Future directions in reducing hepatic lipotoxicity

The role of dietary protein on lipotoxicity

Lipotoxicity is a metabolic abnormality frequently observed during the development of obesity and is the main cause of several changes in the metabolic observed during metabolic syndrome. Consistent consumption of diets high in saturated fat or simple carbohydrates combined with low physical activity are the main causes of obesity and its comorbidities. However, the contribution of dietary protein and, in particular, the contribution due to the type of dietary protein, to the process of obesity and its metabolic consequences are less well-understood. In this review, we showed that the type of dietary protein has a significant contribution to the process of lipotoxicity through the modulation of insulin secretion and the regulation of adipocyte metabolic function. Consumption of soy protein stimulates insulin secretion to a lower extent than casein despite the fact that both are high-quality proteins. The amino acid profiles of soy protein and its isoflavones are responsible for the reduced insulin secretion. Also, soy protein increases insulin sensitivity, whereas casein has the opposite effect. Consequently, soy protein reduces SREBP-1 expression in the liver leading to low accumulation of hepatic triglycerides, despite the consumption of a high-fat diet. Furthermore, soy protein reduces adipocyte hypertrophy, hyperleptinemia, and free fatty acid concentration. Thus, the influx of FA into the liver decreases, and hepatic oxidation of FA increases. These metabolic changes result in a decrease in lipid depots and ceramide which reduce hepatic lipotoxicity, whereas casein produces the opposite effect. This study emphasizes that the type of dietary protein has an important effect on lipotoxicity.

The role of dietary protein on lipotoxicity

The worldwide increase in degenerative diseases is in part due to modifications in the lifestyle including the diet. Epidemiological, clinical, and experimental evidence shows that soy protein may prevent lipotoxicity in non-adipose tissues during obesity. The molecular mechanism by which soy protein prevents lipotoxicity involves a reduction in the insulin/glucagon ratio, resulting in a down-regulation of lipogenic genes mediated by the transcription factor sterol regulatory element-binding protein (SREBP)-1, and up-regulation of SREBP-2 to reduce serum cholesterol. In addition, soy protein maintains the functionality of adipose tissue-liver axis to prevent hepatic steatosis during the development of obesity.

Pancreatic Insulin Secretion in Rats Fed a Soy Protein High Fat Diet Depends on the Interaction between the Amino Acid Pattern and Isoflavones

Obesity is frequently associated with the consumption of high carbohydrate/fat diets leading to hyperinsulinemia. We have demonstrated that soy protein (SP) reduces hyperinsulinemia, but it is unclear by which mechanism. Thus, the purpose of the present work was to establish whether SP stimulates insulin secretion to a lower extent and/or reduces insulin resistance, and to understand its molecular mechanism of action in pancreatic islets of rats with diet-induced obesity. Long-term consumption of SP in a high fat (HF) diet significantly decreased serum glucose, free fatty acids, leptin, and the insulin:glucagon ratio compared with animals fed a casein HF diet. Hyperglycemic clamps indicated that SP stimulated insulin secretion to a lower extent despite HF consumption. Furthermore, there was lower pancreatic islet area and insulin, SREBP-1, PPARgamma, and GLUT-2 mRNA abundance in comparison with rats fed the casein HF diet. Euglycemic-hyperinsulinemic clamps showed that the SP diet prevented insulin resistance despite consumption of a HF diet. Incubation of pancreatic islets with isoflavones reduced insulin secretion and expression of PPARgamma. Addition of amino acids resembling the plasma concentration of rats fed casein stimulated insulin secretion; a response that was reduced by the presence of isoflavones, whereas the amino acid pattern resembling the plasma concentration of rats fed SP barely stimulated insulin release. Infusion of isoflavones during the hyperglycemic clamps did not stimulate insulin secretion. Therefore, isoflavones as well as the amino acid pattern seen after SP consumption stimulated insulin secretion to a lower extent, decreasing PPARgamma, GLUT-2, and SREBP-1 expression, and ameliorating hyperinsulinemia observed during obesity.