In recent years, more people have turned to slower-paced workouts -- the so-called "fatburner" classes -- as a way to lose weight. But a growing number of health experts say it's simply a myth that light exercise will melt away the pounds faster than strenuous exercise.

The idea behind these classes sounds reasonable. In the early stages of exercise, before the body is fully revved up, the greatest percentage of calories burned comes from fat. As the workout picks up, the body burns more carbohydrates.

So it would seem logical that a full workout at a low-aerobic pace would burn more fat and take off more weight -- and everyone from "Fit or Fat" author Covert Bailey to the inter-national association that certifies aerobics instructors jumped on the bandwagon.

But this simply isn't so.

If your goal is to lose weight, doctors say, the key is not what kind of fuel the body uses during exercise -- fat or carbohydrates. It is how many calories you burn, and whether you eat enough to replace them.

The only way to take off pounds is to burn more calories than you take in.

"Finding where the fuel is coming from has spiraled into misinformation promoted by aerobics instructors and other professionals in the industry," says Dave Petrie, director of the Human Performance Laboratory of Union Memorial Hospital's Sports Medicine Center. Mr. Petrie has been noticing this trend for about a year. Patients who came to him for fitness evaluations often ranked low in aerobic conditioning despite investing a lot of time in regular exercise. Worse yet, they weren't losing weight.

In talking with patients, he discovered that they had slowed their workouts to a snail's pace to take advantage of fat burning. What they'd really done, he said, was limit their aerobic fitness, burn fewer calories and make it harder -- not easier -- to lose weight.

Reasonable stress from exercise -- that is, raising the heart rate to between 60 and 85 percent of maximal heart rate -- is healthy, Mr. Petrie says.

It strengthens the heart so it pumps harder and delivers more oxygen to muscles and more nutrients to the body. Not to mention that such aerobic conditioning makes people feel great, while it reduces their risks for heart attacks.

Local aerobics instructors, however, defend the "slower is better" approach, especially as a boon to beginners.

'Fatburner' classes

Twenty percent of the aerobics classes at Merritt Fitness Center in Towson are called "fatburner" classes. Kim Thomas, Merritt's director of aerobics, says she finds many people afraid to try aerobics because they can't last for an hour.

"I tell them it's OK to go at a slower pace, because you won't burn out so fast," she says.

Health experts say that is sound advice for beginners. But working out hard for 10 or 15 minutes, then dropping out, doesn't burn many calories of any kind and can be discouraging. Therefore, a bargain must be struck between going long and going fast.

Many newcomers need a breaking-in period. In a sense, they're in training for exercise, gradually tuning body parts to be able to tolerate a more normal level of activity.

Jack Wilmore, Ph.D., professor of kinesiology and health education at the University of Texas at Austin, agrees that a breaking-in period is needed for beginner exercisers. But he also agrees it shouldn't be an end in itself.

He was a member of the 1992 panel of the American College of Sports Medicine and the Centers for Disease Control, which reached a consensus that some people are so sedentary that a total of 30 minutes a day of any kind of activity, even gardening or strolling to the store, would improve their health.

"But that message was not intended for people who have already started," he says. "Even more [exercise] than that will provide considerably more benefits."

The fallacy of the notion that low-impact workouts help lose weight, he notes, is revealed in one fact.

"Sleep! That's when you burn the highest percentage of fat," Dr. Wilmore says.

Slow burning

He adds that slow-paced workouts don't burn many calories, and that even during the early fat-burning stages of exercise the number of grams of fat the body uses up is very low.

The numbers bear this out. Thirty minutes of walking at 3 mph will burn 260 calories, 50 percent (130 calories) from fat, and 50 percent (130 calories) from carbohydrates.

Pick up the pace to 4 mph for that same 30 minutes and the individual can burn 350 calories, 40 percent (140 calories) from fat, and 60 percent (210 calories) from carbohydrates.

"Exercise in and of itself has little effect on weight loss," Dr. Wilmore notes. "It's a part of a total program, but [losing weight] can't be done without altering diet. Either composition or total number of calories have to be altered."

With more intense exercise, the body burns glycogen, the carbohydrate stored in the muscles and liver. After exercise, the carbohydrates from food will refill glycogen stores. If the food doesn't provide enough calories, the body taps into stored fat to make up the difference.

You will be a few ounces lighter every time the body does that. If, on the other hand, you eat enough to meet all your calorie needs, including the fat and carbohydrates burned during exercise, you won't lose weight.

At Baltimore's Downtown Athletic Club, aerobics director Carol Friedman offers no "fatburner" classes and preaches a steady routine of moderately stressful exercise to help keep the body fat ratio steady.

"Body fat loss should be consistent," Ms. Friedman says. "Once we lose it we should be able to maintain a certain ratio of fat to muscle and not fall into the yo-yo syndrome."

She tells people to stay in the moderate range, about 70-75 percent of target heart rate. Always going at an intense pace can cause injuries or burnout.

6* Then people give up and regain weight.

Comfortable pace

"I'm talking about moderate activity to last a lifetime," Ms. Friedman says.

At that comfortable pace, you could work out for an hour a day, five days a week and burn more calories than if you worked out three days a week at a higher intensity.

Dr. Wilmore concurs. "The most important thing is keeping people in a program doing something they enjoy."

But that doesn't mean promoting a snail's pace as the ultimate activity for fitness and weight control. People who are ready for comfortable increases should be encouraged. As you do more, you enjoy it more. You don't have to be afraid to work a little harder.

Ms. Friedman summed it up neatly: "Challenge yourself without wiping yourself out! You'll get the most out of your workout that way."

For aerobic conditioning, heart rate should stay between 60 percent and 85 percent of your maximum exercise rate, based on your age. For a 51-year-old:

Maximum Heart Rate: 220

Minus Your Age: -51

Maximum Exercise Rate: 169

Times Sixty Percent: x.60

Lowest level for aerobic training: 101 heartbeats per 60 seconds

Maximum Exercise Rate: 169

Times Eighty-five Percent: x.85

Highest Level for Aerobic Training: 144 heartbeats per 60 seconds

Dave Petrie