Walking 20 minutes everyday could help prolong the lifespan of every human being, scientists said Wednesday. In a stark warning against couch-potato lifestyles, they said lack of exercise killed twice as many people as obesity. The Cambridge University study of 334,000 people found that even a modest amount of activity prolonged life. And the least fit had the most to gain. Twenty minutes of walking a day or its equivalent would cut their risk of premature death by almost a third. The researchers could not say how much extra life could be gained through using exercise in this way to move from the ‘inactive’ to the ‘moderately inactive’ part of the population. But even the obese could expect a 16 per cent reduced risk of dying early. Those of healthy weight could profit by 30 per cent. Ulf Ekelund, who led the study, said: ‘‘This is a simple message: just a small amount of physical activity each day could have substantial health benefits for people who are physically inactive. Although we found just 20 minutes would make a difference, we should really be looking to do more than this physical activity has many proven health benefits and should be an important part of our daily life.’’ The researchers estimated that 337,000 of 9.2million recorded deaths of European men and women were attributable to obesity. But twice this number around 676,000 deaths could be blamed on inactivity. Government guidelines advise Britons to do 150 minutes of moderate activity such as gardening, dancing or brisk walking, or 75 minutes of vigorous exercise, including playing sport, running or aerobics, every week. But a recent survey found that a third of people can barely manage to walk for 30 minutes over seven days, even when trips to the shops, work or school are considered. And a report backed by the initiative Walking for Health last year found that physical inactivity cost the economy up to £10 billion a year through sick days, healthcare costs and early deaths. Professor Nick Wareham, Director of the Medical Research Council epidemiology unit at Cambridge, said: ‘‘Helping people to lose weight can be a real challenge, and whilst we should continue to aim at reducing population levels of obesity, public health interventions that encourage people to make small but achievable changes in physical activity can have significant health benefits and may be easier to achieve and maintain.’’ June Davison, senior cardiac nurse at the British Heart Foundation, said: ‘‘The results of this study are a clear reminder that being regularly physically active can reduce the risk of dying from coronary heart disease.’’ The research suggests that just a modest increase in physical activity can have health benefits. Adults should aim to do at least 150 minutes of moderate intensity activity a week, carrying it out in sessions of ten minutes or more. Whether it’s going for a walk, taking a bike ride or using the stairs instead of the lift, keeping active every day will help reduce the risk of developing coronary heart disease. A brisk 20-minute walk would burn between 90 and 110 calories. The study was reported in the American Journal of Clinical Nutrition. The authors defined premature death as dying before the average age for doing so.