According to many doctors, there is a strong correlation between eating disorders and depression. For example, the University of Pittsburgh Medical Center stated that about 25% of bipolar depression patients had eating disorders. In addition, nearly half of all binge eaters have been diagnosed with some sort of depression.
In terms of the correlation there is some debate. Some say that depression leads to eating disorders and other say that eating disorders lead to depression. The jury is out on this issue.
However, some more popular theories out there are as follows. First, some posit that people with eating disorders may not feel good enough. For this reason, they seek to control their lives and it manifests in their physical appearance (hence some sort of eating disorder). Some say that society puts so much pressure on body image and many people try to acheive it. When they fail they fall into depression and some sort of eating disorder.
In the end, there is no one cause of eating disorders. In the end there is some body distortion and it is usually associated with some sort of depression.