A team of university researchers in the United States has been able to significantly reduce the incidence of breast cancer in mice by giving them high doses of vitamin D.
The results of the experiment, conducted at the Georgetown University Medical Center in Washington and released this week at the annual meeting of the American Association for Cancer Research, bolster the striking finding from the field of human epidemiology that women with low levels of vitamin D tend to have a higher risk of breast cancer, while those with more of the sunshine vitamin get less of the cancer.
The new study found that mice with mammary cancers sensitive to estrogen, the most common type in women, had a twofold reduction in tumour incidence and a fourfold reduction in the growth of their cancers if given vitamin D.
Slower cancer growth was also observed in mice with cancers not sensitive to estrogen, the more dangerous and faster-developing kind in people, although this finding applied only to mice given a low-fat diet to make them lean.
The better outcome observed in mice with cancers sensitive to estrogen could be due to vitamin D's ability to inhibit the action of the female hormone. The vitamin is also absorbed by fat, suggesting that among the obese mice, blood levels may not reach a high enough concentration to have a protective effect.
The research helps to strengthen the case that vitamin D is able to prevent cancers or slow their spread once they develop. But, given that the finding is on laboratory animals, it doesn't shed light on a critical question for women. "What is not clear [is]how much humans should be taking daily," said Leena Hilakivi-Clarke, who led the team and is a professor in Georgetown University's department of oncology.
The experiment is one of the first to look at the effects of high doses of vitamin D on mammary cancers in rodents. Until now, most of the lab research in the area has focused on trying to treat cancers using synthetic, drug-like versions of a hormonal form of vitamin D, rather than the commonly available over-the-counter supplement.
"It does seem to show that if you increase [vitamin D in]a diet, you can impact on tumour development," said JoEllen Welsh, a professor in the department of biomedical sciences at the State University of New York at Albany, who wasn't involved in the research.
She said further research is needed to determine "how much is the minimal amount we need and then how much is too much."
Two major human trials are currently under way in the U.S. - one at Creighton University in Omaha, the other a combined effort at Harvard Medical School and Brigham and Women's Hospital in Boston - using higher doses of vitamin D. The trials may be able to shed more light on its use as a possible cancer treatment and appropriate doses, although it is expected to be years before either reports results.
In the meantime, the Canadian Cancer Society has been recommending that, as a precautionary measure, white people take 1,000 international units of Vitamin D a day during the fall and winter and that non-white people take that amount year-round because they make the nutrient less efficiently in their skin.
There have been about a dozen epidemiologic studies of vitamin D levels and breast-cancer incidence in woman. Most have found an association between the vitamin and a lower cancer risk, according to Cedric Garland, a professor at the Moores Cancer Center at the University of California, San Diego.
These studies suffer from the drawback that they show only associations and do not prove that vitamin D has a cancer-preventive effect.
But Dr. Garland said he believes that the human data are strong enough to suggest the use of vitamin D to reduce cancer incidence. He estimates that women need a total of 6,000 IU daily from sun exposure, diet or supplements to achieve the best cancer-prevention effects.
However, many other researchers are more cautious because drug-style human trials have not been done. Many previously touted nutrients, such as vitamin C and beta carotene, have been found on closer inspection to offer no benefits or even be harmful.
As well, an expert panel at the Institute of Medicine in Washington that studied vitamin D for Health Canada and the U.S. government concluded last November that more research is needed to make absolutely sure that higher doses have no harmful effects. However, it said that taking up to 4,000 IU a day is safe.
The Georgetown research also suggested caution. Among the mice with cancers not sensitive to estrogen that were given both vitamin D and a high-fat diet to make them obese, there were slightly more cancers, but the amount was not deemed statistically significant. "There was no indication of a reduction in risk," Dr. Hilakivi-Clarke said.