The rise of cheap junk food and sedentary lifestyles has led to an alarming increase in obesity among children. Picture: Wikimedia

For years, well-off parents had portly children, while those growing up poor were considerably thinner.

But the rise of cheap junk food and sedentary lifestyles has turned this trend on its head, a study has found.

In the decades since the Second World War, disadvantaged children have become more likely than their richer peers to be overweight or obese.

The study by University College London tracked more than 56,000 children born in England, Scotland and Wales in 1946, 1958, 1970 and 2001, and looked at how the height and weight of children and teenagers changed between the post-war years and 2016. The participants’ socioeconomic status was determined by using what their fathers did for a living.

The researchers found that while children with poorer backgrounds used to be shorter, they have narrowed the gap thanks to the easy availability and affordability of food. However, their unhealthier lifestyles have seen their body mass index (BMI) scores increase.

Lead author Dr David Bann blamed the considerable changes to diets and physical activity levels since the post-war years, adding that the study showed previous policies to reduce childhood obesity have not worked.

‘Without effective interventions, childhood BMI inequalities are likely to widen further throughout adulthood, leading to decades of adverse health and economic consequences,’ he said. Bold action is needed, such as creating further incentives for food manufacturers to reduce sugar and fat content in food and drinks, reduce the advertising of unhealthy foods to children and families, and incentivise the sale of healthier alternatives.’

In the post-war years, poorer 11-year-olds were an average of 4.4?lb (2?kg) lighter than their wealthy peers, while poor children born most recently weighed 4.6?lb (2.1?kg) more than those who are better off.

This weight difference increased with age, according to the study published in the Lancet Public Health journal. It found the poorest children still tend to be shorter, but the difference has narrowed in seven-year-olds from 3.9cm in 1946 to 1.2cm in 2001.

A third of children in the UK are overweight or obese by age 11, and the rates are creeping up despite government vows to tackle the crisis. The researchers warn that without drastic action to curb sugar and fat content in food, the widening gap will only get worse.

Tam Fry, from the National Obesity Forum, said the study showed the least advantaged were adopting the least healthy lifestyles. He added: ‘The end of rationing – and simply more food – enabled the poorest substantially to catch up in height. The downside is that their staple diet has become progressively worse.

‘The researchers are quite correct to call for a sea change if further obesity is to be avoided in deprived areas.’