The research, to be presented March 15 at the annual meeting of the American Pain Society in Baltimore, shows that rats with chronic pain resulting from inflammation - similar to the pain experienced by some cancer patients - were more tolerant of painful heat stimuli and had less swelling of the inflamed region when fed a diet based on soy protein.
More than two-thirds of patients with advanced cancer suffer from chronic pain. Managing this is a complex issue for physicians, who struggle to find both the nature of the pain and the most effective treatments. The causes can be a combination of pain resulting from tissue infiltration and inflammation, and neuropathic pain from tumors creeping into a nerve bed. Additionally, when cancer cells spread to bone, they may release chemicals that trigger a painful response. The most effective medications to date have been opioids such as morphine, but the side effects like constipation are so severe that not all patients can tolerate them.
"Our generation is very open to the idea of dietary methods of pain control," says Jill M. Tall, Ph.D., lead author of the study and a research fellow at Hopkins. "We hope to find complementary and alternative treatments to help people suffering from pain."
Researchers studied two groups of 10 rats. For two weeks, the first group consumed a diet based on casein protein (a milk protein found in cheese) while the second group ate a soy protein diet. At random, researchers injected either a placebo or a solution designed to induce inflammatory pain to one of the rats' hind paws.
Paw thickness was measured to assess fluid build-up. Pain tolerance was measured by assessing how long the rats could tolerate a painful heat stimulus before withdrawing the inflamed paw, and how they reacted to varying pressures applied to the paw with a series of nylon filaments. Tests were performed prior to injection and repeated six, 24, 48 and 96 hours after the injection.
Rats on the soy protein diet had significantly less swelling in their paws and a higher tolerance to heat than the casein-fed animals. Diet did not affect the rats' reaction to pressure stimuli. These results are consistent with previous research showing consumption of a soy-containing diet suppressed the development of pain following nerve injury.*1
Further studies will determine if a soy diet can reduce the opioid dosage necessary for treating chronic pain and, therefore, side effects related to the medication.
The study was funded by the National Center for Complementary and Alternative Medicine, part of the National Institutes of Health. Srinivasa N. Raja, M.D., Hopkins' director of pain research, was the co-author.
Poster # 777: Tall, J.M. and S.N. Raja, "Inflammatory pain induced by complete Freund's adjuvant in rats is suppressed by dietary soy."
1* Shir, M.D., Raja, S.N., Weissman, C.S., Campbell, J.N. and D.M.D. Ze'ev Seltzer, "Consumption of soy diet before nerve injury pre-empts the development of neuropathic pain in rats," Anesthesiology (2001) 95: 1238-1244.
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