Feedback Inhibitor of Lactation (FIL) is the whey protein found in breasts that plays a role in the demand and supply process of breastfeeding. When a woman's breasts are full or engorged, they contain more FIL than when they are empty. Repeated instances of engorgement or not emptying breasts can lead to a drop in breast milk supply over time.
To learn more about FIL and its effect on lactation, visit Kellymom, Nurshable, or Low Milk Supply.