Overweight people can still become healthier with excercise (Image: Digital Vision/Getty)

Whether being overweight is an absolute bar to fitness has become one of the most hotly debated questions in exercise science. Steven Blair at the University of South Carolina is one of those who doesn’t accept what might at first sight seems plain common sense – that being fat means you must be unfit.

No one denies that there is a negative correlation between weight and aerobic fitness: overweight people tend, as a group, to be less fit. This is partly because a sedentary lifestyle contributes to weight gain, and partly because fat people may feel discouraged from taking exercise. It can be a vicious circle.

In a study published in 2007 Blair recruited 2600 people of varying weight and timed how long they could run on a treadmill before becoming exhausted, a proxy for fitness (JAMA, vol 298, p 2507). Among those who were mildly obese, only a third met a common definition of being physically unfit, and only half of those who were moderately obese were unfit. Blair points out that measures of aerobic fitness – the body’s ability to deliver oxygen to the muscles – have nothing to do with the amount of fat tissue present.

In the 12 years during which the subjects were followed, Blair’s study found that the risk of dying was more closely linked to fitness than fatness. People who were fit but obese had a lower risk of dying than people who were unfit but of normal weight. That’s important, says Blair, because while many overweight people find it hard to get slim, …