New trial aims to find mechanism underlying low vitamin D and increased breast cancer risk

A recent randomized controlled trial found that vitamin D supplementation did not reduce breast density, a risk factor for breast cancer.

Breast density is measured with a mammogram by comparing the amount of fat to the amount of tissue. Research suggests breast density is an important predictor of a woman’s breast cancer risk. Women with high breast density are four to five times more likely to develop breast cancer than women with low breast density. However, it is unclear whether reducing breast density will decrease breast cancer risk.

A study found that vitamin D intake is linked to reduction in breast density; although not all studies observed this relationship. Furthermore, research has shown that vitamin D deficiency is associated with a faster growth of breast cancer in mice, and that vitamin D supplementation is associated with a 33% increased survival rate in breast cancer.

Therefore, a recent double blind randomized controlled trial assessed whether vitamin D supplementation over a period of one year would reduce mammographic breast density among premenopausal women in Canada.

A total of 405 women were randomized to receive 1,000, 2,000 or 3,000 IU daily of vitamin D3, or a dummy pill daily. The researchers measured breast density before and after the intervention.

The researchers found that all groups experienced the same change in breast cancer density after one year, with the 3,000 IU daily group experiencing the least reduction. No significant differences between self-reported health problems were observed during this time (p > 0.05).

Adherence to vitamin D was not at fault, with a nearly 93% compliance rate.

The researchers concluded,

“In premenopausal women, the reduction seen in breast density after one-year supplementation with 1,000 IU, 2,000 IU or 3,000 IU [daily] vitamin D3 was not greater than that seen in the placebo arm. Thus, at these doses, vitamin D supplementation could not reduce breast cancer risk through reductions in breast density.”

This trial simply eliminated one proposed mechanism underlying the relationship between vitamin D and breast cancer, leaving researchers to continue their search for answers.


Tovey, A. & Cannell, JJ. New trial aims to find mechanism underlying low vitamin D and increased breast cancer risk. The Vitamin D Council Blog & Newsletter, June 6, 2017.


Brisson, J. et al. A randomized double-blind placebo-controlled trial of the effect of vitamin D3 supplementation on breast density in premenopausal women. Cancer epidemiology, biomarkers & prevention, 2017.

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