Glucose: A sugar used by cells for energy, and the sole fuel for the brain and nervous system, while other tissues can also use fats. It comes from carbohydrate digestion and can be stored as glycogen. Blood glucose is mainly regulated by pancreatic hormones insulin and glucagon, with support from the liver, adrenal, and thyroid glands. High levels are associated with diabetes, stress, metabolic syndrome, and diet; low levels may indicate hypoglycemia, excessive insulin, alcoholism, or liver disease.