Folks—I also work in nutritional epidemiology, and there’s lots of evidence moderate dose daily/weekly vitamin D can help. Notably there have been many trials conducted on this, and it shows benefits for colds (some coronaviruses) & flu, esp if deficient. https://news.harvard.edu/gazette/story/2017/02/study-confirms-vitamin-d-protects-against-cold-and-flu/

2) There was a meta-analysis (a combined analysis) of 25 randomized controlled trials including more than 11,000 participants, published in journal BMJ of vitamin D and cold and flu

3) At first there seemed to be conflicting data before the meta. But the meta analysis “investigators found that daily or weekly supplementation had the greatest benefit for individuals with the most significant vitamin D deficiency (blood levels below 10 mg/dl)...”

4) “— cutting their risk of respiratory infection in half (among those deficient) — and that all participants experienced some beneficial effects from regular vitamin D supplementation.

However, occasional high doses of vitamin D did not produce significant benefits.”

5) Bottomline: we believe daily or weekly vitamin D may have benefits for cold and flu. We can’t say certain for #COVID19 yet, but given many common colds are also coronaviruses, and vitamin D helps in general immune system, it could have maybe benefit. https://www.bmj.com/content/356/bmj.i6583

6) Sometimes, vitamin D is listed in micrograms, and the relationship is as follows: 2.5 mcg (micrograms) = 100 IU. 10 mcg = 400 IU.

Many of trials tested 10-50 mcg (400-2000 IUs) per day. Similar effects in daily dosing. But bolus dosing of 10,000 IU not beneficial in study.

7) I personally take 1000-2000 IUs per day depending if I get a lot of sun or not (I sometimes skip if I’m in the sun all day), or 5000 IU two times a week. Not making recommendations or giving advice. Just sharing about myself.

8) Also, vitamin D is well known to have synergy with calcium, hence go together.

But vitamin A can negatively interact with D. Years ago, milk was heavily A+D fortified—but my Harvard SPH colleagues convinced dairy industry to lower the vitamin A dose with combining w/ D.

9) Also vitamin D comes in two main dietary forms:

Vitamin D2 (ergocalciferol)
Vitamin D3 (cholecalciferol)

D3 is better absorbed. vitamin D2 is less effective than vitamin D3 at raising blood levels of vitamin D, 25-OH-D (aka 25-hydroxy-D).