How soon can you start putting food in a breastfed baby's milk?

Why ever????: If you are interested in transitioning baby to solids, they should get the "tutoring" of using their mouth & tongue a different way. It should not be put in a bottle of breast milk. A paste or puree of the material should be offered off a spoon. If baby is not interested, there is no harm in waiting until s/he is. Six months is the time most have lost their tongue thrust & feed easily.