a week or two at most. So, sunbathing in the summer will not get you through the winter. And vitamin D deficiency is a serious problem, as we discussed in this recent blog post. The average American diet provides far less than the recommended 400IU per day, and a consensus is building that even 400IU per day is woefully inadequate.
I just ran across this long but well-referenced article by Krispin Sullivan, posted at the Weston Price Foundation website, that goes into quite a bit of detail on every aspect of this issue. I highly recommend the article for anyone attempting to meet their vitamin D requirements via sun exposure. For example, you'll learn that while you can get a sunburn from both UV-A and UV-B rays, you can only make vitamin D from UV-B.
While UV-A is present all day long, UV-B is only present at mid-day, unless you live in the tropics. So, if you are sunning for vitamin D, you need to do it at mid-day (between 10 am and 2 pm). If you sun yourself before or after that time, you will get a sunburn (from UV-A) before you get enough UV-B to maximize vitamin D production.
Even during those hours, how much vitamin D you can produce depends on your skin type, your age, your latitude, your altitude [with a shout-out to faithful reader and astute observer Jim Kent: you said it first!], how much of your body is exposed, etc. But get this: For less than $200, you can buy a UV-B meter that will actually calculate how many IU of vitamin D3 you are producing per minute!
Here's another fascinating tip: To maximize vitamin D levels, don't take a shower or get in the pool for an hour after sun exposure. Right after you are exposed to the sun, the vitamin D is still in the oils in your skin and can be washed away before it is absorbed into the blood.
Back to your question: Unless you live in the tropics, you will definitely need another source of vitamin D to get you through the winter, either food sources or supplements. And here's a very important point about vitamin D supplementation brought up in the article linked to above.
Sullivan notes that supplementing with vitamin D not only increases your absorption of calcium but will also increase your absorption of lead, arsenic, and cadmium if your levels of calcium, magnesium, and phosphorus are inadequate. So, if you are going to take a vitamin D supplement, be sure that your intake of these nutrients (from either diet or supplementation) is also sufficient!http://blog.nutritiondata.com/ndblog/2008/07/can-you-store-u.html