A major study from Cancer Research UK has found that children who spend more time online and watching commercial TV are more likely to pester their parents for junk food, to buy it themselves and to be overweight or obese.
The charity and researchers from the University of Liverpool surveyed 2,471 children aged seven to 11, as well as their parents, to measure their screen time, purchase requests, pocket money expenditure, consumption and physical activity.
For both TV viewing and internet use (defined as anything other than using the internet for homework), the study divided the children into low (less than 30 minutes per day), medium (30 minutes to three hours) and high (more than three hours) viewing categories. It also separated the results for commercial and non-commercial TV.
The study found that children in the "high" group of commercial TV viewers were 59% more likely to be overweight or obese, while those in the "medium" group were 45% more likely. The impact of internet use was even greater: 79% more likely in the high group and 53% in the medium group.
For non-commercial TV, there was no significant increase in either the medium or high group, suggesting it is advertising that accounts for the increased risk.
Dr Jyotsna Vohra, Cancer Research UK's head of cancer policy research, said the study showed that it was "vital" that the UK government introduced a pre-9pm watershed ban on advertising for foods and drinks high in fat, salt or sugar (HFSS).
The research also found that children with high viewing of commercial TV were far more likely to pester their parents for advertised products, to buy HFSS products with their own pocket money and to consume those products more frequently.
The high group were 166% more likely to pester their parents, according to the parents. Children in the high group were also 193% more likely to say they bought HFSS products with their own money.
They were almost four times more likely to buy chocolate, the biggest increase among the product categories. And when it came to consumption, the high group was 163% more likely to consume sugary drinks, and 109% more likely to consume confectionery.
In each case, children in the medium group were more likely to report the behaviour than the low group, but less likely than the high group.
In most cases, the study found that non-commercial TV had no significant impact on children’s behaviour. The exception to this was on rates of children buying HFSS products with their pocket money: the study found that non-commercial TV also had an impact, although not as big that for commercial TV.
The study found similar impacts from high or medium internet use. Children in the high group were 191% more likely to pester, according to their parents.
But when it came to buying HFSS products with their pocket money, internet use had a much more severe impact on children’s behaviour, with the high group more than six times more likely to buy bakery items, biscuits and crisps.
Dr Emma Boyland, a lead researcher from the University of Liverpool, said: "Young children who spend more time on the internet and watching commercial TV are more likely to pester for, buy and eat unhealthy food and drinks.
"Parents are all too familiar with being nagged for sweets and fizzy drinks in the supermarket or corner shop. Our research shows that this behaviour can be linked to the amount of time children spend in front of a screen and, as a result, the increased number of enticing adverts they see for these sorts of products."
Stephen Woodford, chief executive of the Advertising Association, said that it should come as no surprise to the report's authors that children who spend more than three hours per day online are more likely to be obese or overweight.
"This reinforces the point that exercise and targeted intervention at community level are the absolute priority when tackling UK childhood obesity levels," he added.
Woodford stressed that ad restrictions put in place a decade ago had already led to less exposure by children to HFSS advertising.
"They already ban the advertising of HFSS food or drink products in all media where under 16s make up more than 25% of the audience," he said. "This includes TV, online, social media, on the street or on public transport. But the prevalence of obesity among children has continued to rise consistently over the same period."