In addition, FNDC5/irisin functionally interacts with Fibroblast Growth Factor 21 (FGF21), another metabolic regulator, and bioinformatic studies have identified polymorphisms in FNDC5 that could affect this interaction, influencing browning efficiency and susceptibility to metabolic disorders such as obesity and type 2 diabetes [137]. Here, FGF21 is linked to metabolic disease.