The study referenced above was designed to investigate whether vitamin C intake was associated with survival among Swedish women. The women were divided into four groups (quartiles) based on their vitamin C intake at baseline. The study included 3,405 women with invasive breast cancer in the Swedish Mammography Cohort. A total of 1,055 of the study participants died between 1987 and 2010, of whom 416 died from breast cancer.
Women who had been in the highest quartile of vitamin C intake before diagnosis were found to have a 25% lower risk of breast cancer-specific death compared to those in the lowest quartile. The researchers also found that greater vitamin C intake was associated with lower death from any cause, however this result barely reached statistical significance.
There were 717 study participants for whom data on post-diagnosis supplement use was available. No association was found between vitamin C supplement use (of approximately 1000 mg, on average) and breast cancer-specific mortality. The authors conclude that high dietary vitamin C intake before breast cancer diagnosis may be associated with increased survival. However, vitamin C supplementation after diagnosis (at the level observed in this population) was not associated with survival.
Please see our article on vitamin C and breast cancer for more information.