Pairwise association of key lifestyle factors and risk of solid cancers - A prospective pooled multi-cohort register study

Smoking, alcohol consumption, obesity, and physical inactivity are key lifestyle risk factors for cancer. Previously these have been mostly examined singly or combined as an index, assuming independent and equivalent effects to cancer risk. The aim of our study was to systematically examine the joint pairwise and interactive effects of these lifestyle factors on the risk of a first solid primary cancer in a multi-cohort prospective setting. We used pooled data from seven Finnish health survey studies during 1972-2015, with 197,551 participants diagnosed with 16,373 solid malignant primary tumors during follow-up. Incidence of any cancer was analyzed separately without and with lung cancers using Poisson regression with main and interaction effects of key lifestyle factors. When excluding lung cancer, the highest risk of any cancer in men was observed for smokers with a BMI of >= 25 kg/m2 (HR 1.36, 95 % CI 1.25-1.48) and in women for smokers consuming alcohol (HR 1.22, 1.14-1.30). No statistically significant interactions between any studied risk factor pairs were observed. When including lung cancer, the highest HRs among men were observed for smokers who consume alcohol (HR 1.72, 1.57-1.89) and among women for smokers who were physically inactive (HR 1.38, 1.27-1.49). Smoking combined with other lifestyle factors at any exposure level resulted in highest pairwise risks, both in men and women. These results highlight the importance of smoking prevention, but also the importance of preventing obesity and reducing alcohol consumption.