Ask the doctor
Q. I’ve been taking vitamin D supplements for several years because my doctor said they might help me live longer. Am I wasting my money, or even causing myself harm?
A. You’re right to be skeptical. Vitamin supplements of all types repeatedly have been tested for theoretical health benefits, and often the studies have shown none. There’s no doubt that vitamin D supplements can help prevent or treat osteoporosis. Theoretically, they might reduce the risk of various diseases. What about the question you asked: whether vitamin D supplements help you live longer?
Many studies have found that people with very low blood levels of vitamin D die sooner. (Interestingly, those same studies report that people with relatively high levels of vitamin D might also have shorter life spans, but, for today, that’s a tangent we won’t pursue.) Our blood levels of vitamin D today probably are lower than they were for our forebears. That’s because most vitamin D is produced when sunlight hits our skin, and our forebears were out in the sun a lot more than we are today.
Even if it’s true that people with very low blood levels of vitamin D have shorter life spans, and I think it probably is, that’s no guarantee that raising blood levels by taking vitamin D supplements will extend life. The gold standard for seeing whether a treatment has a benefit is the randomized trial. Recently, scientists pooled the results of 52 randomized trials of vitamin D that involved over 75,000 people, publishing their results in the summer of 2019 in The BMJ.
The scientists concluded that vitamin D supplements did not reduce mortality from all causes, nor mortality from heart and other cardiovascular diseases. However, vitamin D supplements did appear to reduce the risk of mortality from cancer by 16%. This was particularly true for supplements containing vitamin D3 (vitamin D2 also is sold as a supplement).
The doses of vitamin D used by the studies in this analysis were not reported, but probably ranged from around 600 to 2,000 international units (IU) per day. Other recent studies have found that higher doses (above 4,000 IU/day) may even have harmful effects.
So, to answer your question: While you might be wasting your money, there is a reasonable possibility that you are reducing your risk of dying from cancer, and not harming yourself (if your dose of vitamin D3 is in the range of 600 to 2,000 IU/day). And there may be other benefits from vitamin D supplements. Many studies of this are under way, including the longer-term Vitamin D and Omega-3 Trial (VITAL), which is based here at Harvard Medical School. Stay tuned.
— by Anthony L. Komaroff, M.D.
Editor in Chief, Harvard Health Letter
Image: ayo888/Getty Images
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