Which hormone promotes the breakdown of protein in muscle cells?

Cortisol is the hormone that promotes the breakdown of protein in muscle cells. It is a glucocorticoid hormone produced by the adrenal glands in response to stress. When the body is under stress, cortisol levels increase, prompting various metabolic processes, including gluconeogenesis and the breakdown of muscle protein. This catabolic effect allows the body to mobilize amino acids from muscle tissue to be used for energy production or to synthesize glucose, especially during prolonged stress or fasting situations.

Testosterone, while important for muscle growth and repair, generally promotes an anabolic environment, fostering protein synthesis rather than breakdown. Insulin is primarily known for facilitating glucose uptake and promoting anabolic processes, such as protein synthesis within muscle cells, thus serving to preserve muscle mass. Adrenaline, also known as epinephrine, is involved in the body's 'fight or flight' response and helps release energy reserves but does not directly promote protein breakdown in muscle cells like cortisol does. Understanding the distinct roles of these hormones is essential for grasping how they influence muscle metabolism and overall body composition.