Many of us think it's a choice between eating and exercising, but it's not. Eating before exercise is okay--in fact, it can be helpful.

"You'll have a more enjoyable workout and a better workout if you're better fueled. Even dieters need to eat before they exercise," says Nancy Clark, M.S., R.D., author of Nancy Clark's Sports Nutrition Guidebook (Human Kinetics, 2003; www.nancyclarkrd.com) and a sports nutritionist and dietitian in private practice at Healthworks in Chestnut Hill, Mass. "To have breakfast and exercise, or have a snack in the afternoon before going to the gym is very appropriate."

Eating before moderate intensity exercise usually won't cause gastric or other problems. Clark says most people can tolerate consuming 100 to 300 calories within a half hour before exercise--"even five minutes beforehand." Good choices to fill that fuel gap include a banana, granola bar, package of instant oatmeal, 8 ounces of low-fat milk or fruit yogurt.

When you exercise at a comfortable pace, your body easily processes and uses this food. Your blood sugar rises, fueling your brain and giving you the energy to be active. Without food, Clark says, the brain gets no fuel. "Then it says, 'This is not fun.'" That's more likely to defeat your exercise routine.

What about physical activity after, or before, big holiday meals?

Clark advises against "banking calories" by exercising and dieting before the day of a big meal--which some people do to make room for the feast that is to come. That creates strong hunger which can easily lead to significant overeating. Taking a walk after the meal is fine, as an enjoyable family activity for health and fitness.

Do you need to wait an hour or two, or longer, after the last of the pumpkin pie has been cleared away before engaging in moderate-intensity exercise such as walking? "So long as you do what's comfortable, there's no need to wait," Clark says.