Cinnamon Increases Liver Glycogen in an Animal Model of Insulin Resistance

The objective of this study was to determine the effects of cinnamon on glycogen synthesis, related gene expression, and protein levels in the muscle and liver using an animal model of insulin resistance, the high-fat/high-fructose (HF/HFr) diet-fed rat. Four groups of 22 male Wistar rats were fed for 12 weeks with (1) HF/HFr diet to induce insulin resistance, (2) HF/HFr diet containing 20 g cinnamon per kilogram of diet, (3) control diet, and (4) control diet containing 20 g cinnamon per kilogram of diet. In the liver, cinnamon added to the HF/HFr diet led to highly significant increases of liver glycogen. There were no significant changes in animals consuming the control diet plus cinnamon. In the liver, cinnamon also counteracted the decreases of the gene expressions due to the consumption of the HF/HFr diet for the insulin receptor, insulin receptor substrates 1 and 2, glucose transporters 1 and 2, and glycogen synthase 1. In muscle, the decreased expressions of these genes by the HF/HFr diet and glucose transporter 4 were also reversed by cinnamon. In addition, the overexpression of glycogen synthase 3β messenger RNA levels and protein observed in the muscle of HF/HFr fed rats was decreased in animals consuming cinnamon. These data demonstrate that, in insulin-resistant rats, cinnamon improves insulin sensitivity and enhances liver glycogen via regulating insulin signaling and glycogen synthesis. Changes due to cinnamon in control animals with normal insulin sensitivity were not significant.