Being fat is enough to make you even fatter, researchers at Tel Aviv University have found, because of a phenomenon called “cellular expansion.”

According to researchers Prof. Amit Gefen, Dr. Natan Shaked and Ms. Naama Shoham of the Department of Biomedical Engineering, together with Prof. Dafna Benayahu of TAU’s Department of Cell and Developmental Biology, “cellular expansion” plays a primary role in fat production: “fat” cells — that is, cells in the body with a higher than average fat content — get bigger when they are exposed to physical pressure, such as the pressure of sitting down.

“Contrary to muscle and bone tissue, which get mechanically weaker with disuse, fat depots in fat cells expanded when they experienced sustained loading by as much as 50%. This was a substantial discovery,” Gefen added. The research was published this week in the Biophysical Journal.

In the US, over 35% of American adults are considered obese. Add that to those who are “just” overweight, and you find that seven out of every ten Americans have a weight problem. This problem affects children, and even toddlers; over 12% of children age 2-5 years are obese. The situation in Israel is somewhat better — the obesity rate for adults is about 14%, less than the OECD average of 17% — but it’s still substantial.

Because obesity is so expensive — according to experts, the US has been spending as much as $200 billion on obesity-related health care in recent years — insurance companies, health officials, and diet plan companies all have a great interest in figuring out why so many suffer from it. While overeating and a lack of exercise obviously are key factors, everyone knows someone who can eat whatever they want without gaining an ounce. Diet, it would seem, is not necessarily the only variable.

That observation also bothered the Tel Aviv University researchers, who used state-of-the-art technology to analyze the accumulation of fat in the body at the cellular level, in order to detect the structure of “obese” cells.

According to Prof. Gefen, a main question for the researchers — working through a grant from the Israel Science Foundation – was to find out why a sedentary lifestyle often results in obesity. “We found that fat cells exposed to sustained, chronic pressure — such as what happens to the buttocks when you’re sitting down — experienced accelerated growth of lipid droplets, which are molecules that carry fats.”

Apparently, once a cell gets “fat” by accumulating droplets of lipids, the structure of a cell and its mechanics change dramatically. Using an advanced atomic force microscope, the researchers were able to see how cells got stiffer as they expanded.

“When they gain mass and change their composition, expanding cells deform neighboring cells, forcing them to differentiate and expand,” said Gefen. “This proves that you’re not just what you eat. You’re also what you feel — and what you’re feeling is the pressure of increased weight and the sustained loading in the tissues of the buttocks of the couch potato.

“If we understand the etiology of getting fatter, of how cells in fat tissues synthesize nutritional components under a given mechanical loading environment, then we can think about different practical solutions to obesity,” he added.

Using this information, the team will begin investigating the mechanics of the cell changes, and how the fat cells “conspire” to make people overweight or obese. “If you can learn to control the mechanical environment of cells,” said Gefen, “you can then determine how to modulate the fat cells to produce less fat.”