Effects of soy diet on inflammation‐induced primary and secondary hyperalgesia in rat⋆

Abstract

Soy consumption is said to prevent or treat atherosclerosis, cancer, pain, and memory deficits, but experimental and clinical evidence to support these claims are lacking. We used in vivo models of inflammation to determine whether a soy diet reduces primary or secondary hyperalgesia. In all three experiments, rats were fed either a soy- or casein-based diet for at least 2 weeks before induction of inflammation and for the duration of experiments. Mechanical and heat paw withdrawal thresholds and edema were measured before and several times after induction of inflammation. Primary hyperalgesia was assessed in two models: unilateral intraplantar injection with 0.1 ml of 25% complete Freund's adjuvant (CFA) or 0.1 ml of 1% carrageenan. Unilateral injection of the intra-articular knee space with 25% CFA (0.1 ml) was used to determine the effects of soy in a model of secondary hyperalgesia. Following intraplantar injection of CFA, soy-fed animals exhibited significantly less paw edema, mechanical allodynia, and heat hyperalgesia compared to casein-fed animals. In the carrageenan model of paw inflammation, soy-fed animals were also less allodynic to mechanical stimuli, than were caseinfed animals, but showed no diet based differences in paw edema or heat hyperalgesia. Soy diet did not affect any of the outcome measures after the intra-articular injection of CFA. Our results suggest that a soy diet significantly decreases aspects of inflammation-induced primary, but not secondary, hyperalgesia in rats.

Evidence for the Effectiveness of Soy in Aging and Improving Quality of Life

“…An animal study by Borzan et al [93] also supports our clinical findings on soy. The aim of the aforementioned study was to determine if a soy diet could reduce the pain behaviors and inflammation induced by the intraplantar administration of complete Freund's adjuvant.…”

Laboratory environmental factors and pain behavior: the relevance of unknown unknowns to reproducibility and translation

“…Failure to provide bedding material for a rat to make a nest for her pups can lead to hyperalgesia in those pups when they become adults 55,56 , and the handling of young animals (for example, related to injections, and involving the stress of maternal separation) can cause long-term alterations in pain sensitivity 41,[57][58][59][60][61][62][63] . A growing literature also documents the effect of diet on pain sensitivity [64][65][66][67][68][69][70][71][72][73][74][75][76][77][78][79][80] . In one case, differences in the soy content of rat chow turned out to be wholly responsible for a failure to replicate in Baltimore the findings of studies originally performed in Jerusalem 65 .…”

Dietary Influence on Pain via the Immune System

“…2) or carrageenan. 86 In mice, soy diets reduced NFκB activity by decreasing the degradation of IkBα following LPS and reduced blood levels of IL-1β, Il-6, and TNFα. 87 In humans, there has been very little done to examine the potential of soy to alleviate pain.…”

“…Soy diet significantly reduced the peak of the hyperalgesic response to CFA. Adapted from Ref 86. with permission.…”