Vitamin D in winter: the quiet overachiever

A pooled analysis of 25 randomised trials in nearly 11,000 people found that daily or weekly vitamin D lowered the risk of respiratory infection, and the effect was largest in people who started out low on it. One big occasional dose did not work the same way, only steady daily amounts did, which is the shape Moat's 40 mcg of D3 is built around.

Most ingredients come with a pile of caveats. This one mostly does not, which makes it a pleasure to write about.

Of the nine things in our formula, vitamin D has the strongest support behind it, and there is a decent gap between it and everything else.

What did the 25 trials actually measure?

Researchers pooled the individual participant data from 25 randomised, double-blind, placebo-controlled trials, covering close to 11,000 people.

Individual participant data matters here. Most meta-analyses pool the published summaries of trials. This one went back to the per-person records, which lets you ask questions the original papers did not, and makes the result considerably harder to wave away.

The headline: vitamin D supplementation lowered the risk of acute respiratory infection.

Which two details decide whether it works?

This is where most write-ups stop, and where the useful part starts.

Daily or weekly worked. Big occasional doses did not. Trials that gave people a small regular dose saw the benefit. Trials that gave one enormous bolus every month or two did not. Same nutrient, same total quantity, different result.

It helped most in the people who were short on it to begin with. The most deficient participants saw the largest effect. People who already had comfortable levels saw much less.

Put together, those two findings describe a fairly specific product: a modest daily dose, taken by someone who is plausibly a bit low. They do not describe a megadose you take when you already feel a cold coming on.

	What was studied	What worked
Trials pooled	25 randomised, double-blind, placebo-controlled	Individual participant data, not just published summaries
People covered	Nearly 11,000
Dosing pattern	Daily or weekly vs. large occasional bolus	Daily or weekly lowered infection risk; big occasional doses did not
Who benefited most	People already low on vitamin D	Largest effect seen here
Amount in Moat	40 mcg vitamin D3, as cholecalciferol	Daily, in a two-capsule serving

Who is most likely to be low on vitamin D?

Vitamin D is mostly made in your skin from sunlight, which makes it unusual among nutrients and seasonal in a way the others are not. The people most likely to run low are the predictable ones: anyone living far from the equator through winter, anyone who works indoors through the daylight hours, anyone with darker skin at higher latitudes, and anyone who sensibly wears sunscreen.

That is a lot of people, for a few months a year, every year.

How much vitamin D is in Moat, and why that amount?

Our two-capsule serving has 40 mcg of vitamin D3, as cholecalciferol. Daily, small, and boring, which is exactly the shape the evidence rewards.

It will not stop you getting ill, because that is not what the trials showed. What they did show: if you are short on vitamin D, topping up appears to help, and topping up steadily beats topping up dramatically.

If you want the contrast, read what the vitamin C evidence looks like. It is the same standard of review, applied to an ingredient that comes out considerably worse.

The conversion between IU and micrograms, and where each amount sits against the reference intake, is in the vitamin D converter.

FAQ

Does vitamin D actually prevent respiratory infections? The pooled trial data found daily or weekly vitamin D lowered the risk, especially in people who were low on it to begin with. It is not a guarantee for any one person.

Is a big occasional dose as good as a daily one? No. The same body of trials found large, infrequent doses did not produce the same benefit as small, regular ones.

Who is most likely to be low on vitamin D in winter? Anyone living far from the equator through winter, anyone working indoors through daylight hours, anyone with darker skin at higher latitudes, and anyone who wears sunscreen consistently.

How much vitamin D is in Moat? 40 mcg of D3, as cholecalciferol, per two-capsule serving.

Sources

• Martineau et al., Vitamin D supplementation to prevent acute respiratory infections: a systematic review and meta-analysis, BMJ
• NIH Office of Dietary Supplements, Vitamin D fact sheet

These statements have not been evaluated by the Food and Drug Administration. This product is not intended to diagnose, treat, cure, or prevent any disease. Consult a physician if pregnant, nursing, under 18, or managing a medical condition.