Calcium + vitamin D don’t lower breast cancer risk

Some researchers have observed an association between higher calcium and vitamin D intake and a lower breast cancer risk.

Here are the results of the first randomized study to evaluate this apparent association.

First, the details.

  • 36,282 postmenopausal women enrolled in a Women’s Health Initiative study were randomly assigned to take 1000 mg of elemental calcium with either 400 IU of vitamin D3 daily or placebo.
  • Treatment lasted about 7 years.
  • Blood levels of 25-hydroxyvitamin D were measured at the start of the study.

And, the results.

  • The incidence of invasive breast cancer was similar in both groups.
  • Taking vitamin D had no effect the risk of breast cancer risk.
  • 25-hydroxyvitamin D blood levels at the start of the study were not associated with breast cancer risk after adjusting for BMI and physical activity.

The bottom line?
The authors concluded, “Calcium and vitamin D supplementation did not reduce invasive breast cancer incidence in postmenopausal women. In addition, 25-hydroxyvitamin D levels were not associated with subsequent breast cancer risk.”

Why do these findings take precedence over past reports?

Earlier reports that suggested benefit from taking vitamin D and calcium were observational. They differ from this randomized study because participants in an observational study are… you know… observed for certain outcomes. There’s no attempt to affect the outcome by randomly assigning a portion of the participants to a specific treatment.

In a randomized clinical trial, the participants are assigned by chance to separate groups that compare different treatments. Using chance to assign people to groups means that the groups are more likely to be similar, which permits a more objective assessment of the findings.

It’s the process of randomization that leads us down the most direct path to identifying a causal inference from study results. Observational studies report associations, not cause and effect. It’s the latter that we’re interested in.

Dr. Thomas Perneger from the University Hospitals of Geneva in Switzerland has listed 10 reasons to conduct a randomized study.

11/18/08 21:15 JR

Hi, I’m JR

John Russo, Jr., PharmD, is president of The MedCom Resource, Inc. Previously, he was senior vice president of medical communications at, a complementary and alternative medicine website.