The study found that women with blood levels of serum 25-hydroxyvitamin D (OH) -- the main form of vitamin D in blood -- above 60 ng/ml (nanograms per millilitre) had one-fifth the risk of breast cancer compared to those with less than 20 ng/ml.
Thus, researchers from the University of California-San Diego determined that the minimum healthy level of 25(OH) in blood plasma should be 60 ng/ml, instead of the earlier recommended higher than the 20 ng/ml.
"Increasing Vitamin D blood levels substantially above 20 ng/ml appears to be important for the prevention of breast cancer," said lead author Sharon McDonnell from GrassrootsHealth, a non-profit public health research organisation.
The study, published in the journal PLOS ONE, analysed data from two randomised clinical trials with 3,325 combined women and a prospective study involving 1,713 women with average age of 63.
Participants were free of cancer at enrollment and were followed for a mean period of four years. Vitamin D levels in blood were measured during study visits.
"This study was limited to postmenopausal breast cancer. Further research is needed on whether high 25(OH)D levels might prevent premenopausal breast cancer," said Cedric F. Garland from UC-San Diego.
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