Many studies have hinted that vitamin D might help ward off cancer. Some, for example, have found that people with higher blood levels of the vitamin have lower rates of certain cancers, including colon and breast cancers.
But those types of studies cannot prove that taking vitamin D actually causes cancer risk to drop, explained Dr. JoAnn Manson, of Brigham and Women’s Hospital and Harvard Medical School in Boston.
That, Manson said, takes clinical trials that test vitamin D against an inactive placebo.
That’s exactly what the new study did, but it found no significant benefit.
The trial involved 2,300 older women who were randomly assigned to take either high-dose vitamin D plus calcium, or placebo pills.
Over the next four years, about 4 percent of women on the supplements were diagnosed with cancer. That compared with just under 6 percent in the placebo group — a difference that was not statistically significant, the study authors said.
“This is a null study,” said Manson, who wrote an editorial published with the findings. “We can’t make any public health recommendations based on this.”
And it’s not the first such trial to come up empty, she added.
“The clinical trials to date have been disappointing,” Manson said. “Overall, we have no compelling evidence that vitamin D reduces cancer incidence.”
However, she stressed, the new trial is “by far not the final word.”
The problem is, the trials done so far have had limitations — small study groups or fairly low vitamin D doses, for instance.
Two large-scale trials, each involving upwards of 20,000 people, are still ongoing, said Manson, who is leading one of the studies.
Those trials should offer more definitive answers in the next year or two, according to Manson.