Olive oil consumption and risk of breast cancer: Prospective results from the Moli-sani Study, and a systematic review of observational studies and randomized clinical trials

Background: Breast cancer (BC) is the leading cause of cancer-related death among women worldwide. Olive oil, rich in monounsaturated fats and polyphenols, has been linked to a reduced BC risk, but epidemiological evidence remains limited. This study examined the association between olive oil consumption and BC risk in a large cohort of adult Italian women and conducted a systematic review on this association. Methods: Longitudinal analyses were performed on 11,442 women (mean age 54.7 +/- 11.6 years) enrolled in the Moli-sani Study (2005-2010). Cox proportional hazard models were used to estimate BC risk in relation to olive oil consumption. A systematic review was conducted by searching Scopus, EMBASE, PubMed, and MEDLINE databases up to October 2024 for observational studies and RCTs. Results: Compared with lower olive oil consumption (<= 2 tbsp./day), multivariable-adjusted HRs associated with highest intake (>3 tbsp./d) for overall, premenopausal, and postmenopausal BC were 0.71(95 %CI 0.48-1.05), 0.80 (95 %CI 0.28-2.28), and 0.70 (95 %CI 0.46-1.08), respectively. An increase of 1-tbsp./d of olive oil was associated with a lower risk of ER-and PR-breast cancers (HR=0.32; 95 %CI 0.13-0.77), particularly ER-cases (HR=0.32; 95 %CI 0.15-0.69); additionally, a lowered hazard of HER2-BC incidence was observed at highest consumption of olive oil compared to the bottom category (HR=0.54; 95 %CI 0.31-0.96). The systematic review included 13 observational studies (11 case-control and 2 prospective) and 1 RCT. While case-control studies and the RCT suggested a protective effect associated with olive oil consumption, longitudinal studies reported no association. Conclusions: Findings from the Moli-sani Study suggest an inverse association between olive oil consumption and the risk of hormone receptor-negative breast cancers, particularly ER-subtype, while results were inconclusive for overall BC risk. The systematic review revealed that case-control studies more frequently reported a protective association, whereas prospective studies did not consistently support this finding.