One-carbon metabolism (1CM) regulates critical processes in carcinogenesis. However, the influence of 1CM-related nutritional and genetic factors on breast cancer (BC) risk remains unclear.

This study aimed to assess the impact of 1CM-related nutrients and genetic variants with BC risk.

Case-control study nested within the European Prospective Investigation into Cancer and Nutrition cohort, including 3067 incident BC cases and 3067 matched controls from 7 European countries. Individual 1CM-nutrient intakes and nutritional scores for B vitamins and all 1CM-related nutrients were evaluated. Dietary intakes were collected via country- and center-specific questionnaires. Tissue-specific polygenic risk scores (PRSs) for lower expression of 1CM genes were developed using expression quantitative trait loci from BC-relevant tissues (mammary, adipose, hepatic, adrenal, ovarian, and uterine). Odds ratios (ORs) and 95% confidence intervals (CIs) were estimated using conditional logistic regression, adjusting for established risk factors. Interactions between nutritional scores and PRSs and between B-vitamin score and alcohol intake were evaluated.

Per 1 standard deviation (SD) of higher intakes of vitamin B2 (OR_1-SD: 0.93; 95% CI: 0.87, 0.99), vitamin B12 (OR_1-SD: 0.94; 95% CI: 0.89, 0.99), and B-vitamin score (OR_1-SD: 0.97; 95% CI: 0.95, 0.99), inverse associations were observed with BC risk. Lower expression of 1CM genes in adipose tissue was associated with higher risk among premenopausal females (OR: 1.12; 95% CI: 1.02, 1.23) but not among postmenopausal females. The inverse association between B-vitamin score and BC risk varied by alcohol consumption (β = 0.010; 95% CI: 0.001, 0.021; P = 0.044); this association was not observed among females consuming >16 g/d of alcohol. No nutritional score-PRS interactions were observed.

Higher intake of B vitamins was associated with reduced BC risk. Adipose tissue-specific downregulation of 1CM genes may differentially influence BC risk by menopausal status. These findings support the biological plausibility of 1CM in breast carcinogenesis and highlight the potential importance of adequate vitamin B intake.