Youth-Onset Cancer and Modifiable Risk: Quantifying the Combined Impact of Obesity and Alcohol on Breast and Colorectal Cancer in Italy (18-34 Years)

Background Rising rates of cancers diagnosed before the age of 50 have been reported worldwide, including in Italy. Among modifiable risk factors, obesity and alcohol consumption independently and synergistically increase the risk of breast and colorectal cancers, including early-onset disease. However, the population-level burden attributable to these exposures among adolescents and young adults (AYA) remains poorly quantified. Methods We integrated a narrative review with a quantitative Population Attributable Fraction (PAF) analysis to estimate the proportion of early-onset breast and colorectal cancers attributable to overweight/obesity, high-risk alcohol consumption, and their combined exposure in the Italian population aged 18–34 years. Sex- and age-specific prevalence data were derived from the national surveillance systems (“PASSI”), while relative risks were obtained from meta-analyses and large cohort studies. Combined PAFs were estimated assuming independence between exposures and tested through sensitivity analyses varying prevalence and relative risk assumptions. Results Among young adults aged 18–34 years, overweight and obesity together accounted for a substantial proportion of early-onset colorectal cancer cases, while high-risk alcohol consumption showed the largest individual attributable fraction. The combined contribution of excess body weight and alcohol consumption reached 12.2% of early-onset colorectal cancer cases. For early-onset breast cancer in young women, obesity and high-risk alcohol consumption were associated with PAFs of 2.8% and 4.6%, respectively. Sensitivity analyses confirmed the robustness of the estimates and indicated that uncertainty in relative risk assumptions represented the main source of variability. Conclusions Obesity and alcohol consumption contribute meaningfully to the burden of early-onset breast and colorectal cancers in Italy. Integrating weight management and alcohol risk assessment into clinical practice and public health strategies targeting young adults could prevent a measurable proportion of these cancers and help counteract the rising incidence of malignancies at young ages.