Body mass may be more nurture than nature

Research into the genetic causes underlying a person’s body mass index (BMI) score shows that genes have much less influence on body mass than previously thought.

Dr Gibran Hemani, who led the research in Professor Peter Visscher’s lab at the Queensland Brain Institute and Diamantina Institute, says there is debate as to the degree to which genetics contributes to BMI.

“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.

“We wanted to come at the question from a different angle, using very large and complex sets of data and a method that is more reliable.”

Using DNA data on 20,240 pairs of siblings, Hemani and colleagues counted the total number of DNA variants shared across the whole genome between siblings.

On average, if an individual has a variant at a particular position in the genome then his or her sibling will have a 50% chance of sharing that same variant.

By counting the variants, the team could identify how genetically similar (or dissimilar) each pair of siblings was to each other.

They found that although on average siblings were 50% genetically identical, the exact figure varied from about 35% to 65%, meaning that some sibling pairs are more genetically similar than others.

Using this method, Hemani and colleagues calculated whether siblings who were more genetically similar were on average more similar in terms of BMI - they found that 42% of BMI is under the influence of genetics.

Hemani said the study has contributed in other ways to understanding how genetics influence BMI.

“We know that there are a huge number of variants across the genome, but how many of them actually have a direct influence on a complex trait like BMI?

“This is an important question because many researchers who study disease are focused on one particular genetic change or a gene of interest as the cause of the disease; however, there is growing evidence, including from this study, that disease is caused by thousands of small changes and it’s the additive effect of these changes that results in disorder.”

This research has been published online in The American Journal of Human Genetics.