Researchers have observed that the genes active in a person's belly fat are significantly different from those in the person's thigh fat.
The findings of the new study could shift the way we approach unwanted belly fat - from banishing it to relocating it.
Men tend to store fat in the abdominal area, but don't usually have much in the way of hips or thighs. Women, on the other hand, are more often pear-shaped - storing more fat on their hips and thighs than in the belly.
The answer to the question about why are men and women shaped differently still isn't clear, but it's an issue worth investigating, says Steven Smith from Sanford-Burnham Translational Research Institute for Metabolism and Diabetes.
That's because belly fat is associated with higher risks of heart disease and diabetes. On the other hand, hip and thigh fats don't seem to play a special role in these conditions.
In the recent study, Smith and colleagues help explain this discrepancy by determining how belly and thigh fat differ genetically. This research might shift common thinking about fat-rather than focusing on how to banish belly fat, perhaps we need to tip the balance in favour of heart-friendly fat in the lower body.
In that case, the study also provides a first step toward aiming treatments at specific regions of the body, especially those that contribute most to the complications of obesity.
Smith and colleagues first took fat samples from men and women. Then they compared the genes most active in belly fat to those most active in thigh fat.
They found that the genes operating in a person's thigh fat are hugely different from those in his or her belly fat. For men, 125 genes are expressed differently in the belly than in the thighs. For women, it's 218 genes.
The most notable genes that differed are known as homeobox genes. These genes are known for their role in helping shape a developing embryo-determining which cells and organs go where. Many homeobox genes are influenced by hormones such as estrogen.
Why are these homeobox genes important for fat?
"We believe these genes actually program those fat cells to respond differently to different hormones and other signals," Smith said.
In the course of their work, Smith and his team also isolated stem cells from belly and thigh fat and grew them in laboratory dishes. This was a nice control because fat cells in a dish aren't influenced by nerves, hormones, or other outside signals.
Yet the researchers still saw the same location-specific differences in gene activity in the fat that developed from these stem cells. That result told them that the cells are preprogrammed.
In other words, belly fat and thigh fat are genetically destined for their final location during development. It's not a difference that's acquired over time, as a result of diet or environmental exposure.
Medically speaking, says Smith, it's important to understand these differences and how they arise.
"Even though many women hate having large hips and thighs, that pear shape actually reduces their risk of heart disease and diabetes. In fact, women who have heart attacks tend to have more belly fat than thigh fat," he said.
The study has been published in the Journal of Clinical Endocrinology and Metabolism.