Some patients need residential care, others need outpatient care, and still others whose bodies have been impaired medically need physical treatment in the hospital. Often patients need some follow-up care for months or years, and many patients relapse. The nature of both the physical and mental component of eating disorders makes them one of the most complex mental disorders to treat, says Grefe.

Some health insurers don't cover eating disorders at all, trying to minimize their risk of an expensive health problem. Others cover a 30-day residential treatment and/or some outpatient therapy with an eating disorder specialist or psychiatrist, but often with a lifetime limit on visits.

In addition, when insurance companies do cover treatment, they often base "wellness" on the person's body mass index (BMI), not on the psychiatric care needed after the patient reaches the required BMI.

Insurers can also refuse coverage if a patient's BMI is not low enough. The Federal Substance Abuse and Mental Health Administration determined that a BMI of 17.5 is a "strict indicator" of anorexia, yet some insurers require patients to have a BMI of less than 15 to qualify for residential treatment, says MacDonald.

According to BMI calculators, a 5-foot-5-inch woman would weigh 106 pounds with a BMI of 17.6. The same woman would weigh 90 pounds with a BMI of 15.

Yet patients and doctors are left to play an insurance "game" in hopes of scoring treatment coverage. Because eating disorder treatment is covered more fully when physical symptoms are the priority, if someone is seriously underweight they often don't want the doctor to document it as an eating disorder (meaning a mental disorder) because they'll get better coverage under a physical illness.

"You save them physically but don't do anything to help the mental side of it," says Grefe.

"If you want an insurance company to pay, you need to get them to treat the symptoms," says Katherine Woodfield, a New Jersey-based insurance broker. "In some cases, keeping the patient alive and getting the weight on is part of the battle. The mental illness component still exists, but if you want insurance to pay, they need a physical problem."