A new study may have discovered one of the reasons why a bulging belly is a greater health risk than a fat backside.

Researchers have known for some time that fat stored on the thighs, hips and buttocks is healthier than that stored around the abdomen, but they have not known why.

But now scientists from Oxford University, who have been studying the two different types of fat, think lower-body fat produces hormones that help fight disease, like diabetes.

Their work is published in the International Journal of Obesity.

A clinical research fellow at the University of Oxford's Centre for Diabetes, Endocrinology And Metabolism, Dr Konstantinos Manolopoulos, says although we might think all the fat on our body is the same, there are some important differences.

"The fat around the tummy is what we call a metabolically active tissue, which means that it takes up fatty acids from meals quite avidly and releases them in times of exercise, again quite avidly, while the thigh fat is basically long-term storage, so it's much more sluggish in its metabolism," he said.

"Ideally we'd have very flat tummies and very large hips.

"It's a bit difficult to sell, obviously, this way of, let's say, promoting health.

"The saying 'one minute on the lips, forever on the hips' ... there is certainly something true on that."

But despite it being harder to shift, Dr Manolopoulos says the fat on the thighs and hips actually protects the body from diseases related to obesity.

"It is the thigh fat that is the source of some beneficial hormones which are called leptin and adiponectin," he said.

"They help the body metabolise sugars and fats in a better way. In contrast, tummy fat, it's actually known and proven that it secretes a series of deleterious hormones that promotes exactly the opposite."

But he says even then, it is not an excuse for people to develop unhealthy habits.

"Don't get me wrong, I'm not promoting to say to people gain weight," he said.

"Obviously someone who is overweight and who would say 'I have loads of weight around my hips', but probably the same people would have loads of fat around the tummy. Well obviously the [non-]beneficial properties of the tummy fat overrides the beneficial properties of the leg fat.

"The take-home message is not just gain fat but it's important where it's stored."

Dr Manolopoulos says there is certainly a genetic background to this but he says he and his team are trying to find out how the body decides where to store its fat, then they can eventually develop ways of manipulating the body into storing the fat in safer areas.

"The general idea of redistributing fat in our body is a valid one but one should and could explore certainly in the future," he said.