Twelve minutes' exercise per week 'enough to stay fit' Three times a week for 10 weeks, the men undertook bouts of "vigorous" exercise, which involved running on a treadmill at a speed which raised their heart rate to 90 per cent of its maximum capacity. For half of the men the regime involved simple four-minute sessions, three times a week, while the other half completed three sixteen-minute sessions, each of which was divided into four-minute segments. Despite carrying out different amounts of exercise, the results for the two groups were strikingly similar. Oxygen uptake - the amount of oxygen the body can use during exercise - increased by 10 per cent in the four-minute group, and by 13 per cent in the sixteen-minute group. Blood pressure and glucose levels lowered by similar amounts in both groups, but the more intensive sixteen-minute sessions was more effective at lowering cholesterol and body fat. Writing in the Public Library of Science ONE journal, the researchers said that such improvements could lower the risk of death from conditions like heart disease and stroke. "These data suggest that it may be possible to reduce cardiovascular mortality with substantially less exercise than is generally recommended, provided it is performed in a vigorous manner," they wrote. The researchers said the exercises could easily be incorporated into a daily regime, for example by walking quickly up six to ten flights of stairs or by walking up a hill with an eight to 10 per cent gradient. Some doctors have raised concerns that carrying out intensive exercise could pose a health risk to unfit people. Andrew Marr, the BBC presenter, was told that his stroke earlier this year was caused by a work-out on a rowing machine. Speaking at the time Dr Thomas Lee, Professor of Medicine at Harvard Medical School, said: "Short episodes of very intense exercise can raise one's risk for a stroke or heart attack or fatal arrhythmia during or right after the burst of exertion. "I think this approach, which used to be known as "intervals," is very reasonable and accepted among young athletes...I don't encourage it among most of my older patients." But the researchers said their previous work had showed that "most individuals can engage in this type of exercise training as it has been performed without any adverse events." Dr Arnt Erik Tjønna said the programme of brief but intensive exercise sessions could be a "time-efficient" method of staying fit. Previous studies have found that a lack of free time is one of the most frequently mentioned issues when people are asked what prevents them from exercising. "Since we know that more and more people are inactive and overweight, the kind of improvement in physical fitness that we saw in this study may provide a real boost for inactive people who are struggling to find the motivation to exercise," he said.