More than 18 million people in the United States have diabetes mellitus, and about 90% of these have the type 2 form of the disease. In addition, between 17 and 40 million people have insulin resistance, impaired glucose tolerance, or the cluster of abnormalities referred to variably as the metabolic syndrome, the dysmetabolic syndrome, syndrome X, or the insulin resistance syndrome. In all of these disorders, a central component of the pathophysiology is insulin resistance, i.e., reduced responsiveness to insulin in tissues such as muscle, fat and liver. Insulin resistance is also closely...