Here's some welcome news for the 35 percent of Americans who are obese: That extra weight does not automatically translate to a host of health problems. Indeed, doctors are focusing on the small but significant population of otherwise healthy people with a body-mass index (BMI) over 30 to study the mystery behind "metabolically healthy obesity."

"Obesity isn't a homogenous condition," says Harvard's Dr. Frank Hu at Harvard Health. "It appears that it doesn't affect everyone in the same way."

As many as one-third of obese people actually have normal cholesterol and blood pressure levels and show no signs of developing diabetes. These people have long baffled researchers, but a new study in Diabetologia has hit upon a key cellular difference in the metabolically healthy obese population.

The new study examined 16 pairs of identical twins in which one person is obese and the other is not. In the pairs where the obese twin had the same metabolic levels as their thinner twin, the obese twin had 11 percent more adipocytes, or fat cells, in their subcutaneous tissue, maximizing their ability to store fat. But in the other pairs, the obese twin actually had 8 percent fewer fat cells than their thinner twin, even though they had a higher percentage of body fat.

So, what's going on here? Well, remember your high school biology class, where you heard about mitochondria, the "powerhouse of the cell"? Mitochondria apparently lives up to the nickname, especially among the metabolically healthy obese.

Mitochondria in typical obese people tend to swell to their breaking point in their effort to generate new cells to store fat, resulting in inflammation and ectopic fat accumulation, which is the "shuttling of fat into organs where it does not belong, like the liver, heart and skeletal muscles," writes Anahad O'Connor at The New York Times. This is exactly what was seen in an obese twin with higher cholesterol and blood pressure than his thinner twin.

However, in the metabolically healthy obese population, the mitochondria chugs along just fine, exhibiting none of the expected signs of impairments. This suggests "there is a way to safely carry excess fat," says O'Connor.

But don't throw out your Weight Watchers membership just yet. Aside from the fact that only a relatively small portion of obese people are considered metabolically healthy, researchers are not even sure if it is a permanent trait.

A study from the University of Adelaide showed that one-third of people who were once metabolically healthy obese eventually transitioned to unhealthy. "Metabolically healthy obesity may essentially be a transient state," researcher Sarah Appleton tells the New York Times.

While more study is needed, though, medical breakthroughs may result from this revelation about mitochondrial activity. Dr. Jussi Naukkarinen, one of the lead researchers, tells the Times that studying "what goes right" in the minority of people with metabolically healthy obesity may ultimately "teach us a lot about usually obesity."