Two recent studies suggest getting enough vitamin D could help prevent colon, breast, and ovarian cancers. The researchers, from the University of California, San Diego, say taking supplements to get more vitamin D might be an easy, inexpensive way to reduce the number of cancer cases in the US. But other experts say it's too soon to recommend taking vitamin D supplements for cancer prevention.

"These findings are intriguing, but there are still gaps in the scientific evidence," said Marji McCullough, ScD, RD, a nutritional epidemiologist with the American Cancer Society. "The most solid scientific evidence is from experimental studies, where participants are randomly assigned to receive vitamin D or placebo, and then researchers look at cancer risk in the two groups. That level of evidence doesn't currently exist."

The type of studies the UCSD researchers analyzed were observational studies that cannot always control for other factors that might influence cancer risk, like exercise, obesity, or other nutrients in a person's diet, she explained.

Vitamin D is a nutrient that is crucial to bone health. It is found naturally in only a few foods like oily fish (salmon, tuna, mackerel, and sardines) and fish liver oil. It is often added to milk, cereals, and orange juice. But sunlight is the main source for most people; the ultraviolet rays stimulate the skin to produce vitamin D. Many Americans, however, don't have enough vitamin D in their bodies.

Many studies have looked at the relationship between cancer and vitamin D, but they have not been able to pinpoint exactly how vitamin D might influence cancer development, how much of an impact it might have, or how much vitamin D would be necessary to have any effect. The San Diego researchers say their findings are a step in that direction.

Their first study, published in the Journal of Steriod Biochemistry & Molecular Biology, analyzed 18 studies of vitamin D and colon cancer conducted between 1966 and 2004. Most measured how much vitamin D people got from food or supplements. A few looked at blood levels of the vitamin, which could come from either food or sunlight.

Ten of the studies found that people who didn't get enough vitamin D had a higher risk of colon cancer. The analysis also suggested that getting 1000 IU (international units) of vitamin D every day would cut colon cancer risk by about 50%.

Their second paper, published in the American Journal of Public Health, reviewed 63 studies on vitamin D and several cancers: colon, breast, prostate, and ovarian. Most of them found a lower risk associated with vitamin D.

The researchers acknowledge that clinical trials are the best way to know for sure how vitamin D affects cancer risk.

"But such studies could take more than 20 years to complete," said Edward D. Gorham, adjunct professor of family and preventive medicine at UCSD and a researcher on both of the studies.

Until then, he and his colleagues believe there's enough evidence to recommend that people take 1000 IU of vitamin D every day. Doing so would only cost about 5 cents per person per day and could prevent several thousand cancer deaths each year, they say.

That amount of vitamin D is considerably more than what the Institute of Medicine currently recommends for bone health, though it's also well short of the upper limit of 2000 IU the IOM says could be dangerous. Too much vitamin D can cause nausea, vomiting, poor appetite, constipation, and weakness. Excess vitamin D can also increase the level of calcium in the blood, causing heart rhythm abnormalities and kidney problems. The IOM recommends 200 IU daily for children and adults up to age 50, and 400 IU for adults aged 51-70; people over age 70 should get 600 IU daily.

McCullough said getting enough vitamin D may indeed turn out to be relevant for preventing some cancers, but in the meantime there are other preventive measures that people can -- and should -- take.

"Avoiding smoking and maintaining a healthy body weight through regular physical exercise and a healthy diet are important ways to reduce the risk of cancer," she said. "Getting screened for cancer is key for prevention -- for instance, colon polyps can be found and removed before they turn into cancer -- or for detecting cancer early, when it can be effectively treated."

The American Cancer Society does not have an official recommendation on vitamin D supplements, McCullough said, though the evidence is being evaluated as the organization revises its nutrition and physical activity guidelines. People who do choose to take a vitamin D supplement should discuss the decision with their doctor, she said, and be very careful to stay below 2000 IU daily.

Citations: "The Role of Vitamin D in Cancer Prevention." Published online Dec. 27, 2005 and in the February, 2006, American Journal of Public Health (Vol. 96, No. 2:9-18). First author: Cedric F. Garland, DrPH, Department of Family and Preventive Medicine, University of California, San Diego.

"Vitamin D and prevention of colorectal cancer." Published in the October, 2005, Journal of Steroid Biochemistry and Molecular Biology (Vol. 97, No.1-2:179-194). First author: Edward D. Gorham, PhD, MPH, Department of Family and Preventive Medicine, University of California, San Diego.