WASHINGTON – Women who get lots of vitamin D are less likely to develop breast cancer, suggests a pair of studies that add to the already strong evidence that the "sunshine vitamin" helps prevent many types of cancer.
High levels of vitamin D translated to a 50 percent lower risk of breast cancer, one study found. Even modestly higher levels resulted in 10 percent less risk, which would translate to 20,000 fewer cases a year if it were true of all American women.
A second study, by Canadian researchers, found that women who spent time outdoors or got a lot of vitamin D from their diets or supplements — especially as teens — were 25 percent to 45 percent less likely to develop breast cancer than women with less of the nutrient.
"Exposure to vitamin D at the time breasts are developing, particularly around adolescence, might be important," said lead researcher Julia Knight of Mount Sinai Hospital in Toronto.
Both studies were presented Tuesday at a meeting of the American Association for Cancer Research.
The body makes vitamin D from sunlight, but sun exposure is controversial because of the risk of skin cancer. Many health experts see little harm in 15 minutes several times a week.
Vitamin D is found in salmon, tuna and other oily fish, and is routinely added to milk, but diet accounts for very little of the nutrient that actually makes it into the bloodstream.
Supplements contain the nutrient, but most contain an old form, D-2, that is less potent than the harder-to-find D-3. Multivitamins typically contain little D-2 and include vitamin A, which offsets many of D's benefits.
So getting enough D safely and effectively is tough, but important, as the new studies show.
One, led by Cedric Garland of the University of California in San Diego, involved more than 120,000 women participating in two studies at Harvard University and Saint George's Hospital Medical School in London. Blood samples were obtained from 701 with breast cancer and a similar group of 724 healthy women.
Those with the highest blood levels of vitamin D had a 50 percent reduced risk of breast cancer, but very few women are at this level. It would require taking 1,000 international units of vitamin D a day, and most Americans get only about 320, Garland said.
Government advisers can't agree on an RDA, or recommended daily allowance, for vitamin D but 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.
Garland and many other scientists have been advocating 1,000 IUs a day, but warn people not to overdo it because too much can cause a dangerous buildup of calcium in the body.
Still, "it's becoming clearer now that we can take higher doses than people used to think," said Knight, who led the Canadian study.
It involved about 1,000 women with breast cancer and a comparison group of healthy women randomly selected through phone calls.
Those without breast cancer were less likely to cover up whenever they were outside and more likely to get dietary sources of vitamin D or to have had cod liver oil, which is rich in the nutrient, as children.
Diet is one of the few factors women can modify to affect their risk of developing breast cancer, so the vitamin D evidence is important, said Dr. William Nelson, a cancer specialist at Johns Hopkins School of Public Health who had no role in the studies.
"Consumers are looking for guidance" on what foods can help protect against cancer, he said.