Sizzling - exercising at a high intensity - is an attractive idea, since it chews up calories at at lofty rates. On the other hand, sauntering - training at a low intensity - doesn't seem so bad, either, since it permits longer workouts and increases the proportion of fat in the 'fuel mix' used by your muscles for energy. Low-to-moderate intensity workouts are also safer for individuals with potential cardiovascular problems. However, scientific research has been unable to establish firmly which training intensity is best for fat loss.

To establish the actual fat-trimming benefits of highversus low-intensity workouts, scientists at Laval University in Quebec recently asked two groups of people to participate in different exercise programmes. Seventeen subjects (eight men and nine women) trained on an exercise bicycle four to five times per week for 20 weeks. The workouts lasted from 30 to 45 minutes, and exercise intensity ranged from 60-85 per cent of maximal heart rate.

During a 15-week period, a second group of 10 subjects (five men and five women) completed 25 30-minute workouts at an intensity comparable to that attained by the first group. However, the second group also conducted 19 short-interval and 16 longinterval sessions during the 15-week programme. The short-interval sessions consisted of 10 to 15 intervals lasting for 15-30 seconds, while the long-interval efforts were composed of four to five intervals with a duration of 60-90 seconds. Recovery periods between intervals allowed heart beats to drop to 120-130 beats per minute, and the actual exercise intensity within intervals gradually reached 90-95 per cent of maximal as the training programme progressed.

Because the first group worked out for a longer time period (20 against 15 weeks), conducted more total workouts (90 against 60 sessions) and completed longer individual training sessions, total energy expenditure during training was twice as great in the first group as in the second group. However, each group achieved about a 30-per-cent increase in maximal aerobic capacity. Most surprisingly, the second group (the interval-trained athletes who performed less total work) had a NINEFOLD greater loss of body fat than the first group.

Why did interval work lead to larger losses of lard? The intense cycling efforts may have produced a slightly greater gain in muscle mass in the interval athletes. Since muscle tissue ordinarily has a huge appetite for fat, this can have positive consequences for fat loss. Interval efforts may also have aggrandized 'PET' - the post-exercise thermogenesis, or increased burning of calories which takes place after every workout. High-intensity exertions seem to make PET larger. It's also possible that interval training specifically enhances fat metabolism during this PET period, an effect which could have contributed to greater fat loss in the high-intensity subjects.

Some studies have also suggested that athletes' appetites increase to a smaller extent following high-intensity workouts than after easy exertions. And it's also possible that hard workouts do a better job of promoting the release of human growth hormone (HGH) from the pituitary gland. HGH is a potent stimulator of fat metabolism.

Although the Laval researchers weren't certain about the exact fat-loss mechanisms associated with interval workouts, they concluded that fat loss is greater when exercise intensity is high. Individuals who want to lose weight - and who are free from cardiovascular disease - should profit from a step-up in their workout intensities. ('lmpact of Exercise Intensity on Body Fatness and Skeletal Muscle Metabolism, ' Metabolism, vol. 43(7), pp 814-818, 1994)