This article is interesting. It states that a recet study shows that people who stay slim in their forties are the same people who brush their teeth after every meal.

Their reasoning is, "It's a sign that these people are careful about their health -- they want to maintain the appearance of their teeth and prevent bad breath," the paper said. "We think actively encouraging the habit of toothbrushing would play a role in maintaining health and would help prevent obesity."

But I think maybe it's just because they have a definite demarcation between when they're eating and when they're not eating. Think about it! After you brush your teeth you don't want to wreck your just-brushed freshness by grazing on something. Maybe they just spend so much time being minty-fresh that they don't have time to get fat?