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Childhood weight strongly influenced by genes

by Ed Yong | Analysis

7 February 2008

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A pair of scalesToday, a new study from Cancer Research UK shows that a child’s risk of becoming fat is strongly influenced by their genes. Inherited genes account for 77 per cent of the variation in both children’s BMI and the size of their waistlines.

The research was led by Professor Jane Wardle who directs Cancer Research UK’s Health Behaviour Research Centre at University College London. Wardle has had a longstanding interest in obesity and her research often makes the news. For example, she’s also published work on inherited preferences for food and childhood obesity.

In this study, Wardle compared over 5,000 pairs of twins aged 8 to 12. Twin studies are a common way of assessing the effects of genes on physical traits, like body weight. They rely on comparing identical twins, who share all of their genes, with non-identical twins, whose share only half of their genes.

So if genes are linked to body weight, you would expect identical twins to be closer in weight than non-identical twins and that is exactly what the new study found.

Previous twin studies have reported similar results, but the new study adds to this evidence in several key ways:

  • It looked at children, while most earlier work focused on adults
  • Its sample grew up during the modern ‘obesity epidemic’ and the results show that even in an inactive, high-calorie world, genes still have a major effect on body weight.
  • It estimated the contribution of genes to waist circumference, as well as BMI. Waist circumference is a measure of belly fat, which has particularly strong ties to obesity-related health risks.

“In the genes”?

Any study which finds a strong genetic basis for obesity inevitably gets lambasted for letting overweight people “off the hook”. It’s claimed that these studies only serve to provide an excuse for what is essentially a lack of willpower.

But these complaints misunderstand the complex nature of obesity-related genes. By saying that obesity is strongly affected by genes, scientists are not suggesting that genes directly make you fat, or that the effect is inevitable.

The problem is the increasingly passé nature vs. nurture question, which paints a picture of genes and environment tugging at our bodies and behaviour from opposite directions. The modern view has genes and environment working together to shape behaviour – a case of nature via nurture.

Our genes affect the extent to which our environment influences our lives. Obesity-related genes could affect how we metabolise food or grow fat. They could make us more responsive to the smells or sights of food, less responsive to feelings of being full, or give us preferences for fatty foods over healthier options.

The modern ‘obesogenic’ environment amplifies the effects of these small differences in behaviour driven by genes. In a post-war world where food was scarce, it didn’t matter if some people had a natural tendency to eat more. But when calories are free-flowing, these differences show up more clearly.

The family setting

Obesity then, is a complex disease with complex causes and not something that can be boiled down to a simple “blame-the-individual” stance. But Wardle’s new study shows that a “blame the parents” attitude is equally misguided.

Her paper shows that the environmental factors that affect a child’s body weight were mostly unique to each individual. Shared environmental factors like being served similar food or being given the same options for watching TV only accounted for about 10 per cent of the variation in body weight.

These results match those of earlier studies in adults and strongly suggest that family settings and the home environment are not the root cause of obesity. They provide both support and cause for concern for the Government’s new obesity strategy, which is heavily targeted to the parents of young children.

Based on the new results, changing the home environment is certainly important but it’s unlikely to put a big dent in rising levels obesity by itself.

We’ll also need wider societal measures to change the environment at large and support for individuals who may need to make long-term lifestyle changes to offset any inherited predispositions.