Ask an Expert: Can vitamin D prevent breast cancer?

Q: “A friend forwarded an article to me suggesting that vitamin D can reduce the risk of getting breast cancer. Is this true?”

Answer from Alison Conlin, M.D., medical oncologist, Providence Cancer Center: I spoke on this topic at the Komen Issues Conference on March 14, 2010. Vitamin D deficiency has been found to be potentially linked to many conditions, including cardiovascular disease, high blood pressure, diabetes, colon cancer, and now breast cancer. The key word here is potentially. Although associations have been observed, no study to date has been able to prove a link between taking vitamin D and preventing breast cancer.

A lot of the current breast cancer research in this area is observational and associative, meaning that it is based on observations and associations, rather than controlled studies. A number of these studies have looked at a large group of people who have breast cancer and observed that many of them are vitamin D deficient, suggesting a possible association between vitamin D deficiency and breast cancer. More than one study has found that up to 77 percent of women are deficient in vitamin D at the time of their breast cancer diagnosis. However, as it turns out, 66 to 75 percent of all Americans may be vitamin D deficient, so it's hard to know whether the deficiency is causing cancer or is just something that's very prevalent in general.

A stronger association has been found in women who have recurrences of breast cancer. One very well done study found that women who were deficient in vitamin D at the time of their breast cancer diagnosis had worse outcomes, including higher rates of recurrence and death from their cancers. But these findings were also associative, so there may have been other factors that contributed to those outcomes, as well.

The question that these associative studies raise is: Is it the vitamin D, or the company it keeps, that affects the risk of breast cancer? Do people who take vitamin D and other supplements lead healthier lives overall, thereby reducing their risk of cancers? Do people with low levels of vitamin D also have other problems that increase their risk for breast cancer? For example, vitamin D deficiency has also been associated with problems like obesity, which we know is a risk factor for breast cancer.

The gold standard for a study would be to put a large group of women on vitamin D supplements and then compare their rates of breast cancer diagnoses, over time, with a second group of women who don't take the supplements. In fact, several studies have done just that, with conflicting results.

A very large study, called the Women's Health Initiative, compared women on calcium and vitamin D supplements to women taking a placebo and found no differences in the rates of breast cancer diagnoses.

A smaller study in Nebraska divided post-menopausal women into three groups: one group took calcium and vitamin D supplements, one took only calcium, and one took a placebo. This study found that both of the groups that took calcium — whether in combination with vitamin D or alone — had fewer breast cancer diagnoses than the placebo group. It's possible that there may have been a benefit from the vitamin D, but this study offered no proof, since the women who got calcium without vitamin D did better, as well.

Pre-clinical studies of vitamin D's effects on cells do offer reason to believe that vitamin D could be related to cancer cell activity. Vitamin D regulates a great number of genes that control cell growth, proliferation, turnover and turnoff — the same cellular activities that cancer affects. But to date, we still don't have the gold-standard proof to confirm that giving people vitamin D will prevent breast cancer. That evidence just doesn't exist.

So what should women do about vitamin D?
Because we know that vitamin D deficiency is linked to so many problems — not just breast cancer, but also osteoporosis, hypertension, diabetes, and all causes of mortality — it makes sense to make sure that you are not deficient. I recommend that everyone work with their primary care provider to find out what their blood level of vitamin D is, and take supplements if the level is low.

If your vitamin D blood levels are good, you only need to check them about once a year to make sure you're staying on course. Don't measure during the summer and fall, when you're making lots of vitamin D from sunlight. Wait until the darker months of winter and early spring to find out what your lowest levels are.

If you do find that you are deficient, ask your primary care provider to guide you in making changes to bring your levels up, and give the changes some time to take effect before you measure your levels again.

What is considered the ideal blood level of vitamin D?

There is no standard blood level of vitamin D that is best for every person. In general, for bone health, most clinicians recommend a level of 15 to 20 ng/ml or higher.

Because one study did show a higher rate of cancer recurrence in breast cancer patients who had vitamin D levels below 29 ng/ml, a target level of 30 ng/ml might be a good idea for people who have had breast cancer. The same may be true for people at risk for colon cancer. But I haven't seen any studies that would lead me to push for levels higher than 30 for anyone.

What's the best way to increase your vitamin D levels?

Because we live in a place where it's very difficult to get enough sunshine to make vitamin D year-round, we have to get vitamin D from supplements or food. However, it's pretty hard to get vitamin D from food. Oily fish, like salmon (with the skin) and mackerel, are the best food sources. Fortified milk isn't that great — you'd really have to drink a lot of it to get enough vitamin D. So supplements, we're now learning, may be the best way.

The current recommendations for vitamin D intake are pretty low — the National Institutes of Health recommends 200 IU daily until you're 50, then 400 IU from age 50 to 70, and 600 IU after age 70, but those numbers are likely to be increased very soon. They're currently based on the amounts needed to prevent rickets, which occurs when blood levels go lower than 10 ng/ml.

But just because the current recommendations are low, that's not to say that you should start taking massive doses of vitamin D, either. It is a fat-soluble vitamin, and you can overdose on it.

Rather than take a lot more, I recommend that people start by taking it more effectively. Most people don't know that to get the biggest bang for your buck, you should take vitamin D supplements with food — specifically, with fat, and preferably with your biggest meal of the day.
In general, my best advice is to work with your primary care provider to achieve the right level for your optimal health.

April 2010

Ask an Expert is a public education forum only.

Ask an Expert does not respond directly to your questions or provide personal medical advice, diagnoses, treatment recommendations or second opinions through our Web site or by e-mail. Please talk with your health care provider about any questions specific to your medical care.