Bowel cancer is increasing in people under the age of 50, according to an international study released today.
New Zealand was one of seven countries included in the study, which is published in an online edition of the Lancet.
Twelve hundred New Zealanders die from bowel cancer a year and screening is being rolled out in stages to those aged 60 to 74.
The study said the overall incidence of bowel cancer was decreasing in many high-income countries, including New Zealand, but that the cancer in adults under 50 was rising.
In the most recent 10-year period, bowel cancer increased in the age group by just over 3 percent in Denmark, by almost 3 percent in New Zealand and Australia, and close to 2 percent in the United Kingdom.
At the same time, the incidence of bowel cancer decreased in those 50 to 74 by almost 3.5 percent in New Zealand, 1.5 percent in Australia, and almost 2 percent in Canada.
The study said more research was needed to establish the root causes of the rising incidence in young adults so potential preventative and early-detection strategies could be worked out.
It also said the rise in the cancer rate in young people probably reflected a growth in risk factors.
Those could include "increases in the prevalence of excess bodyweight, consumption of red and processed meat, consumption of alcohol, cigarette smoking, cooking meats at high temperatures, and dietary fat intake, and decreases in physical activity and dietary fibre intake.'
The study added: "Evidence also suggests that antibiotic use during childhood ... could be associated with a rise in paediatric-onset inflammatory bowel diseases."
The study said genetics alone could not fully explain the changes.
Dunedin medical oncologist Chris Jackson, a co-author of the study, is also medical director of the Cancer Society.
He said for people over 50, rates of bowel cancer were stable or falling slightly, but the risk was still high.
However, for those under 50, "and particularly those born after 1990, the rates of colon and rectal cancer are on the rise. While this is part of a worldwide trend, New Zealand is showing higher rates than some comparable countries".
Dr Jackson said the cancer typically occurred after 50, with just 10 percent in those under that age.
"So this rise reflects a small number of people affected but [it] is a worrying trend."
Dr Jackson said more research was needed to better understand the causes.
"New research is showing that the bugs that live in our gut, the microbiome, play a pivotal role in cancer risk and immunity against cancer. We need to dedicate more research to understanding the role of the microbiome better."
He said New Zealanders of all ages should be more aware of bowel cancer symptoms and see a doctor if they were concerned.
They should also eat a balanced diet, be physically active, reduce or avoid alcohol, quit smoking and maintain a healthy weight to help prevent bowel cancer.
He said Māori and Pacifica felt the burden of the cancer in particular.
More targeted screening and priority programmes were needed, he said.
"New Zealand has worse cancer outcomes than Australia and the UK, yet incredibly cancer isn't one of the government's health priorities."
Eight district health boards are screening for bowel cancer in those 60 to 74 years of age. Two more DHBs will begin by year end.