Surely more than once it has been called your attention why some people can eat more than others and not get fat, causing envy for many other who fight and fight with various diets and exercise to stay thin. An explanation for this would be given by a special type of proteins that are located inside the cells: uncoupling proteins known as UCP (uncoupling proteins).
These proteins are located in the mitochondria of the cell and its function is to decouple the chemical reactions that normally lead to storage of food energy as ATP, or dissipate that energy into heat. This effect is known as thermogenesis.
To date, five types of these proteins have been reported. The three mains are: CPU 1, that is the most studied (in rodents) and is found mainly in an adipose tissue type known as brown adipose tissue. Another is CPU 2 that is found in most human tissues, and CPU 3 located in the skeletal muscles.
It has been studied that people with visceral obesity have little amount of CPU 1. And people with morbid obesity have less CPU 2 and 3. The lack of such proteins would favor accumulate food energy as fat. While people that have a higher amount of these uncoupling proteins dissipate energy as heat.
To date there is no test being performed in clinical laboratories to determine how much CPU you have. People prone to obesity have lower expression of genes that produce such UCP. This would explain in part the genetic cause of obesity.
Investigations are continuing and it is not weird to think that in the future we could have some medications that precisely do this shift at the mitochondria, thus dissipating the energy contained in food into heat and not accumulate it as fat.