Introduction and objective:
Shift work increases the risk of breast cancer, but the mechanisms is still under discussion. This study evaluates the relationship between breast cancer and shift work on the basis of overweight and obesity among postmenopausal women.

Material and methods:
We examined this association using data from a case–control study carried between 2015 and 2019. The study involved 111 postmenopausal women with breast cancer and the same number of control participants. A self-reporting questionnaire was used for data collection. Multivariate logistic regression was conducted to find correlations between variables and determine the strength of relationships.

A 2.65-fold risk of breast cancer (OR=2.65; 95% CI: 1.34–5.22) was found among shift work women, compared with postmenopausal women not performing shift work. The association was modified by body mass index, showing a risk rate 9.84 times higher (OR=9.84; 95% CI: 2.14–45.19) among shift work and overweight women, compared to non-overweight women who had never been shift workers.

About 49% of controls and 72% of cases had ever worked in a job that required shift work. The risk of breast cancer in postmenopausal women is associated with shift work, especially among overweight women. Some preventive measures to reduce the risk of breast cancer, in particular regarding a healthy lifestyle and weight control in this group of working women, should be implemented.

This research was supported by the funds for the development of young researchers and Ph.D. students at the Medical University of Lodz (502-03/6-024-0/502-64-100-18) and co- financed by Medical University of Lodz (503/6-024-01/503- 66-001 and 503/6-024-01/503-61-001-19-00).
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