New cases of obesity are rising fastest in younger adults in England, a study suggests.

Rates of new diagnoses for people in their 30s were nearly 20% higher in 2024-25 than they were in 2019-20, according to the study published in The Lancet. For people in their 20s, new cases jumped by 16%.

These increases were higher than in older age groups – although the most common ages for diagnosis remained the 40s and 50s.

But experts said the trend towards earlier diagnosis was worrying given obesity increases the risk of illnesses such as diabetes, heart disease and cancer.

Alongside the age-group breakdown, the team also identified significant differences between different ethnic groups – earlier onset of obesity was more common in non-white people – while the areas with the highest levels of deprivation also saw steeper rises.

These patterns, when it comes to obesity, are well established, but the acceleration in new cases among younger adults was more unexpected, the researchers said.

Lead researcher Robert Fletcher said while the study did not investigate the causes behind the rise, he felt there were three key factors.

He said adults in their 20s and 30s were ones who had grown up during a period when there was a boom in the unhealthy food market.

“They have been surrounded by unhealthy food in their formative years. On our high streets there has been a proliferation of takeaways and fast food outlets and unhealthy food has been heavily advertised as these age groups have been growing up.”

He also said the pandemic followed by the cost of living crisis was likely to have had a bigger impact on younger age groups.

“The stress of looking after children, while working from home and then being able to afford healthy food as inflation increased, has made healthier lifestyles more difficult,” he added.

Sarah Perman, from the Association of Directors of Public Health, agreed. She said unhealthier options now dominated “our supermarkets, corner shops and fast food outlets”.

“From infancy, children and young people are bombarded with advertising that encourages an unhealthy diet.

“It is also far cheaper and easier for individuals to consume foods and drinks high in fat, sugar, and salt than healthier alternatives.”

Research by the Food Foundation has found healthy foods are twice as expensive per calorie than unhealthy foods.

Katharine Jenner, of the Obesity Health Alliance, said the environment people in their 20s and 30s have grown up with has had a huge influence on “habit-forming behaviours”.

She said the digital environment was also likely to be playing a role with younger age groups more likely to be using food apps and spending time on social media, while the pandemic disrupted physical activity and other habits at a “crucial moment”.

The team – from the University of Cambridge, British Heart Foundation and George Institute for Global Health – analysed 55 million adult NHS patient records to identify who was being diagnosed with obesity. Those who were already recorded as obese were excluded.

Among those aged 30 to 39 there were 24.1 new cases diagnosed per 1,000 people in 2024-25 compared to 20.3 in 2019-20.

Among those aged 20 to 29 there were 20.3 new cases per 1,000, up from 17.5 five years earlier.

Those were the most rapid rises of all the age groups. In fact rates of new cases fell in those aged 60-79.

Researchers said this could be down to the use of weight loss drugs as these age groups were more likely to be able afford them as access on the NHS is restricted.

Overall the proportion of people recorded as obese rose from 26.2% to 30.3% during the period.

Prof Sir Michael Marmot, from University College London, who is one of the country’s leading experts on inequality, described the findings as concerning.

He said it was “further evidence” of how since the pandemic inequalities had widened.

A Department of Health and Social Care spokesperson said the government was taking “decisive action” on obesity, including new restrictions on junk food advertising and targets on healthy food sales.