Metformin is effective only in the presence of insulin, and its major effect is to decrease hepatic glucose output [3,4]. In addition, metformin increases insulin-mediated glucose utilization in peripheral tissues (such as muscle and liver), particularly after meals, and has an antilipolytic effect that lowers serum free fatty acid concentrations, thereby reducing substrate availability for gluconeogenesis [3-5]. As a result of the improvement in glycemic control, serum insulin concentrations decline slightly [6,7].
Metformin also increases intestinal glucose utilization via nonoxidative metabolism, at least in experimental animals [4]. The lactate produced by this process is largely metabolized in the liver as a substrate for gluconeogenesis [3]. The latter effect could protect against hypoglycemia.
The molecular mechanisms of metformin action are not fully known. Activation of the enzyme AMP-activated protein kinase (AMK) appears to be the mechanism by which metformin lowers serum lipid and blood glucose concentrations [8-10]. Metformin works through the Peutz-Jeghers protein, LKB1, to regulate AMPK [11]. LKB1 is a tumor suppressor and activation of AMPK through LKB1 may play a role in inhibiting cell growth [12]. (See 'Cancer incidence' below.)