like he said its the sugar for the most part which means glucose so not just simple carbs but complex ones too, like the pasta he ate every day as a child. your mouth is like an aquarium. it can tolerate a certain level of exposure to pollutants or sugar but beyond that you will quickly tip the scales in the balance of decay. the proliferation of harmful bacteria has as much to do with gum disease as tooth decay. did you know that your teeth can usually repair themselves if the decay isn't too severe. they can but most people cant realistically implement the diet that would lead to this because it would mean changing their diets from the one they have. one group that is known for their excellent teeth are the agricultural pastorialists of kenya. almost their entire diet is just animals and dairy from the animals. though it was seen in areas where tourists visit and hand out candy, that children from those tribes had started to have dental problems for the first time in their history. any low carb diet like a ketogenic one should give you immunity to dental problems caused by having the wrong kinds of bacteria in your mouth, but im sure you can get away with some carbs if you maintain good dental hygiene. most of all you want to avoid sugary drinks and next stuff like bread. if you want to use a sweetener there's xylitol which actually is good for your teeth, it reduces the harmful bacteria species by a complicated mechanism.

part of why i went on a low carb diet was so my wisdom teeth wouldn't get infected and i wouldn't have to get them pulled. that was a few years ago. before i changed my diet i had one tooth ache or another at all times. when i went to the dentist i always had cavities. i haven't had a toothache in years and even though i go to the dentist less and brush less, i've been free of cavities since i was 17, and im 22 now.