When it comes to obesity, there's more than meets the eye: A new review of studies from Canada suggests that obesity comes in at least 79 different forms that are linked to people's genes, and many are very rare.
Although lifestyle factors, such as diet and physical activity, play major roles in obesity, research has shown that there's a genetic component of obesity as well, and in the new review, the researchers focused on the types of obesity that can be caused by genes. They looked through more than 160 studies on genetic forms of obesity. Previous reviews that tallied up the types have found that there were between 20 and 30 distinct types, the authors wrote.
But in the new review, the researchers identified a total of 79 genetic "obesity syndromes," meaning conditions that result from a genetic change and cause a person to be obese, among other symptoms. For example, one such "obesity syndrome" that has long been known is a genetic condition called Prader-Willi syndrome. People with this syndrome may have obesity, developmental delays, growth hormone deficiency, excessive hunger and abnormally large food intake, according to the review. [The Science of Weight Loss]
Other syndromes may involve intellectual disabilities, facial abnormalities and other symptoms, according to the review.
Of the 79 syndromes, 43 have never been assigned a name, the review found. And scientists have fully figured out the underlying genetics of just 19 of the syndromes. For others, scientists have reported a partial understanding of the underlying genetics, according to the review. However, the researchers identified 22 syndromes for which scientists knew nothing about the gene or the chromosome associated with them.
"A [clearer] understanding of the genetic causes of these syndromes may not only improve the lives of those afflicted with these mutations, but will also help us understand the genes and molecules that are important in obesity among members of the general population," senior review author David Meyre, an associate professor of health research methods, evidence and impact at McMaster University in Ontario, said in a statement.
Studies done in identical twins have found that obesity is estimated to be between 40 and 75 percent genetic, the researchers wrote in the review, published today (March 27) in the journal Obesity Reviews. In the review, the researchers focused on the types of obesity that are associated with one gene and cause syndromes. (Syndromes, by definition, include a collection of symptoms.)
The researchers noted that two other groups of genetic obesity disorders were not included in the review: types of obesity that are associated with multiple genes, and types of obesity that are associated with one gene but do not cause syndromes.
The review also looked into how common these syndromes are, but estimates were available for only 12 of the syndromes, the researchers found. Indeed, these syndromes are quite rare, with their prevalence ranging from 1 in 565 people to less than 1 in 1 million people. One disorder, for example, called Alström syndrome, has been found in about 1 in 900 people worldwide, according to the National Library of Medicine.
The authors of the review noted that they hope their review will boost research into genetic obesity syndromes.
Originally published on Live Science.