Breastfeeding is in no way causing cavities, unless it is somehow heated up or microwaved before feeding. My 2 year old was in this exact situation: breastfed (straight from breast) at night and getting white staining/lines on her teeth. I was stumped and distraught.

We unfortunately had to transition away from breastfeeding and started her on raw cows milk. I started giving her a drop or two of Thorne Research MK-4 in her milk every night, along with daily doses of Fermented Cod liver oil and these stains went away and her teeth are immaculate. Vitamin K2 is vital to dental health, and if you aren't getting enough in your diet, your little one isn't getting enough either.

To summarize: MK-4 and fermented cod liver oil. By the way, I was able to get my little one to take the cod liver oil by syringing about 1/4 tsp and then filling the rest of the syringe with orange juice. She asks for it now every night.