What is the main effect of glucagon in the body?

Glucagon is a hormone produced by the alpha cells of the pancreas and plays a critical role in maintaining glucose homeostasis in the body. Its primary function is to increase blood glucose levels, especially during periods of fasting or low carbohydrate intake. When blood glucose levels fall, glucagon is released into the bloodstream, signaling the liver to break down glycogen stores into glucose through a process called glycogenolysis. Additionally, glucagon stimulates gluconeogenesis, which is the creation of glucose from non-carbohydrate sources. This increase in available glucose is essential for providing energy to the body's cells, particularly the brain, which relies heavily on glucose as its primary energy source.

The other options do not accurately describe the main effect of glucagon. Specifically, decreasing blood glucose levels is the role of insulin, while promoting glucose uptake refers to the action of insulin as well. Inhibiting protein synthesis is not a direct function of glucagon; rather, its main focus is on raising glucose levels in the bloodstream. Therefore, increasing blood glucose levels is the key action of glucagon.