LAUSANNE – Extreme thinness is a disease with diverse, hard-to-identify origins. Once other ailments are excluded, specialists tend to look at the possibility that excessive weight loss is brought on by social and psychological factors, such as those that cause anorexia nervosa.

But now, a team of French and Swiss researchers has turned the focus to genetics. In a study published by the British journal Nature, the research team shows evidence of surplus copies of certain chromosomal sections in the genome of those suffering from the disease.

"Just eat a little something…" Children scorning their food, losing their appetite, becoming nothing but skin and bones, getting worryingly thin. Because of inadequate caloric intake, the body has no choice but to stop growing.

"When faced with these kinds of patients, pediatricians feel like the parents: helpless", said Sébastien Jacquemont of the Medical Genetics Service at Lausanne's University Hospital, and one of the study's main authors. "Being underweight, as is the case for one to three percent of toddlers, is like a black box: we have no knowledge of its diagnostical limits or internal workings".

For adults, extreme thinness is defined by their body mass index (BMI), which is a ratio of weight and height. Being underweight is defined as having a body mass index below 18.5. This is no trifling pathology, as it is said to affect 10 to 15% of the population, surprisingly close to the rate of those overweight: 15 to 20%.

Last year, Professor Jacques Beckmann of the University of Lausanne (UNIL) made a groundbreaking discovery about cases of morbid obesity: having only one copy of a particular fragment of chromosome 16 (instead of the usual two) could be responsible for being overweight.

But what could be the opposite effect of this genetic anomaly on weight? In other words, what might it entail if someone had three copies of this chromosomal fragment –that is, one too many? To answer that question, the team of scientists collected data from about 100 international laboratories. They gathered information concerning more than 100,000 patients, 138 of whom carried this particular genetic anomaly. Researchers demonstrated that such adult patients were 7 to 20 times more likely to be underweight than the rest of the population.

"This factor of genetic risk is four times greater than that of developing cholesterol-related heart diseases", said Philippe Froguel, a professor at the School of Public Health at Imperial College London, another lead researcher on the study. "Such a high level of risk shows that the patient's environment and choices only play a minor part in the evolution of the disease."

But the same is not true when it comes to children. The study showed that 50% of the people carrying that chromosomal anomaly were underweight. However, various factors (like exercising, or a healthy family environment, among others) could still change a lot of things during the evolution of the disease.

Discovering the genetic cause for extreme thinness is a major step for physicians, as it means that they can now rely on a simple diagnostic tool. "It will help prevent family tragedies", according to Froguel. "Sometimes social services suspect domestic abuse and tear the children away from their parents."

With this study, researchers may also be on the path to finding genes that are crucial to the feeling of satiety, and responsible for putting on weight.

Read the original story in French

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