MRS21 studies of glucose disposal in normal humans suggested that skeletal muscle accounts for the majority of insulin-stimulated glucose uptake and that >80% of this glucose is then stored as glycogen.22 The rate of glycogen synthesis in skeletal muscle was ≈50% lower in diabetic subjects than in normal volunteers.22 The only other organ capable of storing a significant amount of glycogen is the liver, and here again, glycogen stores were reduced in diabetics.21,23 Subsequent studies focused on the rate-controlling steps in this pathway. 13C and 31P MRS were used together to monitor intracellular glucose-6-phosphate concentration and intramuscular glycogen synthesis during hyperinsulinemic-hyperglycemic clamps.24 Glucose-6-phosphate is an intermediate between glucose transport into the cell and its subsequent phosphorylation by hexokinase and glycogen synthesis (Figure 1). The increment in glucose-6-phosphate concentration was significantly reduced in type 2 diabetics, suggesting that glucose transport or phosphorylation must be the rate-controlling step in insulin-stimulated glucose disposal in skeletal muscle rather than glycogen synthase.24 Similar observations were also made in insulin-resistant offspring of type 2 diabetics,25 suggesting that this defect precedes the development of type 2 diabetes. Glucose transport in skeletal muscle is largely mediated by a specific insulin-responsive transporter known as glucose transporter 4 (GLUT4), whereas glucose phosphorylation is catalyzed by hexokinase. To determine which of these 2 steps was defective, we used a novel 13C MRS method to assess intracellular-free glucose in muscle,26 the idea being that if hexokinase were rate controlling in insulin-resistant type 2 diabetics, intracellular glucose concentrations should increase substantially. The fact that intracellular glucose concentrations in skeletal muscle from type 2 diabetics (during a hyperinsulinemic-hyperglycemic clamp) were 1/25 what they would have been if hexokinase were the primary rate-controlling enzyme suggested that glucose transport was rate controlling as opposed to hexokinase.26 Together, these data indicate that glucose transport into muscle is the rate-controlling step for insulin-stimulated muscle glycogen synthesis in patients with type 2 diabetes.