I've often wondered if I burn more fat during a high-intensity 45-minute run or a long day of hiking. And judging from e-mails and questions during our Moving Crew online chats, many of you are similarly baffled.

The confusion is due largely to those cardio machines that report when your heart rate enters -- and exits -- that most coveted of spots, the "fat burning zone." Oddly, though, this is a lower level of exertion than you need in order to enter the "cardio zone." So, Mr. Cardio Machine, I burn more fat by working out less hard?

No. Let's settle this once and for all:

If you're trying to manage your weight, you should focus on how many calories you burn, not on what kind of fuel you're burning. In a given period of time you'll burn more calories by working out at a higher intensity (i.e., in the "cardio zone") than at a lower intensity (in the "fat-burning" zone).

Okay, the details: The whole mess stems from the fact that our bodies use different energy sources -- fat, carbohydrate, glycogen -- at different levels of exertion.

When you're working harder, explains Karyn Esser, associate professor in the Department of Physiology at the University of Kentucky in Lexington, your body burns a mix of fuel that's higher in carbohydrate and glycogen and lower in fat. At lower levels of exertion, your body mainly draws on fat stores to feed itself.

Esser, who e-mailed us after a recent online chat to express dismay that so many still misunderstand the concept, said that even when you're working out at higher levels of intensity, "you are still burning fat."

"If you start [exercising] at 50 percent of your maximum heart rate and go up to 75 percent and then all the way up to 90 percent, you do not decrease the quantity of fat burned over time."

Overall, Esser said, if your goal is to lose body mass, "it makes no darn difference . . . what intensity you exercise at" as long as you burn more calories than you consume.

So does this mean you can manage your weight just as effectively by exercising at those lower (i.e., fat-burning) exertion levels?

Yes, but you have to exercise longer. If you walk, hike, jog or sprint, for instance, the equation is simple: A 100-pound person burns 100 calories per mile on a flat surface; a 200-pound person exhausts 200 calories; a 300-pounder, 300 calories, etc. This is (who knew?) a constant equation: For each one-pound increase in weight, the body burns one more calorie per mile.

"It doesn't matter how long it takes you to cover that distance," said Esser. If you take your time, fine. If you cover that mile faster, fine. Either way, the calorie expenditure is the same. And if you want to burn more calories, cover more ground -- at whatever pace you like.

Esser says a moderately active lifestyle will help control body fat, too. "The time you exercise is only a small portion of your day," she said. By making minor changes in what you do the rest of the time, like taking the stairs instead of an escalator, you can use more calories during the non-workout portions of your day.

The little changes "won't totally change your life and allow you to eat Twinkies and Ding Dongs all day, but they'll help," Esser said.

Step up to the chat room Thursday from 11 a.m. to noon for the Moving Crew online: www.washingtonpost.com/wp-dyn/liveonline/health/movingcrew.

-- John Briley