Slim people are lean as the result of genetic advantage and not always because they're more disciplined when it comes to portion control, according to a new study.
New research indicates that the genetic dice are loaded in favour of thin people and against those at the obese end of the spectrum.
It follows the results of Cambridge University researchers studying 2,000 thin people, defined as having a body mass index of less than 18, with no medical conditions or eating disorders.
"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 Cambridge’s Wellcome-MRC Institute of Metabolic Science.
"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."
DNA from 1,622 thin volunteers from the cohort, called the Study Into Lean and Thin Subjects (STILTS), was compared with that of 1,985 severely obese people and a further 10,433 normal weight controls.
Researchers acknowledged that factors such as easy access to high calorie foods and sedentary lifestyles can impact on a person’s weight, but said there is considerable individual variation within a population that shares the same environment.
"We already know that people can be thin for different reasons," said Prof Farooqi.
"Some people are just not that interested in food whereas others can eat what they like, but never put on weight.
"f we can find the genes that prevent them from putting on weight, we may be able to target those genes to find new weight loss strategies and help people who do not have this advantage."
Three out of four people (74%) in the STILTS cohort had a family history of being thin and healthy and the team found some genetic changes that were significantly more common in thin people.
They say this may allow them to pinpoint new genes and biological mechanisms that help people to stay thin.
Dr Inês Barroso of the Wellcome Sanger Institute, which collaborated on the study, said: "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."
The findings are published in the journal PLOS Genetics.