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Cold/Flu Prevention: to D3, or not to D3?

That seems to be the question people are asking after this latest study on vitamin D3 that showed no benefit to preventing the cold of flu compared to placebo. This goes against what most studies have shown, but let's look at the details to see why this may have been the case.

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:
  1. 1000 IU/day of vitamin D3
  2. 1200 mg elemental calcium/day
  3. both
  4. placebo 
All participants were in good health and had serum 25-hydroxyvitamin D levels of 12 ng/mL or higher. Interestingly, only 759 participants completed the study, and from these people, the results showed no significant decrease in the incidence of URTIs (upper respiratory tract infections) or winter days of illness between the treatment groups.

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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1 comment:

  1. Thank you, doc. And you're absolutely right. I'm tired of sloppy studies like this. Why waste the time or money? D3 is protective for colds/flu and I'm living proof. I've been on 2-5K iu for ten years and do very well compared to those around me every cold/flu season. Keep your blood level between 40-60 nanograms per millileter, most experts say. (Mine is about 50.) And take K2 and A (beta carotene) alongside to avoid imbalance which could see your calcium go to the wrong places in your body (arteries, organs). Thank you for your blog and your expert advice!


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