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Obesity Is Not In Our Genes

By Stephen Luntz

Genetic factors have less influence on obesity than previously thought, a new study has concluded. As interesting as the finding may be, the methodology used to reach the conclusion could provide insights into even more exciting questions.

“Some studies claim that more than 80% of BMI is due to genetic factors, with less than 20% being driven by environmental factors, whereas others have put the figure much lower,” says Dr Gibran Hemani of the Queensland Brain Institute, who is lead author of a paper in The American Journal of Human Genetics. Hemani set out to test these figures in a manner that has only recently become possible with the increase in studies of human genome variations.

Hemani investigated the similarity of 20,240 siblings’ genomes to each other. While high school genetics says we share 50% of our genes with our brothers and sisters, Hemani says the truth is more complex, with the figure ranging from 35–65% depending on the pair.

Hemani compared the similarity of each pair’s genes with their Body Mass Index to see whether those with similar genomes are also more likely to have similar weights. The test was only possible as a result of the vast amounts of genetic data now available from single nucleotide polymorphism chips.

Out of this mass of data, Hemani’s estimate for the genetic influence on obesity is 42%. “We know there are a huge number of variants across the genome, but how many of them actually have a direct influence on complex traits like BMI?” he asks.

While epigenetics complicate the situation, Hemani says: “There is evidence epigenetic factors are often caused by other genes. If a methylation down­regulates a gene, something caused that, and if that is inheritable then it is included in the 42%.

“This technique could be applied to a much larger range of topics,” Hemani says, “but as far as we know we are the first to use it. We got data from collaborators across the world. Some people were interested in heart disease, some cancer. They tested for different things so we looked at BMI because it was easy to combine lots of cohorts to get information.”