Patient Summary: The Women's Health Initiative randomized trial of calcium plus vitamin D: Effects on breast cancer and arthralgias
This study looked at the question: would calcium and vitamin D supplements decrease risk of breast cancer ? 32,282 women were randomized to receive either 1000 mg calcium carbonate and 400 IU vitamin D or placebo. The women were postmenopausal, with no history of breast cancer for the past 10 years. The study was stopped after 7 years.
The study found no difference in the rates of invasive breast cancer (528 cases in Calcium/D group vs. 546 cases in the placebo group) or non-invasive breast cancer. They also reported no difference in joint pains, which was a secondary question of the study.
Several previous retrospective studies (looking back on Ca/Vit D use) did show a benefit. Possible explanations for the lack of effect on breast cancer incidence in this study include:
- Too much baseline Vit D use (participants were allowed to have used ~750 IU prior to the study) compared with level of intervention (400 IU) to see an effect from intervention
- Not enough Vit D intervention? Need about 3000 IU to achieve serum level of 42 ng/mL, which has been correlated with greatest breast cancer risk decrease
- Too much baseline Ca use before randomization (mean intake was 1165 mg, which is close to optimal amount, 1200 mg)
- Participation in hormone therapy trial may have confounded effect (this study was part of the WHI hormone trial)
- Intervention in the right age group?
At the present time, Ca and Vit D supplementation can not be recommended to prevent breast cancer.