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Just three, 20-minute sessions a week of interval sprint training on a stationary bicycle has proved to be a quick, effective way to reduce visceral fat and to trim centimetres off the waist, University of NSW exercise scientists report.

Bursts of sprint pedalling for eight seconds, followed by at-ease pedalling for 12 seconds, shifted 3.5cm from the waists of overweight men in six weeks, said study leader Stephen Boutcher, from the university's department of Pharmacology and Medicine.

Associate Professor Boutcher and colleagues, who have worked for years to develop the most time-efficient way to exercise for maximal health benefits think the interval training works by confusing the body into releasing high levels of hormones needed to meet the metabolic needs of the sprinting effort - effectively tricking it into becoming very inefficient and burning up lots of fat.

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"Sprints are a very time-efficient form of exercise," Professor Boutcher said.

"We think the sprints are working very quickly to reduce visceral fat, probably within six weeks."

Other studies using aerobic exercise such as continuous jogging have found that to achieve a similar decrease in visceral fat required one hour of exercise, seven days a week for 14 weeks, he said.

"The sprint program reduced visceral fat with seven times less exercise time and has a much greater impact on cardiovascular health and metabolic health," Professor Boutcher said.

The reduced waist girth in the men studied - 20 men in their 20s with an average weight of 88kg and BMI of 28 - correlated with a significant reduction in the fat coating their internal organs, assessed by abdominal CT scans done before and after the 12-week study. The subjects also increased their trunk and leg muscle mass by 1.2kg and showed significant improvements in their aerobic fitness.

A previous, 15-week study of women by the same researchers, that compared 40 minutes of cycling with 20 minutes of interval sprinting also resulted in the interval training group losing significantly more abdominal fat, reducing their insulin resistance and inflammatory markers, although both groups gained similar aerobic fitness.

"In half the time, the women doing the 40 minutes lost no fat but improved their fitness and insulin resistance a little but those in the interval sprinting had the same increase in aerobic power but lost 2.5kg of fat, a lot of it around the abdominal area. They also reduced both their insulin resistance and their low-grade inflammatory markers," Professor Boutcher said.

Unlike steady aerobic training, the high- intensity, short bursts of cycling prompted the release of "massive" amounts of catecholamines, the hormones that helped burn fat, particularly fat in the abdomen, the researchers were surprised to find.

"So spinning your legs quickly, very fast on the bike results in high levels of catecholamines, and they are the major mediators of fat lipolysis - they get fat out of the adipocytes (fat cells) and induce fat burning in the liver and in the muscle cells - so that is the key," he said.

"When you work hard, such as when running or swimming, you typically don't get high catecholamine levels, even though the exercise is very hard."

And because visceral fat had 10 times the number of catecholamine receptors than subcutaneous fat in the legs, the fat in the gut was much more responsive to the extra levels flooding the system.

"So now we've got it. We have an exercise modality that specifically releases catecholamines and we have a thick visceral fat deposit that is responsive to catecholamines - so that is what we think is the mechanism."

With most people struggling to find the time or ability to do the hours of aerobic exercise required to get the same result, he said interval cycling provided a practical and effective alternative to reduce visceral fat, that could be done indoors and safely, by people who were very overweight, older or had arthritis in their legs.

"You need to get the heart rate over 150 beats per minute (for young adults) and that is very easy on a bike," he said.

"But people don't feel it is that hard when you look at their perceived exertion. That can't be achieved by running or swimming."

Professor Boutcher said other forms of interval sprinting had been successful in clinical groups such as coronary stent patients, diabetics, the elderly and obese children.

"A lot of clinical groups have used this interval sprinting and had very few problems - if they are medically screened and supervised, they enjoy it, they can get these massive health benefits," he said.