Meg Seymour, PhD
Approximately 40% of Americans are low in vitamin D. Low vitamin D has been linked to a number of health problems, including cancer, heart disease, and heart attack.[2, 3]
Sunshine on your skin (without sunscreen) is a great way to get vitamin D (15 minutes between 10 a.m. and 3 p.m is key), but many Americans don’t get that much sun, especially during colder months. Popular foods that provide vitamin D include fish and fortified milks or cereals. Scientists have studied whether taking vitamin D supplements could decrease the risk of developing health problems for people who do not get enough vitamin D from these natural sources.
Do Vitamin D Supplements Help Prevent Cancer?
Researchers have conducted several long-term clinical trials to determine whether vitamin D supplements can decrease the risk of developing cancer. Randomized, controlled clinical trials are the “gold standard” of research. In these trials, some participants were assigned to take vitamin D supplements, and others were assigned to take a placebo (sugar pill). Then, the researchers measured whether or not the people given the supplements were less likely to develop cancer over time.
A 2018 study found that vitamin D supplements did not prevent cancer. Researchers assigned 5,000 people, ages 50-84, to either take 100,000 IU of vitamin D or placebo once a month for 3 years. After 3 years, they found that monthly supplements of high doses of vitamin D did not decrease the percentage of people developing cancer. An even larger study from 2019, of almost 26,000 patients over the age of 50, also found no benefit. The researchers assigned participants to take either 2000 IU of vitamin D or a placebo every day for over 5 years. People taking vitamin D every day were just as likely to develop cancer as the people taking the placebo.
A meta-analysis published in 2019 also looked at the impact of vitamin D supplements on cancer. A meta-analysis is a type of combination study that combines the results of many smaller studies. The study found that although vitamin D supplements did not prevent cancer, people who took daily vitamin D supplements were less likely to die from cancer. The researchers suggested that even though vitamin D supplements do not prevent cancer, perhaps they affect the way that tumors grow. However, a bigger meta-analysis (combining more than 30 studies totaling 18,000 participants) found no difference in deaths from cancer for those who did or did not take the supplements. The two meta-analyses had different methods and looked at different studies, so further research is needed in order to determine whether or not vitamin D supplements actually can prevent deaths from cancer.
Most people taking vitamin D are taking it combined with calcium. For that reason, it is important to look at research that examines the effect of taking them both. A 2017 study looked at more than 2,300 women ages 55 and up. Half were assigned to take 2,000 IU of vitamin D and 1,500 mg of calcium per day, and the other half were assigned to take a placebo every day. The researchers followed them for 4 years and found no difference between the two groups in the chances of getting cancer.
Why is it that people with low vitamin D are more likely to develop cancer, but vitamin D supplements do not prevent cancer? One possibility is that people low on vitamin D might be different from people with enough vitamin D in ways that are related to an increased risk of cancer. For example, people might be low in vitamin D because they do not go outside and exercise regularly, and people who exercise regularly are less likely to develop cancer . Obese people are more likely to develop cancer, and obese people tend to have lower levels of vitamin D.
Are Vitamin D Supplements Good for Your Heart?
People who do not have enough vitamin D are more likely to develop heart disease and have heart attacks.[13,14] Researchers have conducted clinical trials to find out if vitamin D supplements can help prevent heart disease. The same large study from 2019 that measured vitamin D supplements and cancer also looked at whether or not people taking the supplements had fewer heart attacks. The study found no benefit: there was no difference in the number of heart attacks between those taking vitamin D and those taking placebos.
Since people taking vitamin D supplements often take calcium supplements as well, researchers want to understand if taking both of these supplements affects heart health. The results of these studies are not consistent, with some showing an increase in strokes, and others finding no impact on heart health. More clinical trials on the combination of vitamin D and calcium are needed to draw any conclusions about whether these supplements are helpful or harmful to heart health.
Vitamin D and COVID-19
Research has shown that Vitamin D supplements can help protect against acute respiratory infections like the flu. That is why in 2020, research is underway to determine if vitamin D supplements can help protect against COVID-19. Thus far, the research has found that people low in vitamin D are more likely to have tested positive for the coronavirus than people who are not low in vitamin D. This research is based on studies of people who tested positive at a time when testing in the U.S. was relatively rare and many of the people who were tested did so because they had respiratory symptoms such as coughs or flu-like symptoms. Since these were not clinical trials, it is not clear whether being low in vitamin D makes someone more susceptible to COVID-19 symptoms, or whether old age or other traits increases the chances of having low vitamin D levels and also increases the chances of developing COVID-19. Research is being conducted to determine if vitamin D can help prevent serious symptoms or help patients recover.
Potential Risks of Supplements
Some older research found that taking a combination of vitamin D and calcium increased the risk of kidney stones. However, more recent research has found that there is no increased risk of kidney stones in people taking the combination of vitamin D and calcium. A large clinical study conducted in 2019 found that taking vitamin D supplements alone did not increase the chances of developing kidney stones, upset stomach, or hypercalcemia (too much calcium in the blood).
The Bottom Line
There is not enough evidence to conclude whether taking vitamin D prevents cancer or heart problems. The United States Preventive Services Task Force, a federally funded group that analyzes scientific research, has concluded that there is not enough evidence to say that the benefits of taking supplements, including vitamin D and calcium, to try to prevent heart disease and cancer outweigh the risks.
If you are worried that you are not getting enough vitamin D, talk with your doctor about getting your vitamin D levels tested. If you choose to take a supplement, be sure to talk with your doctor about the amount of vitamin D you are taking. Unless you have a medical need and your doctor recommends it, it is not recommended to take more than 4,000 IU of vitamin D per day. It is better to try to get your vitamins from food or the sun. The Food and Drug Administration requires that food packaging in the United States say what percentage of your daily vitamin D needs are included in a serving of packaged food. Read the labels on your food if you are concerned about getting enough vitamin D in your diet. For more information about vitamin D, what it does for the body, and getting vitamin D from food and the sun, click here.
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- U.S. News and World Report. How Much Time in the Sun Do You Need for Vitamin D? Health.usnews.com. Updated July 2018. https://health.usnews.com/wellness/articles/2018-07-18/how-much-time-in-the-sun-do-you-need-for-vitamin-d
- Dietary Guidelines 2015-2020. Vitamin D: Food Sources Ranked by Amounts of Vitamin D and Energy per Standard Food Portions and per 100 Grams of Foods. Health.gov https://health.gov/our-work/food-nutrition/2015-2020-dietary-guidelines/guidelines/appendix-12/
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- Harvard Health Publishing. Harvard Medical School. Taking too much vitamin D can cloud its benefits and create health risks. health.harvard.edu. https://www.health.harvard.edu/staying-healthy/taking-too-much-vitamin-d-can-cloud-its-benefits-and-create-health-risks. Published November 2017. Updated December 2019.