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There are two main types of D. The first is vitamin D3, which is found in animals including fish and is the kind the skin makes when exposed to sunlight. The second is vitamin D2, which comes from plant-based foods including mushrooms. Studies have found that D3 is more effective, and the conclusions of a 2012 meta-analysis argue that D3 is the preferred choice for supplementation.
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When his team analysed raw data from 25 clinical trials involving 11,000 patients from 14 countries, they found a small benefit to taking daily or weekly vitamin D supplements to reduce the risk of respiratory infections, asthma attacks and bronchitis. Although the paper soon attracted robust criticism, Martineau points out that the reduction of risk, while slight, is still significant and comparable to the effects of other health measures: to prevent a single respiratory infection, you’d have to give 33 people vitamin D supplements – compared to, for example, giving a flu vaccination to 40 people to prevent a single case of flu.
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https://www.bbc.com/future/article/20181010-do-vitamin-d-supplements-work
