It's well known that being fat can be a fast track to diabetes and heart disease.

But now Sydney doctors say some obese folk are less at risk from the two potentially deadly illnesses than others - and they've launched a new study to find out why.

Experts at the Garvan Institute, the medical research facility at Sydney's St Vincent's Hospital, say there are "healthy obese people" whose insulin works just as well as in someone who is lean.

These same people also appear to be less at risk of dying from cardiovascular disease.

It remains unclear how many of these so-called insulin sensitive obese people are out there - or even exactly why they are less at risk, according to Jerry Greenfield, St Vincent's Hospital's head of endocrinology.

"A study of these people to examine what protects them from developing diabetes could be very informative in telling us what causes insulin resistance," Dr Greenfield said.

"We're not proposing that insulin sensitive obese people are completely protected from developing heart disease and diabetes.

"Rather, they appear to have a lower risk of these diseases compared to someone who is insulin-resistant, yet as obese."

Dr Greenfield and colleague Dorit Samocha-Bonet have already carried out a review of existing studies on the topic and are now recruiting obese people to try to find out more.

"The studies we reviewed agreed that there was less fat in the liver of the insulin-sensitive obese person, as well as fewer potentially damaging fat metabolites in muscle," Dr Samocha-Bonet said.

- AAP