Vitamin D helps the body absorb calcium, which is essential for good bone health. Vitamin D also helps the immune, muscle, and nervous systems function properly. Most vitamin D is made when an inactive form of the nutrient is activated in your skin when it's exposed to sunlight. Smaller amounts of vitamin D are in fortified milk and other foods, fatty fish, and eggs. As more and more people spend most of their time out of direct sunlight or wearing sunscreen when they are in the sun, vitamin D production from sun exposure is limited.
Vitamin D may play a role in controlling normal breast cell growth and may be able to stop breast cancer cells from growing. Still, research results on vitamin D and breast cancer have been mixed, possibly because the studies have been small. Some studies have suggested a link between low vitamin D levels and breast cancer growth and worse survival, while others have found no link between vitamin D levels and breast cancer risk or survival after being diagnosed.
Now a large study suggests that there is no link between vitamin D and overall survival or disease-free survival in women diagnosed with breast cancer with a high risk of recurrence (the cancer coming back). Still, there are questions about the study.
Overall survival is how long women live, with or without breast cancer coming back. Disease-free survival is how long women live without breast cancer coming back.
The research was presented on May 31, 2014 at the American Society of Clinical Oncology 2014 Annual Meeting. Read the abstract of “Prognostic associations of 25OH vitamin D in NCIC CTG MA.21, a phase III adjuvant RCT of three chemotherapy regimens (EC/T, CEF, AC/T) in high-risk breast cancer (BC).”
The MA.21 trial is looking at several chemotherapy regimens to treat women diagnosed with node-positive and high-risk node-negative breast cancer that hasn’t spread outside the breast to see which regimen offered the best outcomes. As part of the study, 934 of the 2,104 women in the study had blood drawn after surgery to remove the breast cancer but before they started chemotherapy to measure their vitamin D levels.
The women who had their vitamin D levels checked and the women who didn’t have their vitamin D levels checked were different in several ways. Women who had their vitamin D levels checked were more likely to:
- be white
- have had a mastectomy
- be diagnosed with estrogen-receptor-positive, HER2-negative disease
The women who had their vitamin D levels checked had an average level of 27.9 ng/mL (nanograms/milliliter) before they started chemotherapy, which the researchers considered normal. According to the National Institutes of Health, normal vitamin D levels can range from 30 to 74 ng/mL. Many vitamin D researchers recommend a level of 40-60 ng/mL. So while the researchers for this study said the women’s average vitamin D levels were normal, many other researchers would consider them low.
To see if vitamin D levels were linked to outcomes, the researchers used categories created by the Institute of Medicine in 2010:
- deficient (lower than 12 ng/mL)
- meeting the needs of half the population (20 ng/mL)
- meeting the needs of 97.5% of the population (21-50 ng/mL)
- raising a concern of toxicity (higher than 60 ng/mL)
The researchers also put women into categories used before the Institute of Medicine’s categories were created:
Depending on which classification system was used, between 10% and 20% of the women were considered vitamin D deficient.
When the researchers compared the vitamin D levels of the women to their outcomes, vitamin D seemed to have no effect on:
- disease-free survival
- overall survival
- the risk of dying from breast cancer
While this study suggests that vitamin D doesn’t affect breast cancer outcomes, there are some questions about the study:
- The women’s vitamin D levels were measured only once, just before they started chemotherapy. So we don’t know if chemotherapy lowered the women’s vitamin D levels, though other studies have found that chemotherapy is linked to vitamin D deficiency.
- Many doctors consider the women’s starting vitamin D levels inadequate. So in their eyes the study is comparing low vitamin D levels to low vitamin D levels. They say this is why there appears to be no effect on breast cancer outcomes.
- The women who had their vitamin D levels measured didn’t fully represent all the women in the MA.21 trial. This means there could be other reasons why the vitamin D levels didn’t seem to affect outcomes.
If you’ve been diagnosed with breast cancer, you might want to ask your doctor about your vitamin D levels and talk about whether a vitamin D serum level test is a good idea for you. This test is done with a simple blood test that your doctor can order for you when you’re in for a routine physical.
Before you take any supplements, talk to your doctor about the risks and benefits of the product, as well as what a good vitamin D serum level is for you. If your level is low and you start taking a supplement to get back into the normal range, have your vitamin D level checked a few months later and adjust your supplement dose accordingly. Taking too much vitamin D occasionally can cause you to have too much calcium in your blood.
For more information on vitamin D, including daily recommendations and foods rich in vitamin D, visit the Low Vitamin D Levels page in the Breastcancer.org Lower Your Risk section.
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