Just as you can be slim and sedentary, it's possible to be overweight and active. But which is better for your health?

Two months ago, an awful lot of us decided it was time to get off our ever-spreading backsides and do some exercise. This new year's resolution wasn't just about vanity, oh no. The government tells us that obesity has a "severe impact" on our health, and places a "significant burden on the NHS", so slimming is practically our patriotic duty.

If the lard is already melting away, then lucky you. But what if the exercise doesn't seem to be working? What if you can now run a kilometre in a minute and a half, yet your weight has hardly changed? Are you a lost cause? Or is it possible to be both fat and fit – not just fit enough to exercise, but fit enough to live as long as someone a lot lighter?

Not according to a 2004 study from the Harvard School of Public Health, which looked at 115,000 nurses aged between 30 and 55. Compared with women who were both thin and active (ie, who reported taking 150 minutes or more of exercise a week) researchers found that obese but active women had a mortality rate that was 91% higher. Though far better than the inactive obese (142% higher), they were still worse off than the inactive lean (5% higher). "This data does not support the hypothesis that if you are physically active, you don't have to worry about your weight," was the verdict from Frank Hu, who led the study.

A similar picture emerged in 2008 after Harvard-affiliated researchers examined 39,000 women with an average age of 54. Next to active women of normal weight, the active but overweight were 54% more likely to develop heart disease, while the active but obese were in 87% greater danger. "Even high quantities of physical activity are unlikely to fully reverse the risk of coronary heart disease in overweight and obese women without concurrent weight loss," the authors concluded.

That's that settled, then. Or is it? Steven Blair is professor of exercise science at the University of South Carolina. He describes the official focus on obesity as an "obsession ... and it's not grounded in solid data".

Blair's most fascinating study, in the Journal of the American Medical Association in 2007, took 2,600 people aged 60 and above, of various degrees of fatness, and tested their fitness on the treadmill, rather than asking them to quantify it themselves. This is an unusually rigorous approach, he claims, since many rival surveys ask participants to assess their own fitness, or ignore it as a factor altogether. Proper tests, Blair suggests, demonstrate no hard and fast link between excess weight and increased mortality.

"There is an 'association' between obesity and fitness," he agrees, "but it is not perfect. If you look at the normal-weight men and women aged 60 and older, for example, about 90% are fit as demonstrated by a 'maximal exercise' test in the laboratory. This is not asking them if they're fit, or guessing that they're fit – they've proved it on the treadmill. As you progress towards overweight, class I obesity and class II obesity, the percentage of individuals who are fit does go down. But here's a shock: among class II obese individuals [with a body mass index, or BMI, of between 35 and 39.9], about 40% or 45% are still fit. You simply cannot tell by looking whether someone is fit or not."

But doesn't that only prove that some fat people can hold their own on the treadmill? Not at all, Blair says. "In all of these studies, we typically see higher rates of mortality, chronic diseases, heart attacks and the like, in people with high BMI – we see the same thing that everybody else sees. But when we look at these mortality rates in fat people who are fit, we see that the harmful effect of fat just disappears.

"If we look at individuals who are obese and just moderately fit – we're not talking about marathon runners here – their death rate during the next decade is half that of the normal weight people who are unfit. So it's a huge effect."

One day – probably about a hundred years from now – this fat-but-fit question will be answered without the shadow of a doubt. In the meantime, is there anything that all the experts agree on? Oh yes: however much your body weighs, you'll live longer if you move it around a bit.