Both are true.
It's obvious that you don't brush your teeth, then eat, because then you're left with the muck of all the food decaying on your teeth.
Yet, immediately after eating, the food itself attacks your teeth. (especially if it's acidic, like orange juice, or cola).
What is the effect of brushing your teeth immediately after covering your teeth in something corrosive?
You effectively are brushing the acid into your teeth, weakening them more than it would otherwise.
The technically correct answer to this, is that you should eat, wait a while (say half an hour at least), then brush your teeth.
But as you can imagine that isn't exactly convenient.
It's obvious that you don't brush your teeth, then eat, because then you're left with the muck of all the food decaying on your teeth.
Yet, immediately after eating, the food itself attacks your teeth. (especially if it's acidic, like orange juice, or cola).
What is the effect of brushing your teeth immediately after covering your teeth in something corrosive?
You effectively are brushing the acid into your teeth, weakening them more than it would otherwise.
The technically correct answer to this, is that you should eat, wait a while (say half an hour at least), then brush your teeth.
But as you can imagine that isn't exactly convenient.