Scientists have long believed that once we reach adulthood, the number of fat cells in our bodies remains stable. Any fat that we gain, therefore, is not the result of new fat cells, but of old ones expanding in size.
Or so the theory went. For it’s now being challenged. A study published Monday by the Mayo Clinic in Rochester has found, for the first time, that while abdominal weight gain is the result of expanding fat cells, the fat we accumulate in our lower body, or thighs, is the result of added fat cells.
“We suspected that you could make new fat cells, but we were kind of surprised that it was the leg fat that was putting on the new cells,” said Michael Jensen, MD, a Mayo endocrinologist and professor of medicine, in a phone interview.
Gaining just three or four pounds over eight weeks adds about 2.6 billion new fat cells to the lower body, Jensen and his co-authors calculated.
For the study, the researchers gave 28 volunteers (15 men and 13 women) carte blanche to supersize their diets with giant candy bars, ice cream shakes, high-calorie drinks and anything else that would leave them feeling “more full than usual.” (The meals were provided by the Mayo Clinic and could be eaten off campus as long as the study’s volunteers brought back the empty containers.)
On average, the volunteers gained 5.5 pounds of upper body fat and 3.3 pounds of lower body fat over the eight weeks of the study. Jensen and his team measured changes in the volunteers’ body mass as well as in fat-cell size and number. They also assessed some of the characteristics of the volunteers’ preadipocytes (cells that have the potential to mature into fat cells).
What they found is that the weight gain went onto the volunteers’ lower body (hips and thighs) mostly in the form of new fat cells, but it went onto their upper body (abdomens) mostly as expanded fat cells.
This difference matters. It may help explain why other studies have found that fat gained around the waist but not around the hips is more dangerous in terms of increasing the risk of heart disease, diabetes and other chronic diseases. For when fat cells get too big, they can’t store fat properly. “Big fat cells may be responsible for abnormal release of fat into the bloodstream, where you don’t want it,” said Jensen.
“Nobody wants to get fat,” he added, but “it’s much better to gain it in the hips than in the abdomen.”
Of course, we aren’t in complete control of where we put on our pounds. “The major contributors [to how our weight is distributed on our bodies] are genetic,” said Jensen, “and then whether you are a man or a woman.” (Women are more likely than men to carry their added weight on their hips — at least until menopause.)
But certain behavioral factors — most notably, being sedentary, smoking and drinking too much alcohol — can also cause people to gain weight around the waist rather than around the hips, said Jensen.
The next step for Jensen and his team? “To try and understand more about what makes the precursors to fat cells in the leg grow new cells and why the abdomen isn’t able to do so,” he said.
The study appears in the current issue of the Proceedings of the National Academy of Sciences (PNAS).