Obesity is a key factor in rising cancer rates among under-50s, a major new study suggests, but researchers warn it does not tell the full story. The steady increase in overweight and obesity since 1995 is the strongest known clue, yet the rise in body mass index (BMI) alone cannot account for the overall increase in cancer diagnoses among younger adults in England.
Experts at the Institute of Cancer Research, London (ICR) and Imperial College London analysed national cancer registry data from 2001 to 2019, identifying 11 cancers that have become more common in people aged 20 to 49. These cancers — thyroid, multiple myeloma, liver, kidney, gallbladder, colorectal, pancreatic, endometrial, oral, breast and ovarian — are all linked to lifestyle and behavioural risk factors. With the exception of oral cancer, every one of these is known to be associated with excess weight.
In 2023 alone, around 31,000 cancers were diagnosed in people aged 20 to 49 in England — roughly one in every 1,000 people. By contrast, 244,000 cases were diagnosed in those aged 50 to 79, representing about one in every 100. Among the younger group, breast cancer was the most common (8,500 cases), followed by bowel cancer (3,000) and melanoma skin cancer (2,800). The study, published in the journal BMJ Oncology, found that bowel and ovarian cancer were exceptional: they were rising only in younger adults, while the other nine cancers were increasing in both younger and older age groups, suggesting common underlying causes.
The obesity link to early-onset cancers
The researchers found that most established risk factors for these cancers — including smoking, alcohol consumption, red or processed meat intake, low fibre intake and lack of exercise — were either stable or declining in the years leading up to diagnosis. This makes them unlikely to be the primary drivers of the observed increase. In contrast, overweight and obesity have climbed steadily in England since 1995. The largest increases were seen in younger women, who experienced a 2.6% relative rise per year over that period.
Professor Montse Garcia-Closas, from the ICR, said the data suggested around 15% of bowel cancer in younger people could be attributable to being overweight or obese, with roughly 40% to 50% of cases linked to the combined effect of known risk factors such as obesity, lack of exercise, alcohol and smoking. The team estimated that between 2001 and 2019, about 20% of the increase in bowel cancer could be explained by rising BMI. However, she stressed: “Our main conclusion is that although BMI is our best clue, much of the increase still remains unexplained… most likely what’s missing is not just a single cause unexplained, but it’s likely a combination of multiple factors that act together.”
For 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 increased from 6.4 to 9.6 per 100,000. Similar patterns were seen in men. Yet the overall number of BMI-linked cases remained lower than those not linked to BMI, reinforcing the idea that other factors are at work.
How increased insulin and inflammation may drive cancer
Scientists believe the connection between obesity and cancer may be partly explained by higher levels of insulin and chronic inflammation. Professor Marc Gunter, from Imperial College London, explained that obesity is a known risk factor for around 19 different cancers. “For some of these cancers, including colorectal 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,” he said. “We know people with obesity have higher levels of insulin, and insulin is a growth factor and has been linked to cancer.”
Research specifically links insulin resistance, hyperinsulinemia and chronic inflammation to early-onset colorectal cancer. Elevated levels of insulin and the inflammatory marker resistin have been associated with an increased risk of colorectal cancer, particularly in obese individuals. Professor Gunter noted: “In a recent study, we actually found that insulin in particular might be playing a role in early onset colorectal cancer, and this is actually an area of very active research at the moment.” The biological pathway is thought to involve insulin acting directly on cells to promote growth, while inflammation creates a microenvironment that can encourage tumour development.
Despite these insights, the study acknowledges that the observed increase in cancer rates among younger adults is not explained by BMI alone. Several other suspected contributors have been proposed, including ultra-processed foods, antibiotic use, air pollution, sweetened drinks, childhood obesity, changes to the gut microbiome, early-life or prenatal exposures, and shifts in reproductive patterns such as women having fewer children later in life. However, many of these factors have shown stable or declining trends in the UK, underscoring the need to thoroughly investigate all possible biological and environmental pathways.
Broader data underline the urgency. Cancer incidence rates in UK adults aged 25 to 49 increased by approximately 24% between 1995 and 2019. Globally, a study of 42 countries from 2003 to 2017 found that incidence rates for several cancers rose in both younger and older adults, with the cancer types that increased in both age groups all related to obesity. For colorectal cancer, the increasing trends in younger adults were greater than in older adults in 69% of the countries studied. Worldwide, incidence of these cancers in 14- to 49-year-olds rose by around 79% between 1990 and 2019.
Public health recommendations and the need for action
The researchers are calling for large, long-term studies to identify all the biological and environmental factors driving rising cancer rates in young adults. But they argue the evidence on obesity is already strong enough to make it a public health priority. “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,” said Professor Garcia-Closas. She added: “We cannot wait to act.”
Cancer Research UK’s chief executive, Michelle Mitchell, welcomed the study but cautioned that “overweight and obesity doesn’t explain the rise in full”. She noted that improvements in detection are also likely to be playing a part, meaning more people are being diagnosed at a younger age. Mitchell stressed: “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.”
Cancer Research UK also highlights that a child who is obese is around five times more likely to be obese in adulthood, making early intervention critical. The charity advocates for UK-wide restrictions on advertising for foods high in fat, salt or sugar (HFSS) and restrictions on price promotions for such products. Government initiatives such as the Soft Drinks Industry Levy, calorie labelling, and restrictions on the placement of HFSS foods in supermarkets are already in place, alongside plans to ban junk food advertising on television before 9pm and online. The National Institute for Health and Care Excellence (NICE) recommends supporting people to achieve and maintain a healthy weight through dietary approaches, physical activity, behavioural change, weight loss surgery and medications.
Professor Amy Berrington, from the ICR, offered reassurance: “Although rates have been increasing, cancer in young people is still a rare disease.” Yet the trend demands attention. Professor Charles Swanton, also of the ICR, has described the situation as a “scientific conundrum that urgently needs to be solved.” The study’s authors emphasise that most cancers rising in younger adults are also increasing in older adults, suggesting common causes. For bowel and ovarian cancer, which are rising only in younger adults, the role of obesity appears particularly significant — but it is far from the whole answer.