The hormone(s) needed to stimulate the increased levels of blood glucose during fasting or sustained exercise, when glucose is in demand for metabolism, is/are _____.

Glucagon is the primary hormone that increases blood glucose levels during fasting, exercise, or stress. It stimulates glycogenolysis (breakdown of glycogen to glucose) in the liver and gluconeogenesis (formation of glucose from non-carbohydrate sources). Insulin lowers blood glucose by promoting glucose uptake, somatostatin inhibits both glucagon and insulin secretion, so 'all of the above' is incorrect.