Background: Cancer is the leading cause of death in Norway, and lifestyle behaviors are a major contributor to cancer risk. This thesis aimed to investigate the association between a healthy lifestyle index (HLI) and cancer incidence and survival among women in Norway.
Methods: The study used data from the Norwegian Women and Cancer Study, a national prospective cohort of approximately 170,000 randomly selected women. The HLI was constructed based on physical activity, body mass index, smoking habits, alcohol intake, and habitual intake of major food groups. Cox proportional hazard models and restricted cubic splines were used to estimate associations between the HLI score and cancer incidence, HLI score change and cancer incidence, and prediagnostic HLI score and survival.
Results: We observed that a higher HLI score was associated with lower risks of postmenopausal breast, colorectal, lung, postmenopausal endometrial, pancreatic, and kidney cancers, but not postmenopausal ovarian cancer. Regardless of baseline HLI score, greater positive HLI score changes were associated with a lower risk of lifestyle-related cancers combined. Additionally, a higher prediagnostic HLI score was associated with lower all-cause mortality and, weakly, with lower breast cancer mortality among women diagnosed with breast cancer. Associations were also negative, but weak, for women diagnosed with colorectal cancer. No associations were observed for lung cancer mortality. Smoking was particularly strong in driving several associations.
Conclusion: A healthy lifestyle, where smoking avoidance is a priority, should be promoted and facilitated throughout all adult ages to reduce the risk of cancer in the Norwegian general population of women. However, more research is required to understand the potential impact of lifestyle factors and overall lifestyle on cancer survival
