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.
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 relatively large study suggests a link between higher vitamin D levels in the blood at the time of a breast cancer diagnosis and better long-term survival.
The research was published online on Nov. 10, 2016 by JAMA Oncology. Read “Association of Serum Level of Vitamin D at Diagnosis With Breast Cancer Survival: A Case-Cohort Analysis in the Pathways Study.”
The Pathways study was started in 2006 and is ongoing. It is following a group of breast cancer survivors to look at long-term outcomes and factors associated with recurrence and survival. The women were assessed when they first entered the study -- usually about 2 months after being diagnosed with breast cancer -- and then again at 1 year, 2 years, 4 years, 6 years, and 8 years after diagnosis.
One of the factors the researchers measured was the women’s levels of 25-hydroxyvitamin D (25OHD), a metabolite of vitamin D that is in the blood. The more 25OHD in the blood, the higher the women’s vitamin D levels.
In the study reviewed here, the researchers looked at the 25OHD levels of 1,666 women in the Pathways study, as well as the women’s long-term outcomes:
- cancer recurrence
- diagnosis of a second primary cancer
- death from breast cancer
Most of the women in the study were diagnosed with stage I or stage II hormone-receptor-positive breast cancer:
- 824 women had stage I disease
- 606 women had stage II disease
- 202 women had stage III disease
- 34 women had stage IV disease
- 1,226 women had hormone-receptor-positive disease
- 440 women had hormone-receptor-negative disease
Compared to women with lower levels of 25OHD, the researchers found that women with higher 25OHD levels tended to have better outcomes. Specifically:
- Women with higher levels of 25OHD had better overall survival. Overall survival is how long a woman lives, with or without the cancer growing/recurring.
- Premenopausal women with higher levels of 25OHD had:
- 63% better breast cancer-specific survival (meaning a woman didn’t die from breast cancer)
- 48% better recurrence-free survival (how long a woman lived without the cancer coming back)
- 42% better invasive disease-free survival (how long a woman lived without a recurrence and without a second primary cancer being diagnosed)
Even after the researchers adjusted the data for other factors that could affect 25OHD levels, such as age, obesity, race and ethnicity, socioeconomic status, and cancer characteristics, the relationship between higher 25OHD levels and better outcomes was still there.
"We found that women with the highest levels of vitamin D levels had about a 30% better likelihood of survival than women with the lowest levels of vitamin D," said Lawrence H. Kushi, Sc.D., research scientist with the Kaiser Permanente Northern California Division of Research and principal investigator of the study. "The more we know about vitamin D, the more we understand that it may play a key role in cancer prevention and prognosis," he continued. "This study adds to the evidence that vitamin D is an important nutrient."
Still, it’s important to know that this study was an observational study, not a randomized study. So while the results are interesting, more research is needed before all women who’ve been diagnosed with breast cancer start taking vitamin D supplements.
If you’ve been diagnosed with breast cancer, you might want to ask your doctor about the results of this study and whether checking your vitamin D levels makes sense for your unique situation. If your vitamin D level is low, you and your doctor can decide if a vitamin D supplement is right for you. You also can ask your doctor about getting more direct sunlight exposure. Even short periods of direct peak sun exposure -- 15 minutes, 3 times a week, for example -- can give you more than the recommended daily amount of vitamin D. It’s also impossible to overdose on vitamin D from the sun. Still, while sun exposure offers vitamin D benefits, it does have risks. Sun exposure can increase your risk of skin cancer, including melanoma, the most dangerous type.
For more information on vitamin D levels and how they can affect breast cancer, visit the Low Vitamin D Levels page in the Breastcancer.org Lower Your Risk section.