Some people who are overweight or obese are metabolically healthy, while others are plagued with high blood pressure, problems regulating their blood sugar and high levels of fats in their blood.  A new study shows the difference between these groups may be linked to the site of fat in the body.

When people gain fat in their thighs, their bodies typically produce new fat cells, whereas gaining fat in the abdomen usually involves an expansion of existing fat cells, according to researchers at the Mayo Clinic in Rochester, Minn.

The research upends previously held theories that the bodies of adults are largely incapable of producing new fat cells, and weight gain simply causes existing fat cells to swell, said study researcher Dr. Michael Jenson, an endocrinologist at the Mayo Clinic.

Fat cells provide a useful service to our bodies by storing fat in confined areas and keeping it out of other cells, such as muscle or liver cells, where it can be harmful. Some overweight people have "healthy" fat tissue that behaves the same way fat tissue in lean people behaves, Jensen said, they just have more of it. In others who are overweight, the fat is behaving in an unhealthy manner.

"Our findings suggest that this may be because lower-body fat gain is largely the result of gaining more fat cells, each able to function normally, as opposed to the fat cells becoming so big they no longer store and release fat normally," Jensen said.

"People gained a lot of new cells, even without that much weight gain," Jensen said. A gain of about 3.5 pounds (1.6 kilograms) of fat on the thighs meant that about 2.6 billion new fat cells were created for adult participants in the study.

Those who gained more of their weight in their abdomens instead of in their thighs were less likely to produce new fat cells. Instead, their gain came mainly from the expansion of existing cells, Jensen said. Still, some did create new abdominal fat cells, he said.

Losing weight seems to improve the function of fat, he said. But if scientists understood why fat behaves in such different ways, they may be able to develop treatments that coax fat into behaving in a more healthy manner.

The researchers based their findings on 28 people, most of whom were in their 20s, Jenson said. The participants overate for eight weeks, consuming candy bars, milk shakes and almost anything else they wanted to eat.

Seeking to understand why some people gain weight in their thighs while others pack extra pounds into their abdomen, the researchers measured the amount of fat in these regions, and the number of fat cells. On average, the participants gained 5.5 pounds (2.5 kilograms) in their upper bodies, and 3.3 pounds (1.5 kg) in their lower bodies.

People varied greatly in terms of where the weight went, Jensen told MyHealthNewsDaily. Some gained most of it in their abdomen and just a little in their thighs, while others saw the opposite happen.

"We thought for sure we'd see that women gained more weight in their thighs," he said, however, the study did not find any differences between men and women in terms of where the weight went, Jensen said. But the results may be different in older men and women, he added.

The researchers studied the characteristics of fat cells — their genes, cellular chemistry and the ways they interacted with hormones — trying to find the reason fat tended to accumulate in one site or the other, but the cause remains elusive.

They did find some differences in levels of expression of some genes in fat cells in the abdomen that were associated with whether fat cells there grew in number or swelled in size.

Still, "none of the things we looked at was a good predictor of where the weight went," Jensen said.

Future studies will look further into the underlying causes of this, he said.

The findings are published today (Oct. 4) in the journal Proceedings of the National Academy of Sciences.

This article was provided by MyHealthNewsDaily, a sister site to LiveScience.