Scientists say they have destroyed the myth that you can be "fat but fit", with research showing that obese regular exercisers are likely to die before slim unfit people.
Obese people with high levels of aerobic fitness were 30 per cent more likely to die prematurely, compared with those were slim but took little exercise, a study of 1.3 million men found.
The researchers tracked men for 30 years, before coming to the conclusion that being the right weight is the most important factor for long-term health.
Scientists said the findings demolished the myth that being fit could compensate for obesity.
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A number of studies have suggested that obese people who were regular exercisers were at no greater risk of a potentially fatal illness than people of normal weight.
The new research, the largest study of its kind, was based on 18-year-old Swedish military conscripts whose aerobic fitness was tested by asking them to cycle until they had to stop due to fatigue. They were then followed by the health researchers into middle age, for an average of 29 years.
Professor Peter Nordstrom, of Umea University, Sweden, said: "Unfit normal-weight individuals had a 30 per cent lower risk of death from any cause than did fit obese individuals."
He said the findings challenged the idea that obese people could compensate for their mortality risk by taking plenty of exercise.
Nordstrom said: "These results suggest low BMI [body mass index] early in life is more important than high physical fitness, with regard to reducing the risk of early death."
Overall, men in the highest fifth of aerobic fitness had a 48 per cent lower risk of death from any cause compared with those in the lowest fifth, the study, published in the International Journal of Epidemiology, found.
Such men had an 80 per cent lower chance of death associated with alcohol or drug abuse and a 59 per cent lower chance of suicide, and showed a 45 per cent drop in heart disease deaths. But when such men were obese, they were still much more likely to die early compared with slim men.