An international team of researchers found people who develop anorexia are genetically predisposed to have an increased metabolic rate, less body fat and higher physical activity.
It had been assumed that these physical differences were a consequence of people with anorexia starving themselves.
But the new evidence suggests they may in fact be genetic differences in the way the body uses energy which make people more vulnerable to developing it in the first place.
This may explain why existing therapy struggles to treat the serious and life-threatening condition, and point the way to potential drug treatments in future, the authors said.
“A failure to consider the role of metabolism may have contributed to the poor track record among health professionals in treating this illness,” said Professor Cynthia Bulik, one of the study’s co-lead authors from the University of North Carolina Medical School.
“Until now, our focus has been on the psychological aspects of anorexia nervosa such as the patients’ drive for thinness,” she added.
Anorexia has the highest mortality rate of any psychiatric illness and affects between 1-2 per cent of women and 0.2-0.4 per cent of men.
Around 20 per cent of patients will go on to develop the most serious form of the condition and may need to be intensively fed in hospital, though many rapidly lose weight again after discharge.
It is typically diagnosed where individuals have a distorted view of their own bodies, dangerously low weight and a fear of gaining more.
The research, published in the journal Nature Genetics, used genetic data from more than 16,992 cases of anorexia and 55,525 health controls from 17 countries in North America, Europe and Australasia.
They then compared seven million genes across each individual in the study and looked at which variations were more commonly found in people with anorexia.
“Our analyses indicate that the metabolic factors may play nearly or just as strong a role as purely psychiatric effects,” said Dr Gerome Breen, one of the joint lead authors of the study from King’s College London.
“People with anorexia have a set of, what look like, ‘healthy’ genetic correlations. They appear to share genetics with people who have low body-mass index (BMI), people at lower risk of type 2 diabetes, at lower risk of insulin resistance,” he told The Independent.
“They also seem to have this correlation with increased physical activity levels and good cholesterol – HDL (high density lipoprotein) cholesterol.”
These traits suggest that people with anorexia may be using energy from food more quickly, or storing less of it as fat, Dr Breen said.
They also overlapped, but were independent from, those genes already known to influence body mass index (BMI) – which is one of the key traits used to diagnose the condition.
There were also genes implicated in other psychiatric disorders, including obsessive-compulsive disorder (OCD), depression and schizophrenia.
This could suggest that it is the combination of these psychological risk factors which turns these superficially healthy metabolic factors into a serious illness.
“For a long time anorexia hasn’t had any new treatments, it’s been very difficult to develop them,” Dr Breen told The Independent.
“We have hopes to collaborate with people working at the other end of the spectrum, in obesity, to work on the pharmacological implications of this – but that will take quite a long time.”
For now the authors argue that anorexia should be considered a hybrid “metabolic-psychiatric” disorder, and this needs to be factored into treatment.
“This is ground-breaking research that significantly increases our understanding of the genetic origins of this serious illness,” said Andrew Radford, chief executive of eating disorders charity Beat.
“We strongly encourage researchers to examine the results of this study and consider how it can contribute to the development of new treatments so we can end the pain and suffering of eating disorders.”
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