The physiological effects of dietary β-conglycinin (β-CON), one of the major components of soy protein (SOY), were examined in an obese animal model. Prior studies show that β-CON intake decreases plasma triglycerides and visceral adipose tissue weight, and increases plasma adiponectin in rodents. Since plasma adiponectin is known to affect both lipid and glucose metabolism, feeding a diet containing β-CON could modulate insulin sensitivity. Therefore, we examined the effects of dietary β-CON on insulin sensitivity and blood glucose levels, as well as lipid metabolism in obese Otsuka Long-Evans Tokushima Fatty (OLETF) rats (pre-symptomatic stage of type 2 diabetes mellitus). Male OLETF rats (6 weeks old) were fed diets containing 20% protein such as casein (CAS), CAS replaced with soy protein (SOY), or β-CON at a proportion of 50% for 13 weeks. Fasting blood glucose levels were measured every 3 weeks, and an insulin tolerance test (ITT; 0.75 IU/kg body weight) was conducted at week 12. During the feeding period, fasting blood glucose was comparable among the groups. Insulin sensitivity measured by the ITT revealed that the SOY and β-CON diets decreased blood glucose levels at 30 min after intraperitoneal insulin injection (vs. CAS diet). In addition, the β-CON diet increased plasma adiponectin concentrations, hepatic gene expression of insulin receptor substrate (IRS) 2, and muscle gene expression of adiponectin receptor 1 (AdipoR1) and IRS1, and with a decrease in plasma insulin concentration. Finally, the β-CON diet decreased the mesenteric adipose tissue weight and liver triglyceride concentration compared to the CAS diet. These results suggest that the metabolic effects of dietary β-CON are mediated by increasing plasma adiponectin to increase insulin sensitivity and influence the hepatic lipid metabolism in obese OLETF rats.
Keywords: Otsuka Long-Evans Tokushima Fatty rats; adiponectin; adipose tissue; insulin sensitivity; β-conglycinin.