Researchers enrolled 2259 participants aged 45 - 75 years who were also taking part in a colorectal adenoma chemoprevention trial. Participants were randomly assigned to one of 4 groups:
- 1000 IU/day of vitamin D3
- 1200 mg elemental calcium/day
However, there are a few things to consider. First, 1000 IU/day is far too low a dose to study. Yes, some previous studies showed cold/flu prevention at low doses, but it would seem a minimum dose of 2000 IU/day is needed to see a more pronounced benefit.
Second, the drop-out rate was so high that it throws into question the validity of the results. Why did all those people drop-out of the study? If those who didn't finish with study continued right to the end, would we have seen a significantly different result?
Third, by excluding those with the lowest baseline serum 25(OH) vitamin D ( <12 ng/mL), you exclude the group of people who could and would benefit most from vitamin D3 supplementation.
The researchers acknowledge all these limitations to the study's results/conclusions, but also note that self-reported adherence to study protocol (for example, people in the vitamin D3 group may have said they took the supplement when in fact they didn't) and lack of laboratory confirmation of URTI may also have affected the results (for example, those in the placebo group may have gotten sick but never reported it, or it was too mild to report it).
Regardless, many reading this will have likely already started to take higher doses of D3 as we approach the winter months. Keep it up. This study doesn't faze me one bit and the study is not nearly strong enough to throw into question my stance on vitamin D3 supplementation. Further, there are many other health benefits to taking D3 besides cold/flu prevention.
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Source: Vitamin D3 Supplementation and Upper Respiratory Tract Infections in a Randomized, Controlled Trial
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