Scientists think eating problems can start in a lot of ways.
Some scientists think that you are born with them.
Others believe your eating problems are learned from people around you and people who are famous, for example wanting to be like very thin fashion models.
Also, people who don't feel in control of their lives turn to eating, as it's one thing they do have power over.
Eating problems can also start because of other problems, such as unhappiness at home or school, someone you know dying, or a mixture of these and other things.