Scientists have identified several genetic variants that tip the balance in favour of being thin.
In the largest study of its kind, researchers from the University of Cambridge studied people of a range of body types to look for genetic factors linked to thinness. Previous studies have mostly focused on looking for genetic variants associated with being obese or overweight. Many such variants have been found, but few have been identified to be associated with thinness.
'This research shows for the first time that healthy thin people are generally thin because they have a lower burden of genes that increase a person's chances of being overweight and not because they are morally superior, as some people like to suggest,' said Professor Sadaf Farooqi of the University of Cambridge, one of the lead researchers of the study. 'It's easy to rush to judgement and criticise people for their weight, but the science shows that things are far more complex. We have far less control over our weight than we might wish to think.'
The scientists recruited 2000 people who were thin (defined as a body mass index of less than 18) but healthy, with no medical conditions or eating disorders, and analysed their DNA. They compared the DNA of 1622 of these volunteers to 1985 severely obese people and 10,433 people of weight within the healthy range.
They identified several genetic variants that are already known to play a role in obesity, as well as some involved in thinness. They also found that three-quarters of the volunteers in the thin cohort had a family history of being thin and healthy.
'As anticipated, we found that obese people had a higher genetic risk score than normal weight people, which contributes to their risk of being overweight. The genetic dice are loaded against them,' said Dr Inês Barroso of the Sanger Institute, another author of the paper.
Timothy Frayling, professor of human genetics at the University of Exeter Medical School, who was not involved in the research, said: 'The really interesting new thing is that being naturally thin seems to be largely the flip side of the genetic coin to being naturally overweight – the same genes seem to be involved, just in different forms.'
However, experts stress that genetic risk only increases a person's chances of being thin, and environmental factors, such as food and exercise, also play a major role. Tim Spector, professor of genetic epidemiology at King's College London, who was also independent of the study, said: 'About a third of people in most countries manage to remain thin despite exposure to poor food environments. Some of this is down to genes, but other factors like individual differences in lifestyle or gut microbes are likely to also be responsible.'