Study eyes soybean link to lower cancer rates

DAYTONA BEACH, Fla. — Soybean products may help prevent breast cancer and a new laboratory test may enable doctors to better choose therapy for breast cancer patients, according to two preliminary studies released this week.

Rats fed a soybean-containing diet showed lower breast-cancer rates in a laboratory experiment, said Stephen Barnes, an associate professor of pharmacology and biochemistry at the University of Alabama at Birmingham.

He tried the experiment after noting that soybeans are prominent in diets of Asian countries that have lower breast-cancer rates than the United States, he said.

But he stressed that his work only studied rats, and his hypothesis is not ready for experiments in people.

“Don’t get too excited yet,” Barnes told science writers at a seminar sponsored by the American Cancer Society.

“I think what we’ve got here is a possibility that needs to be pursued. … People shouldn’t be encouraged to rush into this.”

Samuel Broder, director of the National Cancer Institute, said he found the soybean idea “fascinating,” but agreed that it is not clear whether it applies to people.

“This is just a hypothesis,” he said. “Please do not write headlines that soybeans are good” for preventing breast cancer.

He also cautioned that many factors other than soybeans could explain the low breast-cancer rates in Asian countries.

In Barnes’ study, 300 rats were fed diets in which amounts of total protein and carbohydrates were kept constant but the amount of soybean protein and carbohydrates varied.

After the animals ate the diets for four weeks, they were given a cancer-causing substance. Tumors began to show up after another four weeks or so.

Animals that ate soybean diets showed only about half as many tumors on average as those on the soybean-free diet, Barnes said.

Barnes said his work was financed largely by the cancer society, with no funding from soybean companies.

He said it is not clear what component of soybeans may suppress breast cancer. But he said molecules of one component, called isoflavones, bear a structural resemblance to tamoxifen, a substance used in treating some breast-cancer patients.

Tamoxifen works by blocking a cancer cell’s ability to absorb and use estrogen, which some cells use to fuel growth. Perhaps isoflavones do the same thing, thus suppressing breast cancer, he said.

The other study presented also dealt with the use of estrogen by breast-cancer cells.

Cells take up estrogen by snaring it with proteins on cell surfaces called estrogen receptors. Doctors routinely check breast-cancer tumors to see if they have these receptors. If they do, doctors prescribe an estrogen-blocking agent such as tamoxifen to be used after surgery.

If the breast-cancer tumors do not show estrogen receptors, doctors prescribe chemotherapy instead as a follow-up to surgery.

However, a woman whose tumor contains estrogen receptors may have only a 40 percent to 80 percent chance of being helped by anti-estrogen therapy, said Christopher Benz of the University of California, San Francisco.

Other tests can identify women with higher chances of responding, but most community doctors decide on therapy after just checking for presence of the estrogen receptor, Benz said.