One possibility is the 'fit and fat' theory that 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 up more blood glucose for fuel – this can help ward off Type 2 diabetes by keeping blood glucose levels down.

But fitness isn't the whole story, says 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 of 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," explains Chen who's trying to find out why some obese people side step the insulin resistance that sets others up for diabetes.

The surplus fat that clings to hips and thighs is different to the fat that hugs the waistline. Called subcutaneous fat, it sits there under the skin doing nothing other than padding out your jeans. But fat around the waist is likely to be visceral fat which is bad news for two reasons, he 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 yet still have visceral fat around the middle," Chen says.

But although some overweight people may be protected from diabetes and heart disease, they're not bulletproof – the extra weight is still going to increase the risk of osteoarthritis, for instance, by adding a burden to hips and knees, he adds.

"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."

Dr Daniel Chen needs more volunteers for his research to find out why some overweight people are protected from diabetes - if science can find a way to identify obese people who are insulin-sensitive more resources can be directed to those who are obese and insulin-resistant too, he says.

If you  have a BMI of 30 or more, are aged between 18 and 70 and live in the Sydney area or in NSW, he’d like to hear from you. Contact (02) 9295 8557 or d.chen@garvan.org.au. If you don't know your BMI, you can use the Better Health Channel's online calculator to work it out.