June 17, 2003 -- The "miso" in Japanese miso soup is a paste made from fermented soybeans -- and it may be a powerful weapon against breast cancer.

Researchers have long pointed to soy products as possibly having a protective effect since women in Japan and other Asian cultures have much lower breast cancer rates than American women.

Soy is an important part of the Asian diet. Isoflavones -- naturally occurring chemicals found in abundance in soy products -- are believed to block the growth of certain tumors.

To further investigate this connection, a group of Japanese researchers has conducted possibly the largest study thus far of soy, isoflavones, and breast cancer.

The study -- led by Seiichiro Yamamoto, PhD, an epidemiologist with the National Cancer Center Research Institute in Tokyo, Japan, appears in this week's issue of the Journal of the National Cancer Institute.

The Evidence

In it, Yamamoto and colleagues identified 21,852 women between the ages of 40 and 59 and surveyed them about various lifestyle issues, including their diet.

They traced the women 10 years later and found that 179 had been diagnosed with breast cancer. Again, they surveyed all the women about their diet -- how frequently they ate miso soup and soy foods such as soybeans and tofu. The researchers then calculated the amount of isoflavones consumed.

Researchers found that:

• 75% of the women consumed miso soup almost daily; of those, 23% had one cup per day, 43% had two, and 34% had three or more per day.
• 45% had soy foods almost daily, 35% ate soy foods three or four times a week, 17% had it one or two times a week, 2% almost never ate soy foods.
• Women with the most isoflavones in their diet had the lowest risk of breast cancer. This was especially true of postmenopausal women.
• Eating soy foods in general, however, did not lower the risk of breast cancer.
• Other traditional eating habits -- eating more rice, pickles, vegetables, and fish -- were also linked with lower breast cancer risk.
• Even those who drank the least miso soup still got 250 times more isoflavones in their diet than does the typical U.S. woman.

It's more evidence that the traditional Japanese diet and lifestyle protect women from breast cancer -- they didn't smoke, they got regular exercise, they maintained a "normal" weight, says Mark Cline, DVM, PhD, with the Soy Research Group at Wake Forest University School of Medicine in Winston-Salem, N.C.

Another study conducted in Shanghai, China, found that women in urban areas tended to have higher breast cancer risk -- they had drifted from the traditional ways, Cline tells WebMD.

Cline's own studies have compared the effects of isoflavone supplements and soy foods in reducing women's hot flashes. Soy foods worked the best, he tells WebMD. "The best thing to do is eat soy products, not go seeking a pharmaceutical, purified compound."

The only caveat: Some 30% of people don't absorb isoflavones because of their intestinal bacteria, Cline says.

Studies analyzing the amount of isoflavones in women's blood have shown "the most profound, protective effects" from soy products, he says.