FRIDAY, April 7, 2006 (HealthDay News) -- A range of foods such as soybeans, fruits and green tea contain powerful antioxidants that help reduce a woman's risk for breast and ovarian cancer, new studies find.
Researchers presenting data this week at the American Association for Cancer Research's annual meeting, in Washington, D.C., also found that vitamin D lowers risks for these cancer.
The new studies build on previous research and do look promising, but it's too soon to change dietary advice, said one expert, Dr. Rowan Chlebowski, a medial oncologist at the Los Angeles Biomedical Research Institute at Harbor-UCLA Medical Center in Torrance, Calif.
"None of the four studies warrant a public health change," Chlebowski said. Still, he called all four studies interesting and valuable.
In one study, postmenopausal women who consumed high levels of flavonoids, a class of antioxidants found in plants, had a 45 percent lower risk of breast cancer, said Brian Fink, a doctoral student at the University of North Carolina, Chapel Hill, who led the study.
He and his colleagues evaluated data from a Long Island study conducted in 1996-1997. That effort looked at the health records of 1,434 women with breast cancers and those from 1,440 women without breast cancer. Those with the highest percentage of total flavonoid intake were at a 45 percent lower risk of breast cancer, although this benefit was restricted to post-menopausal women, the researchers added.
"Flavonoids are in fruits, vegetables, tea and coffee," he said. "We're not sure how much is needed to reduce the risk. We don't really know the exact levels."
Exactly how the flavonoids might work to reduce cancer risk isn't certain, either, he said. Antioxidant properties are thought to play a role, and the flavonoids may be "anti-estrogenic, as well," Fink said. Estrogen activity has long been linked to breast cancer.
Another study found that high intake of flavonoids results in a decreased risk of ovarian cancer.
A team from the Harvard School of Public Health analyzed food intake surveys and data on ovarian cancer from more than 66,000 participants in the Harvard Nurses' Health Study. The researchers zeroed in on a flavonoid called kaempferol, found in caffeinated tea, broccoli and kale. Those who had the highest levels of intake of kaempferol had a 38 percent decrease in the incidence of ovarian cancer compared to women with the lowest levels of this flavonoid.
Another study found that women exposed to the highest levels of vitamin D -- which is manufactured by skin after sun exposure -- had a significant reduction in breast cancer risk.
The researchers, from the Samuel Lunenfeld Research Institute at Mount Sinai Hospital, Toronto, Canada, interviewed 576 people who had been diagnosed with breast cancer and 1,135 women without any history of cancer. They asked each woman if she had worked at outdoor jobs, taken part in outdoor activities or consumed high levels of cod liver oil or milk (both sources of vitamin D) while young.
Those who worked outside between the ages of 10 to 19 had a 40 percent reduced risk of breast cancer, and those who spent a lot of time outdoors between ages 10 to 29 had a 35 percent lowered risk, the researchers found.
Those who took cod liver oil from ages 10 to 19 had a 25 percent lower risk, while women who drank at least nine glasses of milk a week between ages 10 to 29 reduced risk by 35 percent.
In another study focused on vitamin D, a University of California, San Diego, team evaluated the results of two studies and confirmed that those with the highest blood levels of a vitamin D metabolite known as 25-hydroxyvitamin D had a 50 percent reduced risk of breast cancer. The two studies included 1783 women.
"The higher the level of vitamin D, the lower the risk of breast cancer," said Cedric Garland, adjunct professor of family and preventive medicine at the University of California, San Diego, and the Moores Cancer Center at the university. Right now, 320 IUs is about average for Americans, according to the researchers, and 2,400 IUs daily is the upper limit thought to be safe.
"Adding 1000 IUs to ordinary daily intake will decrease your risk of breast cancer by about 10 percent," Garland said. Depending on age, an adequate intake of vitamin D ranges from 200 to 600 IUs. Vitamin D3 is the best form, he said.
The UCSD team also conducted a review of many studies published last year and found that high levels of vitamin D can reduce colon, breast and ovarian cancer risk by as much as 50 percent.
To learn more about nutrition, visit the American Dietetic Association.