Beneficial effects of cod protein on inflammatory cell accumulation in rat skeletal muscle after injury are driven by its high levels of arginine, glycine, taurine and lysine.

We have shown that feeding cod protein, which is rich in anti-inflammatory arginine, glycine, and taurine, may beneficially modulate the inflammatory response during recovery following skeletal muscle injury; however it is unknown if these amino acids are responsible for this effect. This study was designed to assess whether supplementing casein with an amino acid mixture composed of arginine, glycine, taurine and lysine, matching their respective levels in cod protein, may account for the anti-inflammatory effect of cod protein. Male Wistar rats were fed isoenergetic diets containing either casein, cod protein, or casein supplemented with L-arginine (0.45%), glycine (0.43%), L-taurine (0.17%) and L-lysine (0.44%) (casein+). After 21 days of ad libitum feeding, one tibialis anterior muscle was injured with 200 µl bupivacaine while the saline-injected contra-lateral tibialis anterior was served as sham. Cod protein and casein+ similarly modulated the inflammation as they decreased COX-2 level at day 2 post-injury (cod protein, p=0.014; casein+, p=0.029) and ED1(+) macrophage density at days 2 (cod protein, p=0.012; casein+, p<0.0001), 5 (cod protein, p=0.001; casein+, p<0.0001) and 14 (cod protein, p<0.0001; casein+, p<0.0001) post-injury, and increased ED2(+) macrophage density at days 5 (cod protein, p<0.0001; casein+, p=0.006), 14 (cod protein, p=0.001; casein+, p<0.002) and 28 (cod protein, p<0.009; casein+, p<0.005) post-injury compared with casein. Furthermore, cod protein up-regulated (p=0.037) whereas casein+ tended to up-regulate (p=0.062) myogenin expression at day 5 post-injury compared with casein. In the cod protein-fed group, these changes resulted in greater muscle mass at days 14 (p=0.002), and 28 (p=0.001) post-injury and larger myofiber cross-sectional area at day 28 post-injury compared with casein (p=0.012). No such effects were observed with casein+. These data indicate that anti-inflammatory actions of cod protein, contrary to its effect on muscle mass recovery, are driven by its high levels of arginine, glycine, taurine and lysine.