Some research suggests that breastfeeding may actually protect against tooth decay, while formula may play a role in its development.
Antibodies in breastmilk may help to reduce the growth of bacteria (including Streptococcus mutans, the bacterium that causes tooth decay). Lactoferrin, a protein in breastmilk, actually kills S. mutans.
Some formulas dissolve tooth enamel, significantly reduce pH (make it acidic), support the growth of bacteria and can lead to tooth decay.