Overweight children are less intelligent than their colleagues.
Overweight children perform worse in tests
Scientists in the US have tracked more than 5,000 kids aged between nine and 11 and found that those with a higher body mass index (BMI) performed worse in tests.
However, it has not been established if a poor diet stunts brain development or if an under-developed brain leads to excessive eating.
Boffins have warned that a "quick fix environment" of unhealthy diets and sedentary lifestyles is causing problems in the way children develop.
Professor Iain Buchan, a public health expert at the University of Liverpool, told MailOnline: "Life has become so different in terms of pace and food has changed too: we now have unhealthy diets, with high fat, high salt and high sugar junk foods that provide rapid energy, and that's not good for health.
"We have created a quick fix environment that's all about short-term pleasure and it's causing long-term pain."