More Australians under 40 are being diagnosed with cancer than ever before - as experts reveal the possible causes

Published: 04:27, 18 July 2026 | Updated: 04:27, 18 July 2026 Australians under 40 are being diagnosed with cancer more than ever before, with experts pointing to lifestyle and diet changes as potential factors.  Rising rates of bowel and thyroid cancer have largely driven the increase in diagnoses among those in their 30s, according to data from the Australian Institute of Health and Welfare (AIHW) this week. The figures show the number of cases grew from 121 to about 135 per 100,000 people in that age group between 2000 and 2025.  Leading the surge is bowel cancer, with rates rising from 6.3 cases per 100,000 people to 18.9 during that period. Bowel Cancer Australia spokesman Associate Professor Dan Buchanan told the Daily Mail changes to lifestyle, the environment and diet were possible contributors. '[This is] more sedentary behaviours at work and home, modern environmental factors such as microplastics and agricultural chemicals, and a modern western-style diet,' he said. He added that the latter includes diets high in ultra-processed foods, refined sugars and red or processed meats, along with reduced fibre 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,' he said. Rising rates of bowel and thyroid cancer have driven an increase in cancer diagnoses among those in their 30s, according to AIHW data (stock image) Bowel Cancer Australia spokesman Associate Professor Dan Buchanan told the Daily Mail changes to lifestyle, the environment and diet were possible contributors While some assume doctors are simply detecting bowel cancer earlier, Associate Professor Buchanan said it was not driving the increase.  'It is clear the rising rates are not a result of increased detection because many young Australians under 45 do not participate in the National Bowel Cancer Screening Program,' he said.  He urged young Australians to be aware of potential warning signs and seek medical advice if symptoms persist. 'Common 'red flag' warning signs include bleeding from the rectum, blood in the stool, abdominal pain that does not go away, changes in bowel habits, unexplained weight loss and iron deficiency,' he said. '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.' The AIHW data also revealed thyroid cancer saw the second largest surge in diagnosis rates during the 25-year period, more than doubling from seven cases per 100,000 to 15.8.  Head of the University of Sydney Endocrine Surgery Unit Professor Stan Sidhu said this was not due to an actual rise in the disease.  'It reflects increased detection of existing disease due to highly sensitive imaging modalities such as thyroid ultrasound,' he said.  Head of the University of Sydney Endocrine Surgery Unit Professor Stan Sidhu stressed the importance of not ignoring possible warning signs of thyroid cancer  'In Australia, thyroid cancer screening using ultrasound is not encouraged and thyroid cancer guidelines discourage investigation of nodules smaller than 1cm. '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.' Unlike bowel cancer, Professor Sidhu said the only environmental factor known to increase the risk of thyroid cancer was exposure to ionising radiation, similar to that seen after the Chernobyl disaster. He also stressed the importance of not ignoring possible warning signs. 'If you feel a neck lump or report unusual symptoms such as difficulty swallowing or a persistently hoarse voice, see your primary care practitioner, who will examine you and arrange appropriate imaging,' he said. Rounding out the three fastest-growing cancers among Australians in their 30s over the past 25 years was testicular cancer, with an increase of 2.5 cases per 100,000 people. Melanoma, cervical cancer and lung cancer all recorded declines in the number of diagnoses. Cancer mortality rates have also dropped from 18 deaths per 100,000 people in 2000 to an estimated 11 in 2025.