I've heard that you shouldn't brush your teeth after eating something acidic.

Not only is what you've heard untrue, but it's the opposite of what you should actually do. "If you've eaten acidic food, you want to get the acid off your teeth as soon as possible," says Richard Lichtenthal, D.D.S., chairman of the department of restorative dentistry at the School of Dental and Oral Surgery at Columbia University. The bacteria that cause tooth decay, explains Lichtenthal, are more active in an acidic environment. "The acidic environment sets up the atmosphere for the demineralization of the teeth." Toothpaste, he says, will buffer, or neutralize, that acidity; brushing your teeth will help remove it. Acidic foods include not only citrus fruits like oranges and grapefruit but also tomato sauce, vinegar, tea, and carbonated beverages.