We examined effects of dietary soy protein isolate on muscle calpain activity and myosin heavy chain (MHC) degradation in rats performing an acute running exercise.

In rats fed a 20% casein diet, the treadmill running exercise, fixed at 80 kg/m, transiently increased calpain activity in gastrocnemius muscles in parallel with the release of creatine kinase into plasma. The fixed running also caused an accumulation of immunoreactive degradation fragments of MHC in the muscle. Feeding a 20% soy protein isolate diet as opposed to the control casein diet to rats significantly suppressed the running-induced activation of μ- and m-calpains, fragmentation of MHC, and release of creatine kinase into plasma (P < 0.05).

Rats fed the soy protein isolate diet had significantly higher calpastatin activity in gastrocnemius muscle than did rats fed the casein diet (P < 0.05), indicating that this increase ihibits the exercise-induced autoactivation of calpain. Activities of proteasome, cathepsin B + L, and antioxidant enzymes and the levels of glutathione and thiobarbituric acid-reactive substances in the muscle did not differ between the diet groups at the end of the exercise.

Our results suggest that diets containing soy protein prevent exercise-induced protein degradation in skeletal muscle, possibly through inhibiting the calpain-mediated proteolysis.