What's the problem with running (or spinning, or stair-climbing, or Zumba-ing) on empty? We asked Heidi Skolnik, MS, CDN, FACSM, a nutritionist with a private practice who's worked with dancers at the Juilliard School as well as players with the New York Knicks. First, Skolnik wants to know why you didn't have anything to eat. Saving calories?: "You'll probably be so hungry later that you'll eat even more," she says, adding that she sees this over and over with her female clients. Think of this snack as fuel: Skolnik says research supports the idea that having something in your tank will help you work out harder, which will then help you burn even more calories. Worried that it's bad for your body?: Skolnik explains that working out immediately after a big meal will cause blood to be diverted to your muscles instead of your digestive system. But while she agrees this can be uncomfortable, she says it's not physically harmful. Too busy? Skolnik gets that, but she strongly advises against making it a regular habit. You'll be too weak and hungry to get the maximum benefit from your workout, and you'll be setting yourself up for a binge session later that night. And if your workout involves strength-training, your could go into catabolic mode and start breaking down muscle. Isn't that enough motivation to stock your gym bag with energy bars?
Best advice: Eating too much will probably result in an unpleasant workout, but not eating anything can be bad for your workout, your diet—and—your long-term health.