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Background Leucine has unique anabolic properties, serving as a nutrient signal that stimulates muscle protein synthesis. Objective We tested whether the leucine concentration is the only factor determining protein quality for muscle development. Methods We selected three dietary proteins: casein (CAS), egg white protein (EWP), and albumin (ALB), representing the leucine concentrations of approximately 8.3, 7.7, and 6.7% of the total protein (w/w), respectively. In the chronic feeding experiment, these proteins were pair-fed to growing male Wistar rats (110-135 g body weight [BW]) for 14 days as a protein source, providing 10% of total energy intake, after which, soleus and extensor digitorum longus (EDL) muscles were excised to estimate muscle growth. In the acute administration experiment, we injected CAS, ALB, and EWP to rats by oral gavage (0.3 g protein/100 g BW), and after 1 or 3 h, EDL muscle was excised for capillary electrophoresis-mass spectrometry-based metabolomics. In another chronic feeding experiment, rats were pair-fed either CAS or CAS diet supplemented with arginine to the same level as in the EWP diet for 14 days. Results At the end of the 14-day feeding, soleus and EDL muscle weight was 20% and 17% higher, when rats were fed EWP as compared with CAS (p Conclusions EWP promotes rat developmental muscle growth compared to CAS, which can be partly explained by the arginine-rich EWP. Lay summary Leucine has been shown to have unique anabolic properties as a nutrient signal that stimulates muscle protein synthesis. Therefore, we hypothesized that the growth-promoting effect of dietary protein is dependent on the leucine concentrations. To test this hypothesis, we selected three dietary proteins: casein (CAS), egg white protein (EWP), and albumin (ALB) representing the leucine concentrations of approximately 8.3, 7.7, and 6.7% of protein (w/w), respectively. These proteins were chronically fed to growing rats as a protein source of diet for 14-day. As result, the muscle growth was the highest when the rats were fed the EWP diet, followed, in order, by the ALB and CAS diets. Thus, leucine may not be the only factor determining protein quality for muscle development. Meanwhile, the changes in muscle arginine concentration following acute EWP, ALB, and CAS administration paralleled those of muscle growth. Moreover, chronic supplementation of arginine to the CAS diet partly mimicked the EWP-induced muscle growth effect, indicating that arginine derived from EWP plays a significant role in the promotion of muscle anabolism. Translation of these results may allow for improved muscle growth in mammals fed EWP as a dietary protein source. |
