December is a good month to talk about sunshine. It's a good month because, at least here in Western WA, there is so danged little of it! Most of the northern half of the USA is in the belt of deficient sunshine in the winter. What does this deficiency mean? No Vitamin D is getting produced.
Vitamin D gets made in one's skin as a result of exposure to the UV light in sunshine. It takes the body about 15 minutes of full exposure to sunlight to get Vitamin D levels into optimum range. In the winter FEW of us even SEE the sun for 15 minutes, let alone get to "bask" in it! So, the majority of people in northern latitude are running Vitamin D deficiencies. How do you know? The most accurate way is to have blood drawn and test for it. HOWEVER, the test is a pricey one (over $200). A less accurate way (but perhaps reliable way) is to look at your symptoms:
Low levels of Vitamin D are associated with:
1) Osteoporosis -- the loss of bone -- especially in older women.
2) Depression -- seasonal affective disorder. That winter time dole drum/depression that supposedly perks up with expensive full-spectrum light bulbs.
3) Increased risk of cancer: colon, prostate, and breast cancers have all been found less commonly in those carrying higher levels of Vitamin D.
Ways to increase your vitamin D include supplementation or tanning. My observation has been that while supplementation helps, it does not do nearly as well as tanning regularly. "Well doesn't tanning increase skin cancer?" My belief is, NO! What causes skin cancer is over-doing it -- getting burned over and over again. Once more, MODERATION is the key. A carefully done tanning regimen where burning is religiously avoided raises Vitamin D levels and does not increase skin cancer risk.