Children with the gene chose more fatty, sugary foods
People who carry a gene variant linked to obesity eat an average of 100 extra calories per meal, research suggests.
The key variant of the FTO gene is thought to be carried by 63% of people.
The New England Journal of Medicine study, by the University of Dundee, carried out eating tests on 100 children aged four to 10.
Those with the gene variant chose foods with more sugar and fat, suggesting they were instinctively drawn to them rather than healthy options.
Each child in the study took part in three eating tests, offering a range of different food types.
People with the relevant variants on the gene have a trait which may lead them to eat more unhealthy, fattening foods
Professor Colin Palmer
University of Dundee
The researchers found that the gene variant had no impact on the speed at which the body broke down food, or on how active people were.
There was also no evidence that those who carried it had any trouble registering when they were full up and should stop eating.
However, they did seem to be instinctively attracted to more calorific foods.
Lead researcher Professor Colin Palmer said: "This work demonstrates that this gene does not lead to obesity without overeating and suggests that obesity linked to this gene could be modulated by careful dietary control.
"What it effectively shows is that the people with the relevant variants on the gene have a trait which may lead them to eat more unhealthy, fattening foods."
Professor Palmer said the findings also reinforced the idea that soaring obesity rates were closely linked to the widespread availability of cheap, calorie-packed foods.
For people carrying the relevant gene variant, these may simply be too tempting to resist.
Research has shown that people carrying one copy of the key FTO variant (49% of the population) have a 30% increased risk of obesity, while for those carrying two copies the increased risk is almost 70%.
Professor Palmer said it was likely that many different genes were involved in obesity.
Dr Ian Campbell, medical director of the charity Weight Concern, said: "Given that half of us have the FTO gene, making us more prone to eating fatty, sugary foods, this must surely help us to understand how difficult it can be for individuals to simply use will-power to change their behaviour and adopt a healthier diet when their genetic make-up is telling them to do the opposite.
"If we are to tackle this problem adequately, we need to get smarter and start dealing with all the underlying forces that influence our choices."We need to find ways to make a healthier lifestyle a more attractive and, therefore, an easier option."