Short- and Long-Term Soy Diet Versus Casein Protects Liver Steatosis Independent of the Arginine Content

Nonalcoholic fatty liver disease, a major cause of abnormal liver function, is often associated with obesity. Arginine (ARG) plays a role in modulating body weight/fat, but limited data exist as to the role of ARG in soy protein's ability to protect from liver steatosis. We investigated the role of native ARG in the soy protein isolate (SPI) in reducing liver steatosis in male obese Zucker rats. Rats (N=48; 6 weeks old) were randomly assigned to one of three diets for 8 or 16 weeks: the casein (CAS) diet as control (0.6% ARG), CAS diet supplemented to contain 1.3% ARG, or an SPI diet containing isoflavones (1.3% ARG). SPI and ARG rats gained significantly more weight (P<.05) than CAS rats after 16 weeks only. The SPI rats had lower liver steatosis scores after 8 and 16 weeks (P<.05 and P<.001, respectively) compared to CAS and ARG rats. SPI rats had lower serum alanine aminotransferase (ALT) and aspartate aminotransferase (AST) levels (P<.05) compared to CAS after 16 weeks, and AST was lower (P<.05) compared to ARG rats. After 16 weeks, the SPI rats had lower (P<.05) serum ALT and AST levels than at 8 weeks. Our results suggest that a longer period of SPI feeding results in lower liver steatosis and serum ALT and AST levels, while the ARG diet had no effect on steatosis or ALT and AST levels. We found that the SPI diet reduced (P<.001) serum tumor necrosis factor-α (TNF-α) compared to CAS and ARG diets after 8 and 16 weeks. The SPI diet significantly reduced (P<.001) interleukin-6 (IL-6) when compared to the CAS diet at 8 weeks, but there was no significant difference at 16 weeks. Based on the findings of our study, the protective effect of SPI in reducing liver steatosis is not modulated by its native arginine content.