Fat cells are called adipocytes

They are called lots of other things too. Usually nothing too complimentary. When a person gains weight, do they increase the size of their fat cells or increase the number of fat cells they have? Or both?

Once you reach adulthood, the evidence suggests that the number of fat cells is the same in both lean and overweight people, so when you gain weight as an adult you are just cramming more fat into every fat cell such that they enlarge.

Conversely, weight loss in adulthood reduces the size of the fat cells, but not the number. Even those that have undergone surgery to reduce stomach size (gastric stapling) retain the same number of fat cells despite considerable weight loss.

The number of fat cells is determined during childhood
Having fat cells has always been important for human existence. Fat cells became storage packs for reserve energy to help us get through any food shortages we may have encountered. Without fat cells we might be able to live only a day or two without food. With fat cells we have a much longer opportunity to obtain food.

If you cannot change the number of fat cells during the adult years, then any difference in fat cell number must happen during childhood and adolescence. Although there is speculation as to when during our youth we can increase fat cells, it seems that adolescence is a key time of accumulating additional fat cells. You can see why there has been concern about overweight children – three quarters of them become overweight adults. Developing an excess of fat cells during childhood may be one contributing aspect to chunky adults.

Today’s fat is different to 1999 fat
About 10% of your fat cells are renewed every year. Old ones die and are replaced by fresh ones. The average lifespan of a single fat cell is around 8.5 years. Their ability to fill with fat is enormous. Someone at 140 kg having the same number of fat cells as someone at 80 kg just shows you how “efficient” they are at doing their job.

What does it all mean?
The adult body has around 80 billion fat cells. They are there for life, being lost and replaced slowly. You can’t change the number of fat cells you have at your 18th birthday. You can, however, have a big say in their size. The paper did not answer the question whether liposuction permanently reduced fat cell number, although they stressed that there is a “tight regulation of adipocyte number” through life.

Reference: Nature 2008; 453: 783-787