One of several bioactive factors exerting local control of lactation. FIL is a 10 - 30 kDa protein in the whey protein fraction. It is secreted in the mammary gland and works locally (autocrine) when reaching a concentration threshold. It seems to alter the milk secretory pathway by reversibly affecting the Golgi network and reducing the count of prolactin receptors in the alveoli. In short, when FIL accumulates because a breast is not emptied properly or regularly, it will shut down milk production (Weaver & Hernandez, 2016, WHO, 2009).