Diurnal variation in the glycogen content of the human liver using ¹³ C MRS

Glycogen in tissues functions not only as carbohydrate reserves, but also as molecular sensors capable of activating signaling pathways in response to physical activity. While glycogen in the skeletal muscles is mainly a local energy substrate, glycogen in the liver serves as a glucose reserve to maintain normal blood glucose levels in the body, even during the sleep state. The aim of this study is to compare the diurnal variation of glycogen in the muscle and liver of human subjects under normal conditions. The glycogen content was measured in the muscle and liver of 10 young, healthy, male volunteers using ¹³ C MRS, a non-invasive technique. The subjects remained sedentary, and glycogen concentration was measured six times daily. Experimental meals were provided to achieve individual energy balance, estimated according to the energy requirement guideline for patients from Japan. The largest variation in muscle glycogen compared with 1 h after supper (20:00 on Day 1) was 3.1 ± 8.2 mmol/L (16:00 on Day 2). In the liver, however, the glycogen content decreased by 80.6 ± 40.4 mmol/L through the overnight fasting period (07:00 on Day 2). This study demonstrated that the glycogen content in the liver was significantly lower in the morning, while the glycogen content in the calf muscles underwent minimal diurnal variation. The overnight fast is a characteristic daily condition, in which liver glycogen content is low, whereas muscle glycogen content is relatively unaffected.