Obesity may be a key driver of rising rates of 11 cancers in adults under 50, a study has found.

The 11 cancers were thyroid, multiple myeloma, liver, kidney, gallbladder, colorectal, pancreatic, endometrial, oral, breast and ovarian cancers.

All except oral cancer are known to be linked to excess weight, with researchers saying raised insulin levels and inflammation may play a part.

The findings come from researchers at the Institute of Cancer Research, London and Imperial College London, who analysed national cancer registry data for England from 2001 to 2019.

In England, around 31,000 cancers were diagnosed in people aged 20 to 49 in 2023, equal to roughly one in every 1,000 people. This compares with 244,000 cases in the 50 to 79 age group, where the rate is around one in 100.

Concerns have been growing in recent years over rising rates of cancers such as bowel and ovarian in younger adults.

Among the younger group, breast cancer was the most common, with 8,500 cases, followed by bowel cancer at 3,000 and melanoma skin cancer with 2,800 diagnoses.

For nine of the 11 cancers identified, rates are rising in younger adults but also increasing in older adults, who are much more likely to develop the disease. Bowel and ovarian cancer were the exceptions, rising only in younger age groups.

The researchers found that bowel cancer rates in younger women linked to BMI rose faster, from 0.9 to 1.6 per 100,000 people, than those not linked to BMI, which rose from 6.4 to 9.6 per 100,000 people. Similar patterns were recorded for men.

However, the authors noted that the overall number of cases of BMI-linked bowel cancer in younger women remained lower than those not linked to BMI, suggesting other factors must be contributing to the increase.

Several suspected contributors, including ultra-processed foods, antibiotic use and air pollution, have been proposed in recent years. However, many of these factors have also shown stable or declining trends in the UK, the team said.

Despite the rise in several cancer rates among younger adults over the past two decades, most established risk factors, including smoking, alcohol consumption, red or processed meat intake, low fibre diets and lack of exercise, remained stable or even declined in the period leading up to diagnosis.

This suggests these traditional risk factors are unlikely to account for much of the increase in cancer cases.

By contrast, overweight and obesity, which have increased steadily since 1995, could be key factors in the rise in cases. The team suggested that between 2001 and 2019, around 20 per cent of the increase in bowel cancer was explained by increases in BMI over that period.

However, the researchers said rises in BMI alone are not enough to explain the overall increase in cancer among younger adults in England and that there are likely to be other causes.

Data also suggest around 15 per cent of bowel cancer in younger people could be linked to being overweight or obese, with around 40 to 50 per cent in total linked to the combined effect of known risk factors such as obesity, lack of exercise, alcohol and smoking.

Montse García-Closas, professor at the ICR, said more research was needed, but “we cannot wait to act”.

She told a media briefing: “Our main conclusion is that although BMI is our best clue, much of the increase still remains unexplained, and we’ve done some additional analysis that show that most likely what’s missing is not just a single cause unexplained, but it’s likely a combination of multiple factors that act together.”

Amy Berrington, professor at the ICR, said: “Although rates have been increasing, cancer in young people is still a rare disease.”

Marc Gunter, professor at Imperial, said obesity was a known risk factor for around 19 different cancers.

He added: “For some of these cancers, including colorectal (bowel) cancer, we think this could be partly caused by higher levels of hormones such as insulin, which is often elevated in people with obesity, as well as inflammation.

“We know people with obesity have higher levels of insulin, and insulin is a growth factor and has been linked to cancer.

“In a recent study, we actually found that insulin in particular might be playing a role in early onset colorectal (bowel) cancer, and this is actually an area of very active research at the moment.”

The researchers called for large, long-term studies to identify all the biological and environmental factors that could explain rising cancer rates in young adults.

García-Closas added: “Tackling obesity across all ages, particularly in children and young people, through stronger public health policies and wider access to effective interventions, could slow the rise in cancer and prevent many cancers and must become a national priority.”

Michelle Mitchell, Cancer Research UK’s chief executive, said: “Globally, and in the UK, we’re seeing a small increase in cancer rates in adults under 50.

“The picture is complex and we need more research to understand what’s driving the trend, but this study helps to fill in some gaps.

“Overweight and obesity doesn’t explain the rise in full though. Improvements in detection are likely to also be playing a part, meaning that more people are being diagnosed at a younger age.

“Preventing cancer cases must be a priority for the UK government. Smoking remains a leading cause of cancer in adults under 50, which is why the Tobacco and Vapes Bill receiving royal assent this week is such a historic moment.

“Measures to restrict the advertising and promotion of junk food, introducing mandatory reporting and targets on healthy food sales, and making nutritious food more accessible to everyone would all help people keep a healthy weight.”

Ovarian cancer cases are rising among younger adults in England, with bowel cancer showing a similar pattern, a new study suggests.

Researchers said excess weight is a key contributor, but is unlikely on its own to explain the pattern.

The authors wrote: “These patterns suggest that while similar risk factors across ages are likely, some cancers may have age-specific exposures, susceptibilities, or differences in screening and detection practices.”

They added: “Although overweight and obesity are linked to 10 of the 11 cancers evaluated and account for a substantial proportion of cancer cases, both BMI-attributable and BMI-non-attributable incidence rates have increased, though the latter more slowly, suggesting other contributors.”

The study analysed cancer incidence, meaning new diagnoses, in England between 2001 and 2019 across more than 20 cancer types, comparing adults aged 20 to 49 with those aged 50 and over.

Among younger women, cases of 16 out of 22 cancers increased significantly over the period, while among younger men, 11 out of 21 cancers increased significantly.

In particular, there was a significant rise in 11 cancers with known behavioural risk factors among adults under 50. These were thyroid, multiple myeloma, liver, kidney, gallbladder, bowel, pancreatic, endometrial, mouth, breast and ovarian cancers.

Rates of all 11 also rose significantly among adults aged 50 and over, with the notable exceptions of bowel and ovarian cancer.

Five cancers, endometrial, kidney, pancreatic, multiple myeloma and thyroid cancer, increased significantly faster in younger than in older women, while multiple myeloma increased faster in younger than in older men.

The researchers looked at established risk factors including smoking, alcohol intake, diet, physical inactivity and body mass index, a measure used to assess whether someone is underweight, a healthy weight, overweight or obese.

With the exception of mouth cancer, all 11 cancers were associated with obesity. Six, liver, bowel, mouth, pancreatic, kidney and ovarian, were also linked to smoking.

Four, liver, bowel, mouth and breast, were associated with alcohol intake. Three, bowel, breast and endometrial, were linked to physical inactivity, and one, bowel, was associated with dietary factors.

But apart from excess weight, trends in those risk factors over the past one to two decades were stable or improving among younger adults.

That suggests other factors may also play a part, including reproductive history, early-life or prenatal exposures, and changes in diagnosis and detection.

The study noted that red meat consumption fell among younger adults, while fibre intake remained stable or slightly improved in both sexes between 2009 and 2019, although more than 90 per cent of younger adults were still not eating enough fibre in 2018.

Established behavioural risk factors accounted for a substantial share of cancer cases.

Excess weight was the risk factor associated with most cancers in 2019, ranging from 5 per cent for ovarian cancer to 37 per cent for endometrial cancer.

The researchers said the findings were based on observational data, meaning the study could identify patterns but could not prove cause and effect.

They also noted there were no consistent long-term national data for several risk factors, that the analysis was limited to England rather than the UK, and that cancer remains far more common overall in older adults despite the rise in cases among younger people.