I do 200 crunches a day but still have a lot of fat around my midsection. Shouldn't they get rid of that? —S.S., by e-mail 

Alas, no. Crunches and other abdominal exercises can improve the appearance of your midsection by firming muscles and pulling fat in a bit. But the only way to lose fat there—or anywhere else—is to burn more calories than you consume, which generally requires doing aerobic exercise such as biking or walking. (Crunches burn a few calories but not enough to get rid of fat.) Aim for at least 30 minutes of aerobic exercise most days of the week, plus twice-weekly strength training for your abdomen. And there’s really no need to do hundreds of crunches. One or two sets of eight to 12 reps work just as well and pose less risk of an injury.