Obesity experts are getting behind the idea that obesity isn't one disease, but many different subtypes under one umbrella, kind of like cancer. There are rare genetic syndromes that cause obesity, but now Canadian researchers say there are 79 of them documented in medical literature, which is a lot more than they thought.
That's the conclusion of a new analysis, published in the journal Obesity Reviews. The researchers looked at 116 papers and found that doctors can test for 19 of these 79 previously reported syndromes (that is, they've been genetically "solved"), while 11 have been partially clarified, and 27 have been mapped to a specific chromosomal region. Scientists haven't even been able to identify the regions for the remaining 22 syndromes.
In December, Lee Kaplan, director of the obesity, metabolism, and nutrition institute at Massachusetts General Hospital told the New York Times that there are at least 59 distinct kinds of obesity, including at least 25 genes which seem to be directly related to obesity risk. So this new paper suggests that the number of genetic links to obesity is three times higher than that. (There are hundreds of other genes which don't directly cause obesity but can make people gain a few pounds a year; people who inherit several of those and have other health conditions or take medications that can lead to weight gain could still go on to develop obesity.)
Lead author David Meyre, an associate professor at McMaster University's Michael G. DeGroote School of Medicine, told CNN that the paper focused on "monogenic" or "Mendelian" forms of obesity, which mean that people who carry the gene in question only need one mutation in order to develop the disease. Meyre said these genes are probably only present in 0.1 percent of obese people in Canada
Knowing which genes are responsible for all 79 syndromes doesn't solve the problem for people who have those conditions, nor does it help treat non-genetic obesity, which is way more common—at least not yet. But Meyre said that identifying the genes responsible for weight gain is a step forward for understanding the biological causes of obesity.
"If you know the gene and the function of the gene, then you know which biological mechanism is defective," Meyre told CNN. This knowledge could help scientists come up with treatments for the specific syndromes as well as for people whose weight isn't tied to a single gene. One example is the satiety hormone leptin, which the body's fat cells make. Some people with genetic mutations have low levels of it, which means they feel hungry all the time. The gene responsible for making leptin was discovered in 1990 and, since then, doctors have learned that as people lose weight, their bodies make less leptin. They can counsel their weight-loss patients that regaining weight may not be an issue of willpower, but rather one of simple physiology. Some experts are also trying to develop weight-maintenance drugs that mimic leptin's effects.
Learning more about obesity is a worthy cause considering that 36.5 percent of adults in the US are obese (based on body mass index), as are 20.2 percent of adults in Canada, and obesity can increase people's risk of developing sleep apnea, osteoarthritis, heart disease, diabetes, and certain kinds of cancer.
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