One possibility is the ''fit and fat'' theory, which suggests being physically active can protect us from chronic diseases even when we weigh too much. One effect of exercise, for instance, is that it makes muscles use more blood glucose for fuel. This can help ward off diabetes by keeping blood glucose levels down.

However, fitness isn't the whole story, says an endocrinologist, Dr Daniel Chen, from Sydney's Garvan Institute of Medical Research.

''Studies also show that some obese people who aren't physically active are also unexpectedly free of problems like high blood pressure and high blood fats,'' he says.

What's more, they're also insulin-sensitive, which means their body's insulin is doing a good job keeping their blood glucose levels healthy. This is the opposite of insulin resistance, the common condition that means your insulin is struggling to keep the glucose in your blood under control and puts you in the running for diabetes.

''One of the differences that sets these healthier overweight people apart is whereabouts on the body they store their excess fat. In people whose insulin is working well, fat is more likely to be carried around the hips and thighs, rather than around the waist,'' says Chen, who's trying to find out why some obese people sidestep the insulin resistance that sets up others for diabetes.

The surplus fat that clings to hips and thighs is different from the fat that hugs the waist. Called subcutaneous fat, it sits under the skin doing nothing other than padding out your jeans. Fat around the waist, however, is likely to be visceral fat, which is bad news for two reasons, Chen says.

''One is that it secretes inflammatory chemicals that contribute to heart disease and diabetes; the other is that it releases fatty acids which can be harmful, too. They end up being stored in the liver and muscles and this makes it harder for the body to keep blood glucose levels down,'' he says. ''But overweight people whose insulin is working well tend to have less of this 'bad' fat. They might have a lot of fat around the arms, legs and hips but they're not storing it around the waist, so in terms of diabetes it's not a problem.''

Having this visceral fat can give even a relatively slim person a higher risk of diabetes than someone who weighs more, but carries their extra kilos around the hips and thighs.

''People can be slim but still have this harmful fat around the waist, including some Asian people who tend to have a lower BMI [body mass index], yet still have visceral fat around the middle,'' Chen says.

Although some overweight people might be protected from diabetes and heart disease, they're not bulletproof: the extra weight increases the risk of osteoarthritis, for instance, by adding a burden to the hips and knees, he says. ''Whether there's less risk of cancer with healthier obese people isn't clear. So far, we only know there's less diabetes and fewer deaths from heart disease, but we also know there's an increased risk of cancer from diabetes.''

Chen says he needs more volunteers for his research. He would like to hear from those aged between 18 and 70, with a BMI of 30 or more. Phone 9295 8557 or email d.chen@garvan.org.au.

Paula Goodyer blogs at smh.com.au/chewonthis

Correction: Last week's story should have said breads and ''cereals'' are the main sources of salt in children's diets, rather than ''breakfast cereals''. Cereals includes a broad range of cereal-based products, including pizza.