The study, published in Cancer Research, found a chemical in soy -- genistein -- decreased metastasis of prostate cancer to the lungs by 96 percent in mice implanted with an aggressive form of prostate cancer. The amount of genistein in the blood of the animals was comparable to human blood concentrations after consumption of soy foods.

The researchers found that while genistein didn't reduce the size of tumors that developed within the prostate, it stopped lung metastasis almost completely. The researchers repeated the experiment and found the same result.

"These impressive results give us hope that genistein might show some effect in preventing the spread of prostate cancer in patients," study senior investigator, Dr. Raymond Bergan of Northwestern University said in a statement.

"Diet can affect cancer and it doesn't do it by magic -- certain chemicals have beneficial effects and now we have all the preclinical studies we need to suggest genistein might be a very promising chemopreventive drug."