Cancer diagnoses among Australians under 40 have increased, with rising rates of bowel and thyroid cancer driving the trend, according to data from the Australian Institute of Health and Welfare (AIHW). Between 2000 and 2025, the number of cancer cases in this age group grew from approximately 121 to 135 per 100,000 people.
Bowel cancer has seen the most significant increase, with rates rising from 6.3 cases per 100,000 people to 18.9 over the 25-year period. Associate Professor Dan Buchanan, a spokesman for Bowel Cancer Australia, suggested that changes in lifestyle, the environment, and diet may be contributing factors. These include more sedentary behaviors, exposure to modern environmental factors like microplastics and agricultural chemicals, and Western-style diets high in ultra-processed foods, refined sugars, and red or processed meats, coupled with reduced fiber intake.
"Lifestyle, environment and diet exposures can change the type and composition of bacteria in the gut, which may help drive inflammation and tumour development at an early age and is a popular mechanism being investigated for the rise in cases," Buchanan stated. He clarified that the increase is not primarily due to earlier detection, as many young adults do not participate in the National Bowel Cancer Screening Program. Early warning signs for bowel cancer include rectal bleeding, blood in stool, persistent abdominal pain, changes in bowel habits, unexplained weight loss, and iron deficiency. "Being aware of changes in your body, not ignoring them and raising concerns with your GP is important for early detection and surviving an early-onset bowel cancer diagnosis," he advised.
Thyroid cancer showed the second-largest increase in diagnosis rates during the same period, more than doubling from seven cases per 100,000 to 15.8. Professor Stan Sidhu, head of the University of Sydney Endocrine Surgery Unit, attributed this rise largely to increased detection through highly sensitive imaging technologies like thyroid ultrasound. "However, increasingly sensitive neck ultrasounds are being performed when patients present to their primary care practitioner with neck pain, a lump in the neck or enlarged lymph nodes following a viral illness," he noted.
Professor Sidhu emphasized that unlike bowel cancer, the only known environmental factor increasing thyroid cancer risk is exposure to ionizing radiation. He also stressed the importance of seeking medical attention for warning signs such as a neck lump, difficulty swallowing, or a persistently hoarse voice. Testicular cancer rounded out the top three fastest-growing cancers among Australians in their 30s, with an increase of 2.5 cases per 100,000 people. Conversely, melanoma, cervical, and lung cancer diagnoses have declined. Despite the rise in some cancer diagnoses, overall cancer mortality rates have decreased from 18 deaths per 100,000 people in 2000 to an estimated 11 in 2025.