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Being overweight or obese is an important cause of rising cancer rates among younger adults in England, according to a major new study.
Researchers at The Institute of Cancer Research, London, and Imperial College London, found that while rates of several cancers have been increasing in younger adults over the past two decades, most established behavioural risk factors for these cancers – including smoking, alcohol use, red or processed meats, low fibre intake and physical inactivity – remained stable or declined in England in the years leading up to these diagnoses, making these factors unlikely to substantially explain the increase in cancer cases.
Obesity a key factor
In contrast, overweight and obesity, which have increased steadily since 1995, were identified by the researchers as key factors in the rise of cases in England. The largest increases in obesity were seen in younger women, with a 2.6 per cent relative increase per year since 1995.
However, the new analysis, published in the journal BMJ Oncology, showed that increases in body mass index (BMI) alone are not sufficient to explain the overall rise in cancer incidence, pointing to additional, suspected or currently unknown causes.
Using national cancer registry data in England from 2001–2019, the researchers, from the Cancer Epidemiology and Prevention Research Unit (CEPRU) at The Institute of Cancer Research (ICR) and Imperial, examined incidence trends across 22 cancer types in women and 21 in men. They identified 11 cancers that are increasing among younger adults aged 20–49 and are linked to known behavioural risk factors.
Analysing national trends
All of the 11 cancers identified, other than oral cancer, are known to be linked to excess weight. For most of these cancers, increases in younger adults mirrored trends in people over 50 – where the disease burden remains far higher. However, bowel cancer and ovarian cancer were notable exceptions, rising only in younger age groups.
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Established behavioural risk factors for cancer such as smoking, alcohol use, overweight and obesity, physical inactivity, red and processed meat consumption, and low fibre intake together accounted for an estimated 40–50 per cent of cases of bowel, endometrial, oral or liver cancer, in 2019. However, the results of the study showed that trends for most of these risk factors, except for overweight and obesity, were either stable or improving, making them unlikely to substantially explain recent increases in incidence.
Over the past two decades, smoking among younger adults has fallen by around two per cent a year, alcohol consumption has mostly stabilised or declined, and physical inactivity decreased. Intake of red and processed meat has also reduced, while fibre intake, although still low, has shown gradual improvement.
Bowel cancer rates rising
Bowel cancer rates in younger women linked to BMI rose faster (from 0.9 to 1.6 per 100,000 people) than those not attributable to BMI (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 that additional factors must be contributing to the increase.
Several suspected contributors – including ultra‑processed foods, antibiotic use and air pollution – have been proposed. However, many of these factors have also shown stable or declining trends in the UK, reinforcing the need to examine additional biological and environmental pathways. Emerging evidence suggests that other obesity-related factors not captured by BMI, including metabolic dysfunction, inflammation and gut microbiome may influence bowel cancer risk, but further research is needed to understand whether they cause bowel cancer.
Further research needed
The authors say large, long‑term studies that can capture exposures across the life course, and assess emerging risks, are urgently needed to understand what is driving the increase in early-onset cancers identified in this research.
Professor Montserrat García‑Closas, Co-Director of the Cancer Epidemiology and Prevention Research Unit and Group Leader in Integrative Cancer Epidemiology at The Institute of Cancer Research, London, said:
“Our findings show that while cancer rates are rising in younger adults, the trends are unlikely to be explained by changes in most known behavioural risk factors. Smoking, alcohol and other behaviours have been stable or improving for two decades, yet early‑onset cancers continue to increase – particularly bowel cancer.
“Excess weight is an important contributor, although it cannot fully account for the scale of the rise in bowel and other cancers. This tells us that multiple factors – including early‑life exposures – may be acting together. Understanding these patterns is essential for identifying what is truly driving cancer risk in today’s generations. We now need deeper research, better measurement and continued surveillance to uncover the causes behind these worrying trends.
"However, we cannot wait to act. 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."
Professor Marc Gunter, Co-Director of the Cancer Epidemiology and Prevention Research Unit at Imperial College London, said:
“The changes we’re seeing in cancer incidence, particularly the rates of some cancers in younger adults, don’t have a single cause or a simple answer. They reflect a complex mix of generational effects, gaps in long‑term exposure data, and shifts in diagnosis and detection, and show how much more scientists still need to understand about when and how cancer develops across the life course. While rising rates in younger adults are concerning, it remains crucial not to lose sight of cancer trends in older adults, where the absolute burden of disease is still far greater.”
Professor Kristian Helin, Chief Executive of The Institute of Cancer Research, London, said:
“This work highlights a growing public health challenge and the need for urgent action across research, prevention and policy. Although rising cancer rates in younger adults are concerning, the burden remains overwhelmingly higher in older people, which means prevention efforts must span all ages.
“This study makes clear that traditional lifestyle risks alone cannot explain current trends – pointing to the importance of investigating other exposures such as the potential role of the microbiome, while strengthening strategies to address obesity and other established risks. To protect future generations, we must invest in understanding the causes of cancer at all ages and ensure that early diagnosis, screening and prevention strategies keep pace with a changing population.”
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