The regulation of energy in humans is highly complex and not fully understood. A number of appetite stimulating or orexigenic peptides, and a number of satiating or anorexigenic peptides have been identified and shown to be involved in this complex process of energy regulation. However, while many of the hormones and signal molecules that play a role in energy metabolism have been identified, new players in energy metabolism are being discovered continually. For example, three peptide hormones have only relatively recently been discovered to play a role in mammalian energy metabolism. These include preptin, adropin and irisin. Preptin is a 34 amino acid peptide synthesised primarily in the beta cells of the pancreas, the same cells that are responsible for insulin release, while adropin is a 43 amino acid peptide synthesised mainly in the liver and brain tissue. Both preptin and adropin appear to be involved in the regulation of carbohydrate, protein and fat metabolism, a function that they perform through the regulation of the hormone insulin.
Irisin is a 112 amino acid peptide synthesised primarily in the heart, kidney, liver and salivary glands. Irisin appears to be an anti-obesogenic and anti-diabetogenic agent because it can convert white to brown adipose tissue. Brown adipose tissue is highly metabolically active and contains high concentrations of uncoupling proteins, and these function to uncouple substrate oxidation from energy production. In this way irisin may increase heat production, and thus waste energy, causing anti-obesity effects. Irisin therefore may function as a thermogenic regulator and in this way be heavily involved in the natural regulation of body weight through the regulation of energy efficiency. In mice at least, irisin is a product of exercise. Exercise causes an increase in the peroxisome proliferator activated receptor gamma (PPAR-γ, and this results in the production of a precursor peptide (FNDC5) that is cleaved to form irisin. Therefore an increase in thermogenesis may be one of the benefits of exercise that helps with the regulation of body weight, through modulation by irisin.