Can Sunshine Help Prevent Cancer?

Some Scientists Think Too Little Sun Is Worse Than Too Much

(AP) 
"People tend to go overboard with even a hint of encouragement to get more sun exposure," Thun said, adding that he'd prefer people get more of the nutrient from food or pills.

But this is difficult. Vitamin D occurs naturally in salmon, tuna and other oily fish, and is routinely added to milk, but diet accounts for very little of the vitamin D circulating in blood, Giovannucci said.

Most supplements use an old form — D-2 — that is far less potent than the more desirable D-3. Multivitamins typically contain only small amounts of D-2 and include vitamin A, which offsets many of D's benefits.

As a result, pills might not raise vitamin D levels much at all.

Government advisers can't even agree on an RDA, or recommended daily allowance for vitamin D. Instead, they say "adequate intake" is 200 international units a day up to age 50, 400 IUs for ages 50 to 70, and 600 IUs for people over 70.

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 of large groups of people found that those with higher vitamin D levels also had lower rates of cancer. 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 that 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.