The study by Aydin’s group [130] demonstrated that the milk irisin level is not related to the milk maturation stage, being about 550 ng/mL for colostrum, 520 ng/mL for transitional, and 460 ng/mL for mature milk; however, for mothers with GDM, the concentration was significantly lower in colostrum and transitional milk (350 ng/mL and 410 ng/mL, respectively) than in the respective samples of non-GDM mothers. The gene discussed is FNDC5; the disease is gestational diabetes.