Risk Factors for Early-Onset Colorectal Cancer: The Role of Diet, Lifestyle and Obesity

Background. Colorectal cancer (CRC) is the third most commonly diagnosed cancer globally, and remains the second leading cause of cancer-related mortality. The incidence of early-onset colorectal cancer (EOCRC) among young adults before the age of 50 is rising worldwide, with EOCRC rates in New Zealand increasing by 26% per decade overall and by 16% in Maori. The underlying causes remain unclear although environmental and lifestyle factors are suspected contributors. The study investigated associations between known risk factors and the development of CRC in a New Zealand population, with a focus on tumour location and age at diagnosis. Methods. A retrospective case-control study was conducted in Canterbury, New Zealand comparing recently diagnosed CRC patients (n = 304) with age- and sex-matched community controls (n = 627). Data on diet, obesity, physical activity, smoking, alcohol consumption, and family history were collected via a self-reported questionnaire. Logistic regression was used to assess associations between risk factors, tumour location, and age at diagnosis. Results. CRC patients had significantly higher rates of obesity (BMI ≥ 30 kg/m²; OR 1.47, p = 0.020), positive family history (OR 1.49, p = 0.040), sugary drink (OR 1.78, p < 0.001) and fast food consumption (OR 1.57, p = 0.007), heavy alcohol intake (OR 3.05, p = 0.004), and lower levels of physical activity (OR 1.51, p = 0.011) compared with controls. Left-sided tumours (69.1% of cases) were significantly associated with obesity (OR 1.57, p = 0.015), family history (OR 1.57, p = 0.042), physical inactivity (OR 1.56, p = 0.016), and alcohol use (OR 2.46, p < 0.001). Processed meat consumption was significantly associated with EOCRC (OR 2.70, p = 0.019). Conclusions. Modifiable factors, particularly sugary drink and fast food intake, obesity, alcohol use, and physical inactivity significantly associate with CRC risk in New Zealand, particularly for left-sided and early-onset disease. Familial predisposition further compounds this risk. These findings highlight the need for targeted prevention strategies that combine lifestyle modification with genetic risk assessment