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I can't understand how it would make sense to brush before rather than after meals. What's the logic behind that?

This is doubly so if the meal is sugary. By not brushing afterward, you're just letting bacteria process all that sugar and produce plaque and acid.

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My dentist has advised me not to brush after a sugary meal or drink for at least an hour, otherwise you are effectively brushing the sugar into your teeth. Perhaps this also applies (but with a lesser timeframe) for meals that aren't so sugary.

It's more that your enamel is weakened temporarily by acids (which get produced by bacteria in your mouth as a side effect of their consuming the sugar). It is true that you shouldn't brush immediately after a meal -- waiting 30 minutes to an hour is a good practice.

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