Glucagon

Glucagon

Glucagon is a 29-amino-acid peptide hormone produced by alpha cells of the pancreatic islets of Langerhans. Its most well-known function is counterregulatory: glucagon raises blood glucose by stimulating hepatic glycogenolysis (the breakdown of glycogen to glucose) and gluconeogenesis (the synthesis of new glucose from non-carbohydrate substrates). In this capacity, glucagon acts as a physiological counterbalance to insulin, preventing hypoglycemia during fasting and between meals.

Beyond glucose regulation, glucagon has significant effects on energy expenditure and lipid metabolism that are central to its role in retatrutide’s mechanism of action. Glucagon receptor activation in the liver promotes fatty acid oxidation and reduces hepatic lipid accumulation, which is particularly relevant to the reduction in liver fat observed in retatrutide clinical trials. Glucagon also increases resting energy expenditure through mechanisms that may involve thermogenesis in brown adipose tissue and activation of futile metabolic cycles. These calorie-burning effects are thought to contribute to the enhanced weight loss seen with retatrutide compared to agents that target only GLP-1 or GLP-1/GIP.

The inclusion of glucagon receptor agonism in retatrutide was initially viewed as counterintuitive, given glucagon’s glucose-raising effects. However, clinical data have shown that the concurrent GLP-1 and GIP receptor agonism in retatrutide provides sufficient insulinotropic and glucose-lowering activity to offset glucagon’s hyperglycemic effects. The net result is improved glycemic control combined with the metabolic benefits of glucagon signaling, including increased energy expenditure and reduced liver fat.

Glucagon receptor agonism is the defining pharmacological feature that distinguishes retatrutide from dual agonists like tirzepatide (GIP/GLP-1) and positions it within a new class of triple-acting metabolic therapeutics.