It’s important to understand that what your baby is really doing by feeding frequently during a growth spurt is to stimulate your breasts to produce the milk necessary for a baby who, at the end of the growth spurt, is going to be bigger and require more nutrition. When that process of programming the breasts is interrupted by supplementing formula or previously expressed milk, it can cause milk production problems in the future.
SOurce: http://www.breastfeedinghousecalls.com/programming-your-milk-supply.html
Mary Grace Quirimit