Scientists from Johns Hopkins and two Israeli universities have discovered another likely benefit of the much-touted legume, soybeans: They may bring pain relief. A new study shows that laboratory rats fed a diet containing soy meal develop far less pain after nerve injury than their counterparts on soy-free diets.
"In people, strong individual differences exist in the perception of pain," says Hopkins neurosurgeon James N. Campbell, M.D., one of the researchers. "And while this is undoubtedly due to a number of factors, the idea that diet could affect the pain experience offers fascinating possibilities for understanding our vulnerability to it." Enough similarities exist between rats and humans in the biology of pain perception, Campbell says, to make them useful models.
The study, reported initially in a recent issue of the journal Neuroscience Letters, and since supported by additional research, arose from unusual circumstances, Campbell says, when visiting Israeli neuroscientist Yoram Shir of the Hebrew University of Jerusalem came to Hopkins to further study an animal model for pain he'd previously created with collaborators.
Shir's rats were treated to simulate serious painful syndromes that can follow nerve injury in humans. Patients can develop a heightened sensitivity to touch and temperature such that these normally mild stimuli cause pain.
Using anesthetized rats in which the sciatic nerve to one foot is partly severed surgically, the Israeli researchers measured sensitivity to pain by touching the foot with fine probes of varied size.
But when Shir tried to duplicate his model at Hopkins, most of the laboratory rats showed no heightened sensitivity to pain. "We were mystified," says Campbell.
Led by Shir, the research team then systematically tested every possible source of difference in the model, from the strain of rat to possible changes in surgical technique -- to the type of rat chow used.
The conclusion: It was the rat chow. The American chow had higher proportions of soy protein than the Israeli chow. When the researchers returned soy-fed rats to a soy-free diet, the rats' sensitivity to pain also returned. "These studies strongly indicate that diet is an important factor in the expression of nerve-injury pain in rats," says Shir.
At this point, the scientists don't know what component of soy meal suppresses sensitivity or what the mechanism could be. Certain soy proteins may hamper the way cells relay messages internally, Campbell says, or phytoestrogen plant hormones might be a culprit. "I suspect answering these questions will give birth to a whole series of new experiments," he adds, "that could help explain why pain varies in people, and help us develop new therapies."
Models such as Shir's have recently sparked a quiet revolution in scientists' ideas on pain: namely, that pain comes in a variety of forms, each with a specific mechanism in the body. "The concept explains why aspirin isn't particularly useful for pain from nerve injury, for example, but works for inflammation," says Campbell.
Scientists have recently shown that soybeans can lower cholesterol in people. The vegetable is a source of omega-3 fatty acids and calcium and supplies most of the essential amino acids people need to make proteins. Soybeans also may lessen some symptoms of menopause, though studies on that remain to be done.
All of the rat studies met NIH guidelines and have been approved by institutional review boards of both institutions. The work was supported by the USA-Israel Binational Science Foundation and by the Hebrew University Center for Research on Pain.
Other researchers were Srinivasa Raja, M.D., of Hopkins and Alexander Ratner and Ze'ev Seltzer of Hebrew University, Jerusalem.
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