Healthy lifestyle and risk of breast cancer for indigenous and non-indigenous women in New Zealand: a case control study

Background: The reasons for the increasing breast cancer incidence in indigenous Maori compared to non-Maori New Zealand women are unknown. The aim of this study was to assess the association of an index of combined healthy lifestyle behaviours with the risk of breast cancer in Maori and non-Maori women. Methods: A population-based case-control study was conducted, including breast cancer cases registered in New Zealand from 2005-2007. Controls were matched by ethnicity and 5-year age bands. A healthy lifestyle index score (HLIS) was generated for 1093 cases and 2118 controls, based on public health and cancer prevention recommendations. The HLIS was constructed from eleven factors (limiting red meat, cream, and cheese; consuming more white meat, fish, fruit and vegetables; lower alcohol consumption; not smoking; higher exercise levels; lower body mass index; and longer cumulative duration of breastfeeding). Equal weight was given to each factor. Logistic regression was used to estimate the associations between breast cancer and the HLIS for each ethnic group stratified by menopausal status. Results: Among Maori, the mean HLIS was 5.00 (range 1-9); among non-Maori the mean was 5.43 (range 1.5-10.5). There was little evidence of an association between the HLIS and breast cancer for non-Maori women. Among postmenopausal Maori, those in the top HLIS tertile had a significantly lower odds of breast cancer (Odds Ratio 0.47, 95% confidence interval 0.23-0.94) compared to those in the bottom tertile. Conclusion: These findings suggest that healthy lifestyle recommendations could be important for reducing breast cancer risk in postmenopausal Maori women.