The strongest effect was observed in those who had a clear vitamin D deficiency (Table 1) and who received daily or weekly supplements, rather than large bolus doses. A recently published meta-analysis of 25 randomised controlled trials concluded that the risk of acute respiratory tract infections may be reduced by vitamin D supplements (6). There is insufficient evidence that vitamin D alone has a preventive effect.Ī systematic Cochrane review updated in 2014 concluded that vitamin D supplements reduced total mortality by 6 %, but that these findings were insufficiently robust to serve as an argument for recommending vitamin D supplements (5).
This effect has mainly been identified for supplementation with vitamin D and calcium in combination. However, most randomised controlled trials have failed to show a consistent preventive effect of vitamin D supplements, with some exceptions (4): meta-analyses and systematic reviews of randomised controlled trials provide some evidence that vitamin D has a preventive effect on falls and bone fractures. This notion is supported by a number of observational studies (3). It is biologically plausible that vitamin D may help prevent a number of chronic diseases, such as cardiovascular disease, colon cancer and autoimmune diseases. In contrast, according to the Nordic nutrition recommendations, the recommended daily intake of vitamin D for the general population amounts to 10 µg per day for children and adults and 20 µg per day for those 75 years and older (2). High-dosage supplements that contain up to 80 µg vitamin D 3 per tablet are currently available on the Norwegian market. In addition, sales of over-the-counter dietary supplements are high. In 2017, sales amounted to 39.5 million defined daily doses (1). In the same period, the sales of defined daily doses (20 µg) increased more than a hundredfold. According to figures from the Norwegian Prescription Database, the number of users of prescription-only vitamin D 3 supplements increased 85-fold in Norway over the six-year period from 2011 to 2017. A search in PubMed as of January 2019 returns nearly 66 000 hits for 'vitamin D' and more than 5 000 hits for 'vitamin D supplementation'. There is considerable research activity in the field of vitamin D and health.
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