What does glucagon primarily do in the body?

Glucagon primarily increases blood glucose levels, which is essential for maintaining energy balance in the body. It is a hormone produced by the alpha cells of the pancreas and plays a critical role in glucose metabolism. When blood glucose levels drop, such as during fasting or prolonged periods without food, glucagon is released into the bloodstream.

Once released, glucagon promotes gluconeogenesis, which is the process of producing glucose from non-carbohydrate sources, primarily in the liver. It also stimulates glycogenolysis, the breakdown of glycogen (stored glucose) into glucose, thereby increasing the amount of glucose available in the bloodstream. This action is particularly important for maintaining adequate blood sugar levels to provide energy for vital functions, especially in times of need, such as between meals or during physical activity.

In contrast, the other options refer to processes that do not align with glucagon's primary function. For instance, decreasing blood glucose levels is primarily the role of insulin, which is released when blood sugar levels are high. Similarly, stimulating protein synthesis and promoting fat storage are actions associated with insulin and other hormones rather than glucagon. Thus, the role of glucagon as a regulator of blood glucose levels is paramount in understanding its function in energy homeostasis.