A study published in Nature is the first to confirm that severe obesity in otherwise physically healthy individuals can be caused by a rare deletion of DNA, the researchers say. (CBC)

Some severely obese people are missing a set of genes, a new study has found.

Researchers have found a small proportion of obese people are born without 30 or so of the estimated 30,000 genes in the human genome.

This deletion of genes was not found in any subjects of normal weight, the team from Imperial College London and their colleagues in Europe, the United States and Montreal reported in Wednesday's issue of the journal Nature.

The missing genes may account for seven out of 1,000 cases of morbid obesity — people with a body mass index, or BMI, over 40, the study found. The body mass index is a tool used to determine the healthy weight range for a particular height. A BMI over 30 is considered obese and 25 to 29.9 is overweight.

The recent rise in obesity in the developed world has been attributed to an abundance of unhealthy food and too little exercise, but the way people respond to these environmental factors is often genetic, said Prof. Phillipe Froguel, lead author of the study at the School of Public Health at Imperial College London.

"It is becoming increasingly clear that for some morbidly obese people, their weight gain has an underlying genetic cause," Froguel said in a news release.

"If we can identify these individuals through genetic testing, we can then offer them appropriate support and medical interventions, such as the option of weight-loss surgery, to improve their long-term health."

Searched general population

In the first part of the study, researchers looked for the genes in obese teenagers and adults with learning difficulties or delayed development. They found 31 severely obese people with nearly identical deletions in one copy of their DNA.

The second part of the study looked at the genomes of 16,053 European people in the general population, reflecting a range of weights. Nineteen people in this group were missing the same set of genes, and all were morbidly obese.

Those lacking the genes tended to be normal weight as toddlers, overweight during childhood and severely obese as adults, the researchers said.

The study is the first to confirm that severe obesity in otherwise physically healthy individuals can be caused by a rare deletion of DNA, the authors said.

Until now, individual genes linked to weight gain have had a relatively modest effect of about two pounds.

"The genetic change identified in this study is much less common but leads to much more substantial changes in the body weight of the individuals that it affects," study co-author Dr. Robert Sladek of McGill University in Montreal said in a release Wednesday.

Researchers have found a similar modest effect with genes influencing Type 2 diabetes. The approach the obesity researchers used — identifying the deletion in very obese people and then looking for the variant in a much broader population — could help to identify genetic influences on Type 2 diabetes and other diseases, the researchers said.

They now plan to study the function of the missing genes. Previous studies have suggested that deletions of these genes may be linked with delayed development, autism and schizophrenia.