Tracy Rupp, PharmD, MPH, RD and Mingxin Chen, MHS
Although people all over the world can develop cancer, cancer patients are more likely to survive in areas of the world that receive the most sun.1 Since our skin makes vitamin D when exposed to sun, researchers wondered if vitamin D protects against cancer. New research suggests that vitamin D may help women diagnosed with breast cancer to survive the disease.
The evidence for the role of vitamin D in breast cancer
In November 2016, a study published in a major cancer journal looked at the association between vitamin D levels and survival in 1666 women with newly diagnosed invasive breast cancer in California. Among the participants, women with the highest vitamin D levels in their blood (the top one-third among the women in the study) were 28% less likely to die from all causes as compared to women with the lowest vitamin D levels (bottom one-third) in their blood. The association between vitamin D and survival was even stronger in premenopausal women: those with the highest vitamin D levels were 55% less likely to die from all causes and 63% less likely to die from breast cancer, as compared to premenopausal women with the lowest vitamin D levels.2
These results are similar to a study published in 2014, which also found that women with higher levels of vitamin D were more likely to survive breast cancer. This study used meta-analysis to pool the results from 5 previously published studies of the relationship between vitamin D levels and mortality from breast cancer. The study found that among 4443 breast-cancer patients, women with the highest vitamin D levels (about 30 ng/mL) were about half as likely to die from breast cancer as those with the lowest levels (less than 20 ng/mL).3
Since both studies found that women with higher vitamin D levels were more likely to survive the disease, we wonder: could the chances of improving survival really be so simple? Not necessarily. These two studies can’t tell us which came first: breast cancer or low vitamin D levels. For example, it’s possible that breast cancer causes vitamin D levels to drop. That’s one of the reasons it would be premature to recommend more vitamin D for women diagnosed with breast cancer.
The evidence for the role of vitamin D in melanoma
A study published in 2016 found that low levels of vitamin D may result in worse outcomes for patients diagnosed with the type of skin cancer called melanoma.4 In this study, melanoma patients who had vitamin D levels less than 20 ng/mL were more likely to have larger tumors and more advanced disease than melanoma patients with higher levels of vitamin D. The researchers also examined inflammation and found that low vitamin D levels predicted poor outcomes for patients regardless of their levels of inflammation.
This result may seem very surprising, since sunlight exposure increases vitamin D and also increases the risk of developing skin cancer. A study is ongoing in Belgium to see whether vitamin D supplements will reduce the chances of skin cancer returning or worsening.5 While it’s too early to recommend widespread vitamin D supplements for skin cancer, it’s reasonable to check vitamin D levels in patients with melanoma or who have been treated for melanoma. If their vitamin D levels are low, a supplement is an easy way to try to bring levels into the normal range.
What is vitamin D?
Vitamin D helps the body use calcium and phosphorus to make strong bones and teeth. Our bodies make vitamin D when our skin is exposed to direct sunlight. We can also benefit from the vitamin D that is added to milk and cereals.
How much vitamin D is recommended for healthy people?
Approximately one-third of children and adults in the U.S. (over 1 year of age) do not get enough vitamin D.6 The Institute of Medicine recommends the following daily amount of vitamin D for average healthy adults:7
- For those between 1 and 70 years of age, including women who are pregnant or lactating, the recommended dietary allowance (RDA) is 600 IU per day.
- For those 71 years or older, the recommendation is 800 IU per day.
Experts agree that just 15 minutes of sun at mid-day in the summer is enough. Of course, this varies based on how much skin is exposed (darker skinned people may need more time), the time of the day (mid-day is best for vitamin D), altitude (the higher the altitude you are at the more vitamin D your body can make). It is also more difficult to get enough make enough vitamin D from the sun during the winter. If you live anywhere north of Los Angeles, then you really can’t get much vitamin D from November to March when the sun is very low in the sky. Thus, we have to rely on the vitamin D we were able to store up during the summer or the vitamin D we can take in through our diets and supplements.
How much vitamin D is too much?
Given the possible link to reducing cancer, you might wonder if you should take vitamin D supplements even though the results of these studies are not conclusive. It is important to remember that too much of any nutrient, including vitamin D, can be unhealthy. The safe maximum of vitamin D for adults and children older than 8 years of age is about 4000 IU per day.8
Dietary supplements are more likely than foods to provide too much vitamin D. Although too much sun exposure is dangerous because of skin cancer, it will not cause vitamin D toxicity.