FGF21, or Fibroblast Growth Factor 21, is a protein that plays a crucial role in regulating metabolism, particularly in the liver, adipose tissue, and skeletal muscle. It is involved in processes such as glucose and lipid metabolism, helping the body manage energy balance and respond to fasting or high-fat diets. Research has shown that FGF21 can influence weight loss and improve insulin sensitivity, making it a potential target for treating metabolic disorders like obesity and type 2 diabetes. Its effects on appetite regulation and energy expenditure are also being studied for their implications in overall health and longevity.