Seventy-nine rare genetic syndromes in which obesity is a major feature have been catalogued by Canadian researchers.
Previously it was widely believed there were 20 to 30 obesity-associated syndromes, but no-one had systematically trawled the published literature to evaluate them. The team found reports of 79 obesity syndromes in the scientific literature.
'These syndromes, although individually rare, are much more numerous and diverse than anticipated,' said Yuvreet Kaur, who began working on the study while still an undergraduate and is the study's first author.
The team, led by Dr David Meyre, associate professor at McMaster University's School of Medicine, Ontario, collaborated with the University of British Columbia to search seven databases and analyse 161 papers.
They focused on monogenic forms of obesity in which there is a 100 percent chance of developing obesity. Obesity caused this way is rare and affects only 0.5 percent of the obese population, and individuals with these syndromes also often have other symptoms such as heart and kidney malformations or cognitive impairment.
Due to the rarity of these conditions, doctors may only encounter one or two patients with these syndromes in their career. As a result, the conditions often go undiagnosed and patients don't get the help they need.
'Identifying genes is very important for the families,' Dr Meyre told CNN. He added, 'We also hope that our study will help clinicians to recognise these syndromes.'
Some common obesity syndromes, such as deficiency of leptin hormone, can be easily treated: replacement of the hormone causes dramatic weight loss. If genes are identified for all these syndromes, researchers will be able to work towards appropriate treatments.
Mary Freivogel, president of the National Society of Genetic Counsellors, who was not involved in the research, praised the comprehensive nature of the study but emphasised to CNN: 'Most of the obesity in the United States is not syndromic', saying that the overwhelming majority of cases are polygenic and result from a combination of multiple genetic and lifestyle factors. She also advised anyone wanting to undergo genetic testing to discuss this with a genetic counsellor beforehand, to ensure that testing is appropriate and any results are interpreted correctly.
Professor Phil Beales, head of genetics and genomic medicine at University College London, also praised the authors' methodology and was pleased someone had updated this 'specialised category of obesity'. He explained that rare examples like these can help to understand and potentially treat the more common forms of obesity.
The study was published in the journal Obesity Reviews.