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

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 casein-fed 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.