The number of under-50s being diagnosed with bowel cancer is increasing worldwide, according to research that also reveals rates are rising faster in England than almost any other country.
For the first time, global data suggests doctors are seeing more young adults develop early-onset bowel cancer, from Europe and North America to Asia and Oceania.
An increase in rates was reported in 27 of the 50 countries examined, with the greatest annual increases seen in New Zealand (4%), Chile (4%), Puerto Rico (3.8%), and England (3.6%).
Experts are still in the early stages of understanding the reasons behind the rise. The authors of the study, published in the Lancet Oncology, said consumption of junk food, high levels of physical inactivity and the obesity epidemic were likely to be among the factors.
"The increase in early-onset colorectal cancer is a global phenomenon," said Hyuna Sung, a senior principal scientist in cancer surveillance research at the American Cancer Society and lead author of the study. "Previous studies have shown this rise in predominately high-income western countries, but now it is documented in various economies and regions worldwide."
The escalating trend of bowel cancer among young adults is now so significant that it could also lead to higher incidence in older people, among whom rates have been stable or falling - potentially reversing decades of progress made against the disease.
"The global scope of this concerning trend highlights the need for innovative tools to prevent and control cancers linked to dietary habits, physical inactivity and excess body weight," said Sung.
"Ongoing efforts are essential to identify the additional factors behind these trends and to develop effective prevention strategies tailored to younger generations and local resources worldwide."
The study found bowel cancer rates in people aged between 25 and 49 rose in 27 of the 50 countries studied in the decade to 2017, the most recent year for which figures were analysed.
Young women were found to have faster increases in early bowel cancer rates than men if they lived in England, Norway, Australia, Turkey, Costa Rica or Scotland.
Bowel cancer is the third most diagnosed cancer and the second most common cause of cancer death, responsible for more than 1.9m new cases and almost 904,000 deaths in 2022 worldwide.
Michelle Mitchell, the chief executive of Cancer Research UK, said: "This flagship study reveals that increasing rates of early-onset bowel cancer, affecting adults aged 25 to 49, is a global issue.
"Concerningly, this research has revealed for the first time ever that rates are rising more sharply in England than in many other countries around the world. A cancer diagnosis at any age has a huge impact on patients and their families - so while it's important to note that rates in younger adults are still very low compared to people over 50, we need to understand what's causing this trend in younger people."
There were several limitations to the study. It reported the rates of bowel cancer only up to 2017, so may not accurately reflect current trends. The study also used data from subnational registries that often represent a small fraction of a country's population, which may limit the generalisation at the population level.
David Robert Grimes, an assistant professor of biostatistics at Trinity College Dublin, who was not involved with the research, urged caution in interpreting the findings. "Comparing international data about cancer rates is a difficult undertaking, as there is considerable variation in data quality and availability ... we have to resist the urge to jump to conclusions, especially with conflicting and complicated data," he said.
The rate at which bowel cancer in the under-50s was increasing was also much smaller than England in Wales (1.55%), Scotland (0.64%) and Northern Ireland (0.54%), raising further questions about the data.
Katrina Brown, a senior cancer intelligence manager at Cancer Research UK, said it was "hard to say for certain" why rates in England were increasing faster than the other UK nations. "More research is needed to understand whether there are genuine differences between the nations, and how to address them," she said.
She added that the overall number of cases in young adults was still low, with only about one in 20 bowel cancers in the UK diagnosed in people aged under 50.
Sung said it was critical that more people know the symptoms. "Raising awareness of the trend and the distinct symptoms of early-onset colorectal cancer (eg rectal bleeding, abdominal pain, altered bowel habits, and unexplained weight loss) among young people and primary care providers can help reduce delays in diagnosis and decrease mortality," she said.