When you look at the chemical structure of breast milk, it will change as the baby grows. At first, it will be a thick, fat milk, full to the brim with nutrients and other things the baby needs to survive their first weeks. You could say that it's a health boost that the mother gives her baby. Later, it will become a thinner milk, that satisfy the baby's hunger. The formulas is a imitation of this late-stage kind of milk, which means that the babies that only get feed formula misses out on the natural health boost.