Scientists at the Karolinska Institute in Sweden say that the number of fat cells in the body is set during adolescence and remains the same, regardless of obesity later in life.

The study tested patients who lost huge amounts of weight. Even though they lost the weight, researchers found that the level of fat cells had stayed constant.

Researchers say this explains why it is so difficult to lose weight and keep it off.
They also say this study gives solid evidence that they can use in future research into obesity and its causes.