For some time, the buzzword in fitness circles has been intensity. High-intensity workouts, conventional wisdom has it, deliver better results faster. They are the surest route to building muscle, shedding fat, fortifying the heart and lungs, and improving stamina and endurance.

But does intensity merit the hype? And how intense must intensity be before it really makes a difference? Is low-intensity exercise a waste of time? And does high-intensity exercise deliver superior benefits?

Those were questions professor Robert Ross and his colleagues at the Queen's University School of Kinesiology and Health Studies in Kingston, Ontario, wished to explore. The results of their study were published last week in the Annals of Internal Medicine. In some respects, the research confirmed what one might expect; in others, it was surprising.

The study involved 300 sedentary, abdominally obese adults. (In the U.S., an estimated 60 percent of men and 45 percent of women between ages 35 and

65 are abdominally obese, meaning fat concentrated around the waistline). They were divided into four groups and instructed to eat a healthy, balanced diet, neither increasing nor decreasing their caloric intake over time, and to continue their normal daily physical activity.

The control group did no exercise.

The three intervention groups exercised five days a week, in accordance with international fitness guidelines, for the six months of the study.

One of them did a low amount of exercise at low intensity (about 120 heartbeats per minute, or 50 percent of maximum oxygen uptake; the men burned 300 calories per session; the women, 180).

A second group did a high amount of low-intensity exercise (enough for the men to burn 600 calories per session, and the women 360 calories).

A third group did a high amount of high-intensity exercise (140 heartbeats per minute, or 75 percent of maximum oxygen uptake. Again, they exerted themselves enough for the men to burn 600 calories per session; the women, 360).

It took the low-amount, low-intensity exercisers about 30 minutes to complete their workouts. It took the high-amount, low-intensity exercisers about 60 minutes, and it took the high-amount, high-intensity exercisers about 40 minutes. (Because they were exercising harder, it took less time to expend the same energy.)

Current fitness guidelines suggest 75 minutes of weekly high-intensity exercise is equivalent to 150 minutes of weekly low-intensity exercise. The scientists wanted to find out whether that was true - and furthermore, whether high-intensity exercise makes a difference, especially with respect to waist circumference (a measure of abdominal obesity) and glucose tolerance (a measure of how well you manage blood sugar and a predictor of diabetes and cardiovascular disease).

The exercise was performed on a treadmill. The low-intensity exercisers walked at a comfortable pace. The high-intensity group walked briskly and purposefully, with their arms swinging and pumping. Some elevated the grade of the treadmill to 1 percent or 2 percent.

None broke into a jog.

The study participants, though Canadian, were representative of "a large proportion of the American population," said Ross, the lead author. Their average age was 52.

"That's important to note," Ross said, "because all of the participants found the exercise palatable. The high-intensity exercise was high only in relation to the low-intensity exercise. None of the participants was ready to run a marathon or do a triathlon or climb Mount Everest."

What did Ross and his colleagues discover?

Both low-intensity and high-intensity exercisers achieved the same results with respect to reducing waistline (about two inches) and weight (10 to 12 pounds). So intensity didn't matter.

"The good news there is that it gives people who want to reduce their waistlines options as to how hard and how long they want to work," said Ross, 60, a kayaker and cyclist.

When it came to glucose tolerance or blood-sugar management, however, intensity mattered very much. Among the high-intensity exercisers, glucose levels after two hours "came down very nicely," Ross said. Low-intensity exercise did not have the same effect.

High-intensity exercise, not surprisingly, proved to be superior in one other crucial respect - cardiorespiratory fitness, which is a fancy way of saying aerobic capacity, your ability to walk up a flight of stairs without becoming breathless.

Cardiorespiratory fitness is a strong predictor of disease and death, Ross said. Contrary to the guidelines, both the amount and intensity of exercise mattered. All three exercise groups improved their cardiorespiratory fitness, but the improvement was more dramatic among those who were most active.

Declared Ross: "The amount of exercise and intensity of exercise both matter when it comes to improving your fitness level." Optimal strategy: Exercise at a heart rate of 140 beats per minute for 40 minutes a day, four to five times a week.