Overweight children have lower intelligence and are more likely to be depressed, controversial study finds.

Scientists in the US, who tracked more than 5,000 nine to 11-year-olds, found pupils with a higher body mass index (BMI) performed worse in tests.

And they were also more likely to have symptoms of depression, researchers said. 

But it is not clear if a poor diet harmed brain development or if an under-developed brain caused over-eating.  

Previous studies have also suggested BMI is associated with alterations in the brain's prefrontal cortex development. Deficits in working memory may contribute to poor dietary decisions. 

The researchers from Washington University in St Louis, Missouri assessed data from more than 2,500 girls and 2,700 boys between 2016 and 2018. 

Over a follow-up of two years they discovered kids who scored one point lower on picture and vocabulary tasks had a higher BMI of 0.012 (1.6 per cent) on average. 

Children classed as overweight or obese — who had a BMI above the 85th percentile — were almost twice as likely to have 'more problems annually than those with normal weight', scientists said. 

Writing in the journal JAMA Pediatrics, researchers said the findings highlight the 'importance of cognitive and mental health' to weight gain.

They also called on clinicians to monitor children who are overweight or obese 'for increased depression problems'. 

Future research could explore why this is the case and why some do not adhere to a healthy diet or lifestyle. 

Experts have long warned that changes to kids diets and a lack of physical activity have created 'the perfect storm' in hindering child development. 

Professor Iain Buchan, a public health expert from the University of Liverpool, told MailOnline: 'If you look at the change in our energy balance, the way we eat, what we eat, the way we move around and relate to each other, all these things have changed dramatically in the last 20 years.

'Although we are more connected we are also more sedentary as everything is done sitting down, online, or fixed to a device.

'Our bodies didn't evolve to spend so little energy. We need to move around and interact physically and spend time disconnected in order to reflect.'