Do you get enough sun?

Over the past many years I've slowly been lead to believe, mostly by way of media reports, that basically any sun exposure is bad - everyone knows that the sun causes skin cancer! Turns out it is worth it to seek out a little exposure several times a week, because too little exposure may be even worse for you than too much:

Dr. Edward Giovannucci, a Harvard University professor of medicine and nutrition who laid out his case in a keynote lecture at a recent American Association for Cancer Research meeting in Anaheim, Calif.

His research suggests that vitamin D might help prevent 30 deaths for each one caused by skin cancer.

"I would challenge anyone to find an area or nutrient or any factor that has such consistent anti-cancer benefits as vitamin D," Giovannucci told the cancer scientists. "The data are really quite remarkable."
[...]
Many scientists think adults need 1,000 IUs a day. Giovannucci's research suggests 1,500 IUs might be needed to significantly curb cancer.

How vitamin D may do this is still under study, but there are lots of reasons to think it can:

_Several studies observing large groups of people found that those with higher vitamin D levels also had lower rates of cancer. For some of these studies, doctors had blood samples to measure vitamin D, making the findings particularly strong. Even so, these studies aren't the gold standard of medical research - a comparison over many years of a large group of people who were given the vitamin with a large group who didn't take it. In the past, the best research has deflated health claims involving other nutrients, including vitamin E and beta carotene.

_Lab and animal studies show that vitamin D stifles abnormal cell growth, helps cells die when they are supposed to, and curbs formation of blood vessels that feed tumors.

_Cancer is more common in the elderly, and the skin makes less vitamin D as people age.

_Blacks have higher rates of cancer than whites and more pigment in their skin, which prevents them from making much vitamin D.

_Vitamin D gets trapped in fat, so obese people have lower blood levels of D. They also have higher rates of cancer.

_Diabetics, too, are prone to cancer, and their damaged kidneys have trouble converting vitamin D into a form the body can use.

_People in the northeastern United States and northerly regions of the globe like Scandinavia have higher cancer rates than those who get more sunshine year-round.

The National Institutes of Health says:

Sun exposure
Sun exposure is perhaps the most important source of vitamin D because exposure to sunlight provides most humans with their vitamin D requirement [13]. UV rays from the sun trigger vitamin D synthesis in skin [13-14]. Season, geographic latitude, time of day, cloud cover, smog, and sunscreen affect UV ray exposure and vitamin D synthesis [14]. For example, sunlight exposure from November through February in Boston is insufficient to produce significant vitamin D synthesis in the skin. Complete cloud cover halves the energy of UV rays, and shade reduces it by 60%. Industrial pollution, which increases shade, also decreases sun exposure and may contribute to the development of rickets in individuals with insufficient dietary intake of vitamin D [15]. Sunscreens with a sun protection factor (SPF) of 8 or greater will block UV rays that produce vitamin D, but it is still important to routinely use sunscreen to help prevent skin cancer and other negative consequences of excessive sun exposure. An initial exposure to sunlight (10 -15 minutes) allows adequate time for Vitamin D synthesis and should be followed by application of a sunscreen with an SPF of at least 15 to protect the skin. Ten to fifteen minutes of sun exposure at least two times per week to the face, arms, hands, or back without sunscreen is usually sufficient to provide adequate vitamin D [14]. It is very important for individuals with limited sun exposure to include good sources of vitamin D in their diet.

Updated: Another study, this time by the Australian National University, comes to similar conclusions: Between two and 14 minutes of midday summer sun three or four times a week on the face and arms will produce an adequate dose of the vitamin for those living in Australia.

Updated2: Latest studies question the above.