A genetic predisposition to weight gain and obesity in later life can in part be explained by a lack of feeling full after eating, according to a new study in JAMA Paediatrics.

Although there has been some study of why some people appear more likely to gain weight, researchers from UCL and King’s College London were interested in how 28 of the 34 identified “obesity genes” affected children. They found that although someone might be predisposed to becoming obese, it was also due to a tendency to eat more. In other words, predisposition doesn’t mean growing large is inevitable, and recognising it may help avoid overeating.

“Our findings suggest that satiety responsiveness [how full we feel after eating] is one of the mechanisms through which obesity genes influence weight,” Clare Llewellyn, research associate in the Health Behaviour Research Centre at UCL and a co-author of the study said. “Obesity genes influence satiety responsiveness, and through influencing satiety responsiveness, they indirectly influence your weight … This study indicated that the reason why some people feel full and other don’t is due to differences in our genes.”

The paper is based on the participants to the Twins Early Development Study – a long-term study into more than 16,000 pairs of twins born in the UK between 1994 and 1996. But rather than compare siblings, the researchers chose to study 2,258 participants who weren’t related. The average age of participants was nine. And they used a polygenic (multiple genes) risk score (PRS), which adds up the number of genetic variants someone has that puts them at increased risk of obesity.

For each of the 28 known obesity-related genes, you can score between zero and two. If you have all 28 genes, your score can go up to 56, where the higher the score the higher your genetic predisposition is to getting obese. They found that those children with a higher PRS also tended to have a lower response to feeling full and a larger BMI and waist circumference. More children in the top 25% of obesity risk were overweight than in the lowest 25%.

“These findings indicate that some children are less sensitive to internal ‘fullness’ signals in response to food intake, by virtue of their genetic endowment,” Llewellyn said. “These children are at risk of overeating and gaining excessive weight, putting them at greater risk of overeating, and obesity.”

Parents were often aware of their child’s “fullness” sensitivity, she said, but the study findings should give them more confidence in saying “no” if a child demands more or using ways to help children acknowledge fullness. “For example, if at the end of a meal a child is asking for more food, but the parent believes that the child has had an ample amount to eat, the parent may wish to say ‘no’ to seconds.”

“There are a number of good habits that can be established within the family home to aid responsiveness to internal fullness signals; these include eating at a slower pace (so that natural satiety mechanisms have time to come into play), not eating in front of the television (so that children pay attention to internal fullness feelings), and not routinely having second helpings.”

Twin studies are being increasingly used to work out the broad extent to which differences in characteristics are inherited or gained from our environment. In an accompanying study published in the same journal, this time of non-identical twin pairs, researchers found that babies with bigger responses to food (sight or smell) and lower fullness response grew faster. By 15 months they were up to around 2lbs heavier than their sibling.

Jane Ogden, professor of Health Psychology at Surrey University, said this is why portion size – and parental input – matters.

“Fullness is not only the result of brain and gut chemicals but a perception that is influenced by learning, emotion and distraction,” she said. “If parents feed their children on the go, or in front of the TV they will feel less full and not learn the association between food and mealtimes. But if they are fed at the table, with a plate and at a time called ‘dinner’ they will feel more full.”

In a post on The Conversation, she writes that dealing with child obesity also required other approaches such as exercise. “Childhood obesity is an increasing problem and is caused by a simple imbalance between energy in (food) and energy out (exercise and activity) and any attempt at prevention or cure needs to address this imbalance,” she said.