How low you can go to lose weight and feel great—plus, a spicy snack we love. BY CYNTHIA SASS, R.D.

A CLIENT RECENTLY TOLD ME SHE WAS TRYING TO LIMIT HER CALORIES TO 1000 A DAY. In her mind, the math was simple: If eating 1500 calories was helping her slim down, slashing her intake by a third would give her even better results. Seems logical, but when it comes to calories, there is a tipping point. When your fuel intake dips below what you need to support the activity of your muscles, bones, and organs, your cells simply can’t function properly (just imagine how less productive your office would be if 30 percent of the employees called in sick!) First, your body will make the shift into conservation mode, causing you to burn fewer calories. Under-eating will also affect your energy level and your looks (even short term, your skin can get dry and flaky, and your hair will fall flat).