Every fall, the same products return to the shelves, and every winter, the same people get sick. The problem isn't really the choice of supplements: it's that we use them almost all at the wrong time and in the wrong form.
This article is based on five meta-analyses, two of which together involved more than 86,000 participants, to answer three specific questions. What actually reduces the risk of respiratory infection, and by how much. Under what conditions, because they change everything and are almost always overlooked. And when to start, which, for one of these nutrients, matters now rather than in December.
Vitamin D is the best-documented lever. A meta-analysis from the BMJ involving 11,321 participants and another from Lancet Diabetes & Endocrinology involving 75,541 participants show a reduction in the risk of respiratory infection. But the effect depends entirely on the dosing regimen : daily intake daily, the risk drops significantly ; with spaced loading doses, the effect disappears completely. And in people with severe deficiency, the reduction reaches 70%.
Zinc works in ways we don't expect. This is not about prevention but about treatment : zinc acetate lozenges, taken within 24 hours of the first symptoms, shorten colds by 2.7 to 2.9 days over an average duration of seven. By the fifth day, 70% of patients were cured compared to 27% on placebo. Vitamin C: no preventive effect in the general population, but a risk cut in half in endurance athletes.
- Why start now and not in December
- Vitamin D, the best-documented lever
- The deciding condition: daily, not injections
- Zinc: what works, and which exact form
- Vitamin C: for whom it makes a difference
- What matters more than all supplements combined
- The plan, in concrete terms
- Frequently asked questions
Why start now and not in December
Vitamin D is produced by the skin under the effect of solar radiation, and this synthesis depends on the angle of the sun. At our latitudes, around the 49the parallel for mainland France, this angle becomes insufficient starting in autumn: for several months, the skin produces practically no vitamin D, regardless of time spent outdoors.
The body then draws on its reserves, built up during the summer. These reserves decrease progressively, and it is at the end of winter that blood concentrations reach their lowest point.
Many people begin supplementation in December or January, when they start to feel tired or see those around them fall ill. At that point, reserves have already dropped significantly.
Starting at the beginning of autumn amounts to maintaining a level rather than trying to raise it once it has fallen. On a nutrient whose effect, as we will see, depends on regular and prolonged intake rather than a one-time boost, this timing difference is not trivial.
Our complete guide to vitamin D details the question of dosages, and our article on when and how to take it addresses the time of day.
Vitamin D, the best-documented lever
It is by far the nutrient with the most solid data, and they come from two large-scale studies.
| Meta-analysis | Scope | Overall result |
|---|---|---|
|
BMJ, 2017 Individual participant data |
25 randomized trials 11,321 participants, ages 0 to 95 |
Risk of acute respiratory infection reduced, odds ratio of 0.88 (0.81 to 0.96) |
|
Lancet Diabetes & Endocrinology, 2021 Expanded update |
46 randomized trials 75,541 participants |
Odds ratio of 0.92 (0.86 to 0.99), real but modest reduction |
Two points deserve to be highlighted. The first is that the authors of the meta-analysis in the BMJ qualify all contributing data as high quality, which is rare in this field. The second is that supplementation proved to be safe in both studies, with no increase in the proportion of participants experiencing a serious adverse effect.
Let's be precise about the magnitude: an odds ratio of 0.92 corresponds to a risk reduction of approximately 8% on average. It's not spectacular, and the authors of the Lancet say so themselves in describing a reduction of modest magnitude.
But this average masks what's essential, because it aggregates very different situations. As soon as you look at how vitamin D was administered and to whom, the figures change dramatically. This is the subject of the next section, and it's the most useful information in this entire article.
The decisive condition: daily, not in ampoules
Here's what distinguishes effective supplementation from useless supplementation, and it has nothing to do with brand or price.
| Mode of administration | Measured effect | Verdict |
|---|---|---|
| Daily or weekly intake, without loading dose | Odds ratio 0.81 (0.72 to 0.91) | Effective |
| Spaced loading doses, vial type | Odds ratio 0.97 (0.86 to 1.10) | No effect |
| Daily intake, 2021 meta-analysis | Odds ratio 0.78 (0.65 to 0.94) | Effective |
| 400 to 1,000 IU per day | Odds ratio 0.70 (0.55 to 0.89) | The most favorable window |
The contrast is clear. Daily intake reduces the risk of respiratory infections by 19 to 22% according to the meta-analysis. Spaced loading doses give an odds ratio of 0.97 with a confidence interval that widely crosses 1: in other words, nothing.
This is a point of considerable practical importance, because spaced vial administration remains a very common practice. It has its reasons, notably adherence: one vial every two or three months is easier to remember than a daily tablet. But on this specific criterion, the prevention of respiratory infections, the data do not support it.
Also note the dose window: 400 to 1,000 IU per day is the range where the effect is most pronounced, with a 30% reduction in risk. More is not better, which is rather reassuring.
The factor that multiplies everything: your baseline status
This is the second major finding of the BMJmeta-analysis, and it explains why the overall average seems modest.
| Baseline vitamin D status | Effect of daily supplementation |
|---|---|
| Very low concentration, below 25 nmol/l | Odds ratio 0.30 (0.17 to 0.53), representing a risk reduction of approximately 70% |
| Higher concentration, starting from 25 nmol/l | Odds ratio 0.75 (0.60 to 0.95), representing approximately 25% |
Zinc: what works, and which form exactly
With zinc, the reasoning error is different: it's used to prevent, whereas the strongest data concern treatment.
Two meta-analyses of individual data from three randomized placebo-controlled trials in 199 cold patients yield remarkable results.
| Criterion | Result |
|---|---|
| Duration of cold | Shortened by 2.73 days (1.8 to 3.3) to 2.94 days (2.1 to 3.8) depending on the method, over an average duration of 7 days |
| Recovery speed | Rate ratio of 3.1 (2.1 to 4.7), representing recovery approximately three times faster |
| By the fifth day | 70% of patients taking zinc were cured, compared to 27% on placebo |
| Factors modifying the effect | None: neither age, nor sex, nor smoking, nor allergies, nor initial severity |
| Safety | No serious adverse effects observed in the three trials |
These results concern zinc acetate lozenges, at 80 to 92 mg of elemental zinc per day, started within 24 hours of symptom onset.
Three elements therefore matter simultaneously. The form : a lozenge that dissolves slowly in the mouth, which suggests a local action at the throat level, not a capsule that is swallowed. The dose, much higher than the usual intake of a daily supplement. And the timing, from the very first hours.
We must be clear about our own product: our zinc comes in capsule form, at a nutritional dosage. It does not reproduce the protocol of these trials and we therefore do not apply these results to our product. Zinc contributes to normal immune system function, and it is on this basis, as a foundational support, that it has its place here.
This is an important and rarely made distinction: the same mineral can have a documented and recognized foundational nutritional use, and a specific therapeutic use that falls under an entirely different format and dosage.
Vitamin C: for whom it makes a difference
It is the most ingrained and least supported reflex. The Cochrane review, involving 11,306 participants, is unambiguous on prevention in the general population: risk ratio of 0.97, interval of 0.94 to 1.00, in other words no effect.
But it contains a result that almost no one cites, and which identifies a very specific audience.
| Population | Effect on the risk of cold |
|---|---|
|
General population 10,708 participants |
Risk ratio 0.97 : no preventive effect |
|
Intense physical exertion 598 marathon runners, skiers, soldiers in subarctic conditions |
Risk ratio 0.48 (0.35 to 0.64): risk cut in half |
|
Duration of episodes 9,745 episodes |
Reduced by 8% in adults and 14% in children, severity also reduced |
|
Supplementation started at first symptoms 3,249 episodes |
No consistent effect |
What matters more than all supplements combined
Let's put the magnitudes in perspective. The effects presented above are real and measured, but they amount to modest percentages. Four factors matter more, and none of them can be purchased.
| Lever | Why it matters |
|---|---|
| Sleep | It is the most powerful determinant of immune function, and the most often sacrificed. No supplement can compensate for an accumulated sleep debt |
| Hand hygiene | The transmission of respiratory viruses occurs largely through hands and surfaces. It is the gesture with the best effort-to-benefit ratio on this entire list |
| Vaccination | For the flu, it falls into a different category of effectiveness than anything discussed in this article, particularly for people at risk |
| Overall diet | Adequate and varied intake does more than an isolated nutrient. Supplements correct deficiencies, they don't build a foundation |
Because a reader who would buy supplements thinking they had protected themselves for the winter would be misinformed, and would eventually realize it. The honest place for the products we discuss is that of a supplement that comes after these four levers, not in their place
It's also the reading most faithful to the data: vitamin D acts mainly when it's deficient, which makes it a correction of a deficiency rather than an enhancement
The plan, concretely
The three nutrients in this article work, but under precise conditions that are almost always overlooked. Vitamin D as a daily intake, not in ampoules, especially if you're starting from a low level. Zinc as a baseline supplement, with spectacular performance on colds coming from a very different form and dose. Vitamin C regularly, and especially if you train hard. And all of this comes after sleep, hand washing, and vaccination, which carry more weight and cost nothing.
Frequently Asked Questions
Vitamin D
Does vitamin D really reduce respiratory infections?
Yes, with good quality evidence. A BMJ meta-analysis of 25 trials and 11,321 participants finds an odds ratio of 0.88, and an update in Lancet Diabetes and Endocrinology of 46 trials and 75,541 participants finds 0.92. Both are statistically significant. The BMJ authors describe the overall body of data as high quality, which is rare in this field.
Why do people say the effect is modest?
Because an odds ratio of 0.92 corresponds to a risk reduction of approximately 8% on average, and the Lancet authors themselves speak of a reduction of small magnitude. But this average aggregates very different situations: as soon as you distinguish the dosing schedule and baseline status, the figures change dramatically, up to 70% reduction in people who are severely deficient.
Do spaced-out ampoules work?
Not on this criterion. The BMJ meta-analysis explicitly distinguishes the two schedules: with daily or weekly dosing without a loading dose, the odds ratio is 0.81, so effective. With one or more spaced loading doses, it is 0.97 with an interval of 0.86 to 1.10, in other words no effect. This is the most important and most overlooked result in this review.
What daily dose?
The 2021 meta-analysis identifies a window: doses equivalent to 400 to 1,000 IU per day give the most favorable odds ratio, at 0.70. More is not better on this criterion. That said, the appropriate dosage depends on your status, your age, and your situation: this is a question to ask your doctor, ideally after a blood test.
Why start in autumn rather than winter?
Because skin synthesis of vitamin D depends on the sun's angle, which becomes insufficient at our latitudes from autumn onwards. The body then draws on its summer reserves, which reach their lowest point in late winter. Starting in early autumn amounts to maintaining a level rather than trying to raise it once it has fallen.
Should you get your vitamin D tested?
This is the most rational approach, because the effect depends massively on the starting point. In people whose initial concentration was below 25 nmol/l, the odds ratio drops to 0.30, or approximately 70% risk reduction. Above this threshold, it is 0.75, or approximately 25%. Testing also allows you to adjust the dosage with your doctor.
Is supplementation safe?
Both meta-analyses conclude that supplementation is safe, with no increase in the proportion of participants who experienced a serious adverse effect. That said, vitamin D is fat-soluble and can accumulate, unlike water-soluble vitamins: prolonged very high doses are not harmless and require medical advice.
Zinc
Does zinc prevent catching a cold?
That's not where the strongest data lies. The most striking results concern treating an already-declared cold, not preventing it. As a baseline supplement, zinc contributes to normal immune system function, which is a recognized effect, but different from demonstrated cold prevention.
What exactly has research shown about zinc?
Two meta-analyses based on individual data from three randomized placebo-controlled trials in 199 cold patients found colds shortened by 2.73 to 2.94 days over an average duration of seven days, recovery approximately three times faster, and by day five 70% of zinc patients recovered versus 27% on placebo. No serious adverse effects were observed.
Why does form matter so much?
Because these trials used zinc acetate lozenges, at 80 to 92 mg of elemental zinc per day, started within 24 hours of first symptoms. Slow dissolution in the mouth suggests a local action in the throat, which a swallowed capsule does not reproduce. The dose is moreover much higher than that of a daily supplement.
Do your zinc capsules reproduce these results?
No, and we prefer to be clear about it. Our zinc comes in capsules at a nutritional dosage: it reproduces neither the form, nor the dose, nor the protocol of these trials, and we therefore do not transpose these results to our product. Zinc contributes to normal immune system function, and it is as this baseline support that it has its place.
Does zinc's effect depend on the profile?
No, and that's noteworthy. The authors note that the effect was not modified by age, sex, origin, smoking, allergies, or initial cold severity. This consistency across subgroups suggests that the estimate applies broadly.
Vitamin C
Does vitamin C protect against colds?
Not in the general population. The Cochrane review, with 10,708 participants for this analysis, concludes with a risk ratio of 0.97 with an interval of 0.94 to 1.00: no preventive effect. This is one of the most entrenched and least supported reflexes regarding supplements.
So why mention it?
Because of a result that almost no one cites. In 598 marathon runners, skiers, and soldiers subjected to intense exercise in subarctic conditions, the risk ratio drops to 0.48, with an interval of 0.35 to 0.64: the risk of catching a cold is cut in half. If you train intensely, you fit precisely this profile.
And on cold duration?
Duration decreases by 8% in adults and 14% in children, over 9,745 episodes, and severity is also reduced. This is modest but consistent from one study to the next. Conversely, supplementation started at the moment of symptoms shows no consistent effect over 3,249 episodes.
In Practice
Should you take all three together?
They do not act at the same time or in the same way. Vitamin D is a baseline supplement dependent on timing and status. Zinc and vitamin C are foundational supplements contributing to normal immune system function. If you had to choose just one, the strongest data concerns vitamin D as a daily intake.
What matters more than supplements?
Sleep, which is the most powerful determinant of immune function and most often sacrificed. Hand hygiene, since respiratory viruses spread widely through hands and surfaces. Flu vaccination for those concerned, which falls into a different category of efficacy. And generally sound overall nutrition.
When should you consult a doctor rather than take supplements?
When experiencing persistent fever, respiratory discomfort or shortness of breath, and when infections recur in an unusual pattern. This last point warrants medical evaluation: recurrent infections have identifiable causes that a supplement cannot treat. Persistent fatigue also justifies a medical assessment.
Key takeaways from this article
That vitamin D is the best-documented lever, provided it is taken daily and not in spaced-out doses, in a range of 400 to 1,000 IU, with benefits being greater when starting from a low baseline. That zinc shortens colds, but in a specific form and dose. That vitamin C prevents nothing except in endurance athletes. And that sleep and hand washing matter more than all of this combined.
Glossary
- Acute respiratory infection
- Term encompassing respiratory tract ailments, from the common cold to more severe lower respiratory forms.
- Odds ratio
- A measure comparing the probability of an event between two groups; a value of 1 means no difference.
- Loading dose
- A substantial amount administered at once, typically an ampoule, as opposed to daily intake.
- 25-hydroxyvitamin D
- The circulating form of vitamin D measured in a blood test, expressed in nmol/l or ng/ml.
- Elemental zinc
- The actual amount of zinc provided, distinct from the total weight of the salt carrying it.
- Lozenge
- A form that dissolves slowly in the mouth, allowing localized action in the throat.
- Individual participant data
- A meta-analysis method that combines data from each patient rather than aggregated results.
- Number needed to treat
- The number of people who need to be treated to achieve one additional benefit compared to placebo.
Sources
The studies cited below were identified via PubMed.
- Martineau AR, et al. Vitamin D supplementation to prevent acute respiratory infections: a systematic review and meta-analysis of individual participant data. BMJ, 2017;356:i6583. DOI
- Jolliffe DA, et al. Vitamin D supplementation to prevent acute respiratory infections: a systematic review and meta-analysis of aggregated data from randomized controlled trials. The Lancet Diabetes & Endocrinology, 2021;9(5):276-292. DOI
- Hemilä H, et al. Zinc acetate lozenges may improve the rate of recovery from cold symptoms: a meta-analysis of individual participant data. Open Forum Infectious Diseases, 2017;4(2):ofx059. DOI
- Hemilä H, et al. Zinc acetate lozenges for treating the common cold: a meta-analysis of individual participant data. British Journal of Clinical Pharmacology, 2016;82(5):1393-1398. DOI
- Hemilä H, Chalker E. Vitamin C for the prevention and treatment of the common cold. Cochrane Database of Systematic Reviews, 2013;(1):CD000980. DOI
