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It’s obvious that obese people more have fat than non-obese people, but it’s not as clear how it happens. Do obese individuals have more adipocytes (fat cells) than lean people, or do they have the same number of adipocytes, just larger ones? It turns out to be both. But the way that comes to pass is just being worked out by scientists. Engineering Professor Dr. Amit Gefen and his colleagues at Tel Aviv University recently demonstrated in a mouse cell line model that preadipocytes (precursors to fat cells) subjected to prolonged periods of “mechanical stretching loads” — the kind of weight we put on our body tissues when we sit or lie down — differentiate significantly faster, and retain significantly larger fat droplets, than those that are not. The research was published in the American Journal of Physiology — Cell Physiology .
Osteoblasts, or bone cells, have been shown to grow into mature bone when subjected to mechanical stress. Scientists hypothesized that the same might hold true for other cell types. Adipocytes seemed the ideal test system for a number of reasons. Firstly, they are derived from the same embryonic progenitor cells as osteoblasts, so they might share similar properties. Moreover, adipocytes in the buttocks are known to be exposed to a large mechanical strain in a physiological setting — when we sit down. And lastly, obesity is such an immense health problem that any insight into its development could be clinically valuable.
To test their theory, researchers created a unique experimental apparatus in which two groups of cells could be cultured under identical conditions, but one group would be stretched and the other would not. The stretching of the cells mimicked the about half the degree of tissue compression that occurs in weight bearing postures. All of the cells were induced to differentiate by the addition of insulin, and cells were inspected almost daily. The researchers measured the numbers and sizes of lipid droplets in the two cell populations every two to three days over the course of three to four weeks. Within the first ten days of culture, the stretched cells differentiated about twice as fast as the non-stretched cells. Thereafter, the stretched cells contained lipid droplets that were about one and a half times as large — 50% more fat — as those in the non-stretched cells.
This is the first study to look at fat cells as they develop under sustained mechanical loading. The authors are quick to point out that cyclic or intermediate stretching — such as that attained during physical motion — has been shown to have the opposite effect; it actually inhibits the differentiation of preadipocytes and lipid production. They also note that future studies should try to determine if these results hold true in a more physiological setting, and if the cells might respond differently to different concentrations of insulin and glucose such as those that might be representative of a high calorie diet.
Nevertheless, the study is highly suggestive of yet another damaging effect of a modern, sedentary lifestyle. Dr. Gefen notes :
Obesity is more than just an imbalance of calories. Cells themselves are also responsive to their mechanical environment. Fat cells produce more triglycerides, and at a faster rate, when exposed to static stretching.
Indeed, the findings indicate that we need to take our cells’ mechanical environment into account as well as pay attention to calories consumed and burned. Even people that eat a healthy diet and exercise will be negatively impacted by long periods of inactivity.