Are eating disorders an illness or a sin? Should they be considered distinct from a disease like cancer or diabetes, where a person can be affected but not necessarily “cause” the disease?
To the extent that the person has free will to choose to cause harm to his body, he can be culpable for sin. That freedom of the will can be compromised as an eating disorder (such as bulimia, for example) progresses, so a person might not be as culpable later as he might have been during the initial stages. In any event, such a person would be well-advised to go to confession. Whether or not the sin is mortal, the graces of confession—along with the proper medical treatment—can heal a person of the temptation to abuse his body.