Australia's bowel cancer rates are world's highest for under 50s. Scientists wonder if the gut microbiome is to blame

The Guardian

28-05-2025

Australia's rates of bowel cancer in people under 50 are the highest in the world, though the reason why remains unclear, experts say.
As incidences of what's known as early-onset bowel cancer are increasing worldwide, a study of 50 countries – published recently in Lancet Oncology – revealed Australia was ranked worst.
An early release of findings from an Australian study published in medRxiv this week showed the incidence of bowel cancer is up to three times higher among Australians born in the 1990s compared with the 1950s cohort.
The study, led by the University of Melbourne, analysed government data on all diagnoses of bowel cancer (also known as colorectal cancer) in Australia from 1990-2020.
The study, yet to be peer-reviewed, found 28,265 cases of early-onset bowel cancer over the 30 years studied. An estimated 4,347 additional cases were attributable to the rising rates of the disease.
While over 50s make up the majority of diagnoses since the most significant risk factor for cancer is age, these rates have begun to decline. National screening was lowered from 50 to 45 in 2023.
Early-onset cases, however, are increasing by up to 8% per year, the study found.
Dan Buchanan, an associate professor and co-author on the medRxiv paper, said Australia's world-leading rates of early-onset bowel cancer were 'not a good gold medal to have'.
He said there was no solid evidence, only 'guesses' as to what had changed over time.
The cancer registries where the data came from did not capture individual-level risk factors, which was a limitation of the study, authors said.
While rising rates of obesity, diabetes and increasingly sedentary lifestyles have been hypothesised to be contributing to the increase, Buchanan said he and other researchers were particularly interested in the role of the gut-microbiome.
'We think that the lifestyle dietary changes are … allowing nasty bacteria to colonise in our gut, and that then is contributing to the development of a substantial proportion of early onset bowel cancer.'
Buchanan said Australians' diets could be playing a role. Compared to other nations, Australians have high consumption rates of processed foods which increasing evidence indicates negatively affects the microbiome.
Increased antibiotic use in childhood and other changes to how humans get their foods, including pesticides and microplastics, have also been proposed as reasons, but more research is needed, Buchanan said.
Buchanan and his colleagues are attempting to 'work backwards' from analysing the tumours to understand the source of the DNA damage, and to discover a mechanism rather than just an association driving the disease.
In the absence of that knowledge, it is important to raise awareness of the signs and symptoms, Buchanan said, particularly among young people.
Bowel cancer is now the leading cause of death in people aged 30 to 45 in Australia.
Julien Wiggins, the CEO of Bowel Cancer Australia, said she was concerned about the rising early onset cases. 'The data is now starting to really unfortunately confirm what we've been seeing and catching up.'
Dr Lisa Mielke, the head of the mucosal immunity and cancer laboratory at the Olivia Newton-John Cancer Research Institute, said: 'This is one of the most comprehensive studies to break down the characteristics of this increase in bowel cancer using data from Australians.
'The study highlights that it is not just one type of bowel cancer that is on the rise in younger people. Overall, the incidences of bowel cancers in all regions of the large bowel, rectum and anus are increasing,' Mielke said.