Finally, we have the results of a large scale randomized, placebo-controlled clinical trial to tell us whether or not vitamin D can reduce the risk of breast cancer.
The study, reported in today’s issue of the Journal of the National Cancer Institute, concludes that there is no evidence that vitamin D decreases breast cancer incidence in post-menopausal women.
But I will bet you dollars-to-doughnuts (well, maybe not doughnuts—they are fattening) that this study isn’t going to provide closure to the hotly-debated question of whether or not vitamin D reduces breast cancer risk.
The study was part of the larger Women’s Health Initiative which was designed to look at the impact of hormone therapy on the health of post-menopausal women. As part of that study, close to 18000 post-menopausal women were randomly assigned to take 1000 mg of calcium and 400 IU (international units—the recommended daily allowance for vitamin D) daily. The other 18,000 women took placebos.
The problem is that both groups of women were allowed to take extra vitamin D and calcium, and a good number of them did just that—although the number of women who did so was basically the same in both groups. By year 6 of the trial, about half of the women in both groups were taking extra vitamin D.
The end result was that over the seven years of this study, there was no difference in the frequency of breast cancer between the two groups. The vitamin D did not make a difference in how often women developed breast cancer.
An editorial that accompanied the report pointed out some of the problems with the study, and which I would also consider important:
- There were large number of women took vitamin D and calcium on their own, even if they were selected as part of the vitamin D treatment group or as part of the placebo group.
- Breast cancer takes a long time to develop in most women. Seven years may simply not be enough time to detect a benefit that may have been seen if the study was longer.
- It is possible based on the results of other studies that vitamin D would be more effective if given to pre-menopausal women as opposed to post-menopausal women as was the case in this study.
- Some experts recommend a fairly high dose of vitamin D to prevent cancer which is much greater than that studied here (however, if women were taking vitamin D on their own, it is quite possible that they were taking doses greater than 1000 units a day).
In the same editorial, the authors wrote:
“Because preclinical, epidemiological, and clinical trial results of vitamin D supplementation are conflicting, additional studies will be needed to determine whether vitamin D plus calcium will prevent breast cancer…Future clinical trials should address the above questions to help determine whether higher does of vitamin D supplements will be cancer preventive. The potential health benefits of vitamin D and calcium may yet have a bright future.”
Another interesting finding pointed out by the authors of the research report was that vitamin D blood levels measured at the beginning of the study didn’t necessarily relate that closely to the amount of vitamin D that the women had been taking in the past. High vitamin D intake did not necessarily mean high vitamin D levels in the blood. That raises the question of how much vitamin D our bodies absorb when we take supplements. Those amounts may be different in different people, and may be controlled by other factors such as our genetic makeup.
The authors concluded:
“Such results suggest that factors other than dietary and supplement intake of vitamin D likely influence (blood levels of vitamin D)…Before future clinical trials of high-dose vitamin D regimens to reduce breast cancer risk are implemented, it will be important to demonstrate that the selected vitamin D dose can definitively increase (blood vitamin D) levels to the projected target level. Definitive assessment of factors that influence the relationship between vitamin D supplement use and subsequent changes in circulating (vitamin) D levels are therefore a research priority.”
The bottom line?
When it comes to vitamin D there are a lot of questions and not many answers.
For now, we would be hard-pressed to conclude that vitamin D reduces the risk of breast cancer in post-menopausal women.
There is still a lot we don't know: we don't know how much vitamin D is healthy in our bodies, and if in fact there are risks associated with too much vitamin D (see prior blog). We don't know whether or not we should be routinely measuring vitamin D in our bodies with blood tests. We don't know how much vitamin D we should be taking every day, and whether each person responds the same way to the same dose of vitamin D. We don't know whether vitamin D reduces cancer risk or influences outcome after cancer is diagnosed and for which cancers, if any.
Unfortunately, when it comes to vitamin D, confusion reigns supreme. We deserve better.