WASHINGTON (Reuters) - A diet rich in soy that reduced pain and swelling in rats could one day be used by humans to manage chronic pain in diseases such as cancer, US researchers said on Friday.

Scientists at Johns Hopkins University in Baltimore said in a study that rats on a soy-based diet experienced "significantly less" swelling and were able to tolerate more pain than another test group given a milk protein.

"I was skeptical to say the least when I began my work in the lab looking at complementary and alternative therapies for pain," Jill Tall of Johns Hopkins, co-author of the study, said in a telephone interview.

"But for humans in the long run, this is going to offer some additional therapies" in managing pain and inflammation, she said.

Managing chronic pain is difficult for many patients using existing treatments. The most effective medication given to patients to control pain is morphine, but often side effects such as constipation are too severe to continue.

The research was to be presented at the annual meeting of the American Pain Society in Baltimore on Friday.

Researchers gave 10 rats a soy protein diet and another 10 a milk protein. Rats were then randomly given either a placebo or a solution that would cause inflammation in the rat's hind paw.

Tall and her colleagues measured paw thickness to gauge fluid build up. Pain tolerance was measured by assessing how long rats could tolerate pressure and heat stimulus before removing their paw from the heat source.

Rats given the soy protein had "significantly less" swelling in their paws and were able to withstand more pain for a longer period of time than the milk protein group.

Tall said additional research will be necessary to determine if the soy protein works in humans, and, if it is effective, why it causes inflammation to recede and pain tolerance to increase.

A standard dietary supplement also would have to be designed because existing over-the-counter products do not all contain the same type of soy protein, Tall said. The researchers did not specify which type was used in the study or the amount given.

"Unless there is a standard way of distributing these dietary supplements...you may not have consistent beneficial effects," she said.

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