People with lower blood levels of vitamin D are at higher risk of various serious illnesses, including several types of cancer. But that doesn’t mean that raising blood levels by taking vitamin D supplements would necessarily reduce cancer risk. Harvard researchers set out in 2011 to test this strategy with a randomized trial of vitamin D supplementation (2,000 IU per day) involving 25,871 people; the results were published online Nov. 2, 2020, by JAMA Network Open. The participants were ages 50 or older, were free of cancer at the start of the study, and were followed carefully for an average of 5.3 years. The rate of metastatic or fatal cancers was 17% lower in those who took the vitamin D supplement. However, the benefit was restricted to people who were not overweight or obese. In healthy-weight people, the risk fell by 38%; in contrast, among those who were overweight or obese, there was no reduction in risk. What do the results of this large and careful study tell us about what we should do? For healthy-weight people at risk for developing cancer (because of lifestyle or a family history of cancer), it would be reasonable to take daily vitamin D supplements starting at about age 50.
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