Recent research indicates that a child’s local environment may inhibit their natural urge to exercise.
It is often suggested that environmental factors affect children’s exercise, but experts have found it hard to quantify. Studies which altered the environment to increase exercise opportunities have been inconclusive, so Professor Jane Wardle from University College London, UK and her team tried a new approach, looking at twins.
In the journal PLoS One, they explain that twin studies are a type of “natural experiment” that can show the effect of environmental factors, while controlling for genetic differences.
If identical twins have more similar activity levels than non-identical twins, it means their genes are playing a role. When they are living apart, any further difference in their activity levels is likely due to their environment.
Twin studies of activity in adults point to a “heritability” rate of 48 to 71 per cent, with a relatively small influence from the environment. But this cannot be assumed to generalize to children, say Professor Wardle and colleagues, because “the genetic contribution may show increasing expression with age”. They believe adult twin studies are likely to underestimate the environmental effect on children’s activity levels.
So the researchers measure the environmental and genetic influence on fidgetiness (parent-rated), enjoyment of activity (parent-rated), and physical activity (measured by a worn device) for 117 pairs of 9 to 12 year-old twins. They predicted that fidgetiness and enjoyment of activity would be heritable, but that objectively-measured daily activity would show a strong environment effect.
The results agreed with this prediction. Most of the variation in how fidgety the children were and how much they enjoyed exercise depended on genes, but the actual amount of exercise they got was mainly influenced by their family, neighborhood or school environment. It accounted for 73 per cent of the variation in activity levels.
“Whether a child enjoys being active or not may be influenced by temperament or physical skills which themselves show genetic influences,” say the researchers.
But they found that objectively-measured activity was entirely environmentally determined. This finding agrees with two previous studies and is consistent with a recent review which concluded that alterations to the environment can increase children’s physical activity.
“This does not mean that genetics do not influence childhood physical activity in any circumstance,” they add, “simply that the environment was the dominant influence on day-to-day activity in our study.”
“This research shows us how important it is to encourage exercise in schools and at home,” said Professor Wardle. “Some children may inherit versions of genes that make them naturally more likely to enjoy sports and exercise. But their environment is the most powerful factor in determining how active they actually are.”
Professor Wardle previously found that the genetic risk for obesity is transmitted to the next generation “partly through differences in activity preferences”.
She carried out a study of 214 pairs of twins aged 4 to 5 years, whose parents were either obese/overweight or normal/underweight. “Activity preferences” were measured by parents completing questionnaires.
Children from the obese/overweight families had a much stronger preference for sedentary activities, and spent more time in sedentary pastimes. The researchers say this places children at risk of becoming overweight.
These children also had a greater preference for fatty tastes, which “would increase the risk of overeating when foods are as plentiful and palatable as they are in Western industrialized countries today”.
“It might also be less risky if matched by a desire for high levels of physical activity, but these results suggest the opposite – the high-risk children preferred sedentary activities and spent more time engaged in sedentary pastimes, as indicated both by time spent at the TV and computer, and parent ratings of activity levels.”
“Because the children from the families with obese parents were not yet overweight, differences observed in the two types of families are more likely to be causes than effects of obesity,” the authors write in The International Journal of Obesity.
They conclude: “If behavioral risk factors were found to mediate genetic influences on eating and weight, it might point to new behavioral interventions that could disrupt the pathways between genes and obesity. Finding genetic markers predicting obesity risk more precisely could facilitate environmental and behavioral interventions targeted to children most likely to profit from them.”
Fisher, A. et al. Environmental influences on children’s physical activity: quantitative estimates using a twin design. PLoS ONE, published online April 21, 2010.
Wardle, J. et al. Food and activity preferences in children of lean and obese parents. The International Journal of Obesity, Vol. 25, July 2001, pp. 971-77.