Supplement use, vitamin C intake and breast cancer risk in UK women

Background: When analysing relationships between breast cancer risk and vitamin C intake, few prospective studies have included vitamin C intake from supplements, or excluded general supplement users in dietary only analyses. For the first time these relationships are explored in UK women, and from diary recordings.

Methods: The UK Women’s Cohort Study was used in prospective breast cancer risk analyses examining exposures from: general supplement use; fruit and vegetable intake; and dietary only vitamin C intake recorded at baseline using FFQs and additional questions for 33,000 women (~1,000 cases); vitamin C contained in supplements recorded by diaries at phase 2 for 11,000 women (239 cases); and total vitamin C intake from diet and supplement recorded by diaries in pooled UK nested case-control studies (851 cases 2727 controls).

Results: There was no evidence of dose-response relationships between breast cancer risk and vitamin C intake from diet, supplements or both, or from fruit and vegetable intake; risk estimates were non-significant and generally close to unity. There was some evidence that risks differed by menopausal status and supplement use. There were no significant associations for non-users of supplements or post-menopausal women by continuous estimate or intake category. Risks were raised for pre-menopausal women who were: frequent users of supplements containing low vitamin C (1-60mg/d) (HR=2.37; 95% CI: 1.32, 4.27; p=0.004); daily multivitamin users (HR=1.51; 95% CI: 0.90, 2.54); or general supplement users (HR=1.14; 95% CI: 0.91, 1.43), compared to non-users of these supplements. Women with a history of breast cancer were significantly more likely to be high dose vitamin C users (≥1000mg/d).

Conclusion: There was no evidence of significant associations between vitamin C intake per se and breast cancer incidence, even at high doses. The increased risk for pre-menopausal women taking supplements containing low dose vitamin C may be due to other ingredients.