Whatever changes in body fat we may experience as adults, it is only the filling state of our fat cells that varies. The number of fat cells is set during childhood and adolescence and remains constant throughout later life.

 

Understanding the natural comings and goings of body fat may hold the key to better tackle the rising rates of obesity. On these grounds, researchers from the Karolinska Institute and Stockholm University in Sweden collaborated with colleagues from the USA, Germany and Greece in a study assessing the fat cell (adipocyte) turnover in humans at different life stages. The scientists made use of the carbon isotope ¹⁴C, which is incorporated into our DNA in proportion to its environmental concentration. Because ¹⁴C largely stems from the nuclear bomb tests performed in the middle of the last century and levels have dropped steadily since then, the quantity of ¹⁴C found in an adipocyte reflects the date the cell was synthesised.

 

Being able to identify the birthday of adipocytes, the researchers assessed whether major weight loss after surgical reduction of stomach size (so-called bariatric surgery) impacted on their number. Body mass index (BMI) and fat cell volume decreased markedly upon bariatric surgery, yet fat cell number remained constant. They then went on to calculate that roughly 10% of fat cells are replaced each year, irrespective of whether people are lean or obese. This means that after about 10 years, half the adipocytes will have been exchanged for new ones.

 

Although obese people started expanding their fat cell number earlier than lean people, this process also ended earlier in the obese. The main difference between obese and lean people occurred in the rate of producing adipocytes during childhood and adolescence. Obese individuals generated twice as many fat cells over the same period, ending up with a significantly larger capacity for fat storage.