Cancer rates are rising among adults aged 20 to 49, with obesity emerging as a significant factor behind the increase, a study suggests.
Researchers found that 11 cancers are becoming more common in this age group: thyroid, multiple myeloma, liver, kidney, gallbladder, colorectal, pancreatic, endometrial, oral, breast and ovarian cancers.
Of those 11 cancers, all except oral cancer are already known to be linked to excess weight, the researchers said.
While the team cautioned that obesity does not fully explain the trend and that more evidence is needed, they said being overweight or obese appears to play an important role, potentially through raised insulin levels and inflammation.
Concerns have been growing in recent years over the rising incidence of cancers such as bowel and ovarian cancer in young adults.
In England, about 31,000 cancers were diagnosed in people aged 20 to 49 in 2023, or roughly one in every 1,000 individuals.
This compares with 244,000 cases in the 50 to 79 age group, where the rate was about one in 100.
For nine of the 11 cancers, rates are increasing in younger adults but are also rising in older adults, who are much more likely to develop the disease.
Bowel and ovarian cancer were the exceptions and were rising only in younger age groups.
Using cancer registry data for England from 2001 to 2019, the team found that bowel cancer rates in younger women linked to body mass index, or 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.
BMI is a measure based on height and weight used to estimate whether someone is underweight, a healthy weight, overweight or obese.
Similar patterns were recorded for men.
Experts from the Institute of Cancer Research, London, and Imperial College London conducted the study.
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 ultraprocessed 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, reinforcing the need to look closely at all possible causes, the team said.
Prof Montse García-Closas from the Institute of Cancer Research said more research is needed, but “we cannot wait to act”.
García-Closas said the data suggested around 15 per cent of bowel cancer in younger people could be down to being overweight or obese, with around 40 per cent to 50 per cent in total linked to the combined effects of known risk factors such as obesity, lack of exercise, alcohol and smoking.
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.”
Among the younger cohort, 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.
The researchers found that 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 factors are unlikely to account for much of the observed 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 researchers found.
However, the study found that increases in BMI alone are not enough to explain the overall rise in cancer among younger adults in England.
There must be other additional causes behind the rise, whether suspected factors or ones not yet identified, the team said.
Most cancers with rates rising in younger adults are also increasing in older adults, suggesting there may be common causes affecting both age groups.
The team suggested that, between 2001 and 2019, around 20 per cent of the increase in bowel cancer was explained by rising BMI over that period.
Prof Amy Berrington said: “Although rates have been increasing, cancer in young people is still a rare disease.”
Prof Marc Gunter said obesity is 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 biological and environmental factors that could explain rising cancer rates in young adults.
However, they said the case is already strong enough on obesity to make it a public health priority, particularly when it comes to bringing down obesity rates in children and young people.
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.”
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