Acetic acid (AcOH), which is a short-chain fatty acid, is reported to have some beneficial effects on metabolism. To test the hypothesis that feeding of AcOH exerts beneficial effects on glucose homeostasis in type 2 diabetes, we fed either a standard diet or one containing 0.3% AcOH to KK-A(y) mice for 8 weeks. Fasting plasma glucose and HbA1c levels were lower in mice fed AcOH for 8 weeks than in control mice. AcOH also reduced the expression of genes involved in gluconeogenesis and lipogenesis, which is in part regulated by 5'-AMP-activated protein kinase (AMPK) in the liver. Finally, sodium acetate, in the form of neutralized AcOH, directly activated AMPK and lowered the expression of genes such as for glucose-6-phosphatase and sterol regulatory element binding protein-1 in rat hepatocytes. These results indicate that the hypoglycemic effect of AcOH might be due to activation of AMPK in the liver.