If you didn't eat a decent breakfast (say, muesli, fruit and yogurt, or oatmeal with nuts), you'll need a bite of something—not because you can't get through the class but because waiting until you get back to your desk will make you so hungry that you'll probably overeat, says Rosenbloom.
The snack: A few whole-grain crackers (again, 30 grams is the magic number of carbs) and maybe a piece of string cheese. The key to not letting your snack sabotage your weight is to count these bites toward your lunch. So if you opt for cheese now, skip piling it on your turkey sandwich.
When: 20 minutes before hitting the mat.