Every fall, the same promises return: boost your defenses, strengthen your immunity, get through winter without getting sick. And every fall, the same active ingredients are highlighted in the same order—that of revenue rather than evidence. So we did the opposite: we reviewed the available meta-analyses, classified them by level of evidence, and published the hierarchy that emerges, even when it challenges our own shelves.
The result boils down to one sentence: sleep and physical activity rank ahead of all supplements, vitamin D is the only nutrient with high-quality evidence on respiratory infections, probiotics follow with a respectable body of evidence, vitamin C does not prevent colds in the general population, zinc has no demonstrated preventive effect, and echinacea has not proven itself. Here are the details, with numbers to back it up, and what this concretely means before winter.
Vitamin C won't prevent you from catching a cold, and we say so in this article. What it does, when taken regularly, is shorten episodes and reduce the risk by half in people subjected to intense physical effort. Organic acerola, without additives.
Ranked by level of evidence, the immune levers are not the ones marketing pushes forward. Sleep comes first: its deprivation impairs innate and adaptive immunity and increases infectious risk. Moderate and regular physical activity follows, with documented effects on immunosurveillance and vaccine response, while intense and prolonged efforts temporarily depress it. On the nutrient side, vitamin D is the only one with high-quality evidence: across 25 trials and 11,321 participants, it reduces the risk of acute respiratory infection, provided it is taken daily or weekly, and the effect is four times stronger in severely deficient individuals. Probiotics reduce the number of episodes, their duration, and antibiotic prescriptions, with weak to moderate level of evidence. Vitamin C does not prevent colds in the general population but shortens episodes and cuts the risk in half for endurance athletes. Zinc has no demonstrated preventive effect and its use as a treatment relies on weak evidence. Echinacea has not proven itself. Finally, no supplement replaces hand washing, ventilation, and vaccination for those concerned.
- The ranking by level of evidence
- Why "boosting immunity" means nothing
- Sleep, the primary immune lever
- Physical activity: the dose makes the poison
- Vitamin D: the only nutrient with high-quality evidence
- Probiotics: the second strongest case
- Vitamin C: what it does and doesn't do
- Zinc: the nuance no one makes
- Echinacea and winter plants
- The micronutrients whose deficiency matters
- What works best and costs almost nothing
- When weakened immunity should prompt a doctor's visit
- Building your winter strategy
- Frequently asked questions
1. Classification by level of evidence
| Lever | What the data shows | Level of evidence |
|---|---|---|
| Adequate sleep | Its deprivation impairs innate and adaptive immunity, establishes chronic inflammation, and increases infection risk | Strong convergence, experimental and epidemiological |
| Moderate physical activity | Better immune surveillance, less inflammation, improved vaccine response including in older adults | Large and consistent body of evidence |
| Vitamin D | Reduced risk of acute respiratory infection, especially in deficient individuals and with regular intake | High quality (25 trials, individual data) |
| Probiotics | Fewer episodes, shorter episodes, fewer antibiotics prescribed | Low to moderate depending on criteria |
| Vitamin C | No prevention in general population; reduced duration with regular intake; risk halved in endurance athletes | Solid, but limited and conditional effect |
| Zinc | No demonstrated preventive effect; possibly reduced cold duration with treatment, with more adverse effects | Weak, very high heterogeneity |
| Echinacea | No demonstrated benefit for treatment, non-significant trend for prevention | Insufficient |
A word on the methodology, because it explains this ranking. We did not include laboratory studies showing that an extract "activates NK cells" in a test tube: this type of result is easy to obtain and predicts almost nothing about what happens in humans. We included randomized trials and their syntheses, with a criterion that matters to the reader: getting sick less often, for shorter periods, or less severely.
2. Why "boosting your immunity" means nothing
The verb "boost" implies an accelerator you would press to go faster. This is not how the immune system works, and the image is misleading on two counts.
First, the immune system is a balance, not power. It must react quickly to pathogens, and stop just as quickly to avoid destroying healthy tissues. An excessive response has specific names: allergy when it targets a harmless allergen, autoimmune disease when it targets the body itself, cytokine storm when it spirals out of control during an infection. Nobody wants an immune system that is "stronger" in that sense.
Next, no European health claim is formulated in these terms. The only authorized statements say that a nutrient "contributes to the normal functioning of the immune system," which is quite different. The word normal is not rhetorical softening: it designates exactly what science establishes, namely that a deficiency degrades the immune response and correcting it restores it to normal.
Rather than "how do I boost my immunity?", two questions produce actionable answers. What in my lifestyle is currently degrading my immune response? Sleep deprivation, sedentary behavior, tobacco, alcohol, chronic stress, impoverished diet. And do I have a deficiency to correct? Vitamin D in winter is by far the most common case in France. These two questions cover the essentials of what is truly modifiable.
3. Sleep, the primary immune lever
Modern societies are experiencing a growing trend toward reduced sleep duration, with nighttime sleep times below the ranges recommended for health. Epidemiological and laboratory studies have demonstrated the deleterious effects of sleep deprivation on health. Sleep serves an immune support function, promoting host defense against infection and inflammatory injury. Sleep deprivation has been associated with alterations in innate and adaptive immune parameters, leading to chronic inflammatory state and increased risk of infectious and inflammatory diseases, including cardiometabolic, neoplastic, autoimmune, and neurodegenerative diseases.
Garbarino S, Lanteri P, Bragazzi NL, Magnavita N, Scoditti E. Commun Biol 2021;4(1):1304. DOI: 10.1038/s42003-021-02825-4
This finding deserves to be put into perspective with the rest of the article. We are talking here about a factor that simultaneously modifies innate immunity, adaptive immunity, and baseline inflammatory state, with documented consequences extending to chronic diseases. No nutrient tested in randomized trials approaches this magnitude. And yet, sleep is almost always relegated to the end of the list, after supplements.
In practice, three guidelines suffice: aim for seven to nine hours for an adult, maintain regular schedules including weekends, which matters just as much as duration, and treat sleep disorders rather than simply endure them. Undiagnosed sleep apnea, chronic insomnia, or night shift work cannot be corrected with a supplement, but they can be medically managed.
4. Physical activity: the dose makes the poison
Exercise has a profound effect on normal immune system function. It is generally accepted that prolonged periods of intensive training can depress immunity, while regular exercise of moderate intensity is beneficial. Single prolonged sessions can impair T lymphocyte, NK cell, and neutrophil function, and blunt immune responses. Elite athletes frequently report symptoms associated with upper respiratory tract infections during periods of intense training and competition. Conversely, single sessions of moderate intensity are immunostimulating and have been effectively used to increase vaccine response in at-risk patients.
Simpson RJ, Kunz H, Agha N, Graff R. Prog Mol Biol Transl Sci 2015;135:355-380. DOI: 10.1016/bs.pmbts.2015.08.001
A more recent review on the link between physical activity and the defense system confirms this picture and highlights the interest of exercise onimmunosenescence, that is, aging of the immune system (Nieman and Wentz, J Sport Health Sci, 2019). This is a point that matters particularly after age 60, when vaccine response weakens.
The practical translation is reassuring: it's not about performance. Thirty minutes of brisk walking per day produce the effect described in these studies. And if you train intensely, our article on the difference between fatigue and overtraining details the warning signs to monitor during heavy training periods, when vulnerability to infections increases.
5. Vitamin D: the only nutrient with high-quality evidence
This is the centerpiece of this entire dossier, and it deserves to be read in detail rather than summarized as "vitamin D boosts immunity."
Twenty-five eligible randomized trials were identified, totaling 11,321 participants aged 0 to 95 years; individual data were obtained for 96.6% of them. Vitamin D supplementation reduced the risk of acute respiratory infection in all participants (adjusted odds ratio 0.88). In subgroup analyses, protective effects were observed in those receiving vitamin D daily or weekly without additional loading doses (0.81), but not in those receiving one or more loading doses (0.97). Among those receiving vitamin D daily or weekly, protective effects were more pronounced in people whose baseline 25-hydroxyvitamin D was below 25 nanomoles per liter (0.30) than in those whose baseline level was at or above 25 nanomoles per liter (0.75). Vitamin D did not influence the proportion of participants who experienced at least one serious adverse event. The body of evidence contributing to these analyses was judged to be of high quality.
Martineau AR, Jolliffe DA, Hooper RL, et al. BMJ 2017;356:i6583. DOI: 10.1136/bmj.i6583
Three key lessons, and they are all actionable.
Trials show that consistency makes the difference, not massive doses. Our plant-based Vitamin D3 in drops, derived from lichen, allows for precise daily adjustment rather than spaced-out doses. To be discussed with your doctor, ideally after testing.
Discover plant-based Vitamin D3 →To learn more, read our complete vitamin D guide and discover our immunity collection.
6. Probiotics: the second-best dossier
Twenty-three randomized trials were meta-analyzed, totaling 6,950 participants including children, adults, and elderly people. Probiotics could reduce the number of participants who experienced at least one upper respiratory infection episode (relative risk 0.76; low certainty evidence) and probably reduce the number of participants who experienced at least three (0.59; moderate certainty evidence). They could reduce the incidence rate of infections (0.82), could reduce the average duration of an episode (mean difference of minus 1.22 days), and probably reduce the number of participants who used prescribed antibiotics (0.58; moderate certainty). They probably do not increase the number of participants who experienced at least one adverse effect. Reported adverse effects were minor, most often gastrointestinal.
Zhao Y, Dong BR, Hao Q. Cochrane Database Syst Rev 2022;8(8):CD006895. DOI: 10.1002/14651858.CD006895.pub4
The most interesting result is not the one we expect. Reducing prescribed antibiotic use by 42% is a hard endpoint, which does not depend on the participant's subjective perception of symptoms, and which has public health significance in a country where antibiotic consumption remains high.
Two important limitations. The certainty of the evidence is weak to moderate according to the criteria, which the authors explicitly point out. And the strains vary enormously from one study to another: most trials used one or two lactobacillus strains at doses in the range of one billion colony-forming units per day, for more than three months. In other words, an effect observed with a specific strain does not automatically apply to any product on the shelf. Our article on probiotics and strain selection details this point.
7. Vitamin C: what it does and does not do
In trials conducted in the general population, including 10,708 participants, the relative risk of developing a cold with regular supplementation was 0.97. Five trials totaling 598 marathon runners, skiers, and soldiers in subarctic exercise gave a pooled relative risk of 0.48. Thirty-one comparisons examined the effect of regular vitamin C on cold duration: in adults, duration was reduced by 8%, and in children by 14%. In children, 1 to 2 grams per day shortened colds by 18%. Severity was also reduced. Seven comparisons examined the effect of vitamin C taken as a therapeutic agent: no consistent effect was observed on duration or severity. The failure of supplementation to reduce cold incidence in the general population indicates that routine supplementation is not justified.
Hemilä H, Chalker E. Cochrane Database Syst Rev 2013;(1):CD000980. DOI: 10.1002/14651858.CD000980.pub4
We sell acerola, and this conclusion is still included in our article, because it is what it is. What remains true after complete reading nonetheless deserves to be highlighted: the reduction of risk in half in people undergoing intense physical exertion is a clear result, across five convergent trials. If you are a long-distance runner, cyclist, or exposed to prolonged cold, you are in the population where vitamin C has shown a real preventive effect.
And for everyone, the reduction in episode duration with regular intake remains a modest but measurable benefit. The authors themselves conclude that, given the consistency of this effect, low cost, and safety, it may be worthwhile to try individually. This is an honest recommendation, far removed from the usual "cold shield" claims. Our article on choosing a natural vitamin C covers the available forms.
8. Zinc: the nuance that nobody makes
Zinc is probably the supplement most associated with colds in the public mind, and it is the one whose evidence file has deteriorated most with the latest available synthesis.
Thirty-four studies were included, fifteen on prevention and nineteen on treatment. Zinc supplementation could have little or no effect on the risk of developing a cold (relative risk 0.93; low certainty evidence) and little or no effect on the average number of colds occurring over five to eighteen months of follow-up. When colds occur, there is probably little or no difference in their duration (minus 0.63 days; moderate certainty). When used as treatment, zinc could reduce the average duration of cold (minus 2.37 days; 97% heterogeneity; low certainty evidence), but it is uncertain whether it reduces the risk of having another cold at the end of follow-up. There is probably an increase in the risk of non-serious adverse effects when zinc is used as treatment (relative risk 1.34; moderate certainty).
Nault D, Machingo TA, Shipper AG, et al. Cochrane Database Syst Rev 2024;5(5):CD014914. DOI: 10.1002/14651858.CD014914.pub2
A heterogeneity of 97% on the main treatment endpoint means that the studies do not say the same thing, to the point that the calculated average has limited value. Some trials show a marked effect, others none, and the protocols differ considerably: gluconate or acetate lozenges, doses of 45 to 276 milligrams per day, durations of 4.5 to 21 days, oral or intranasal routes.
Should we discard zinc altogether? No, and we must distinguish between two questions. Zinc as a nutrient remains essential for immune function, and its deficiency, which exists notably in vegetarians, elderly people, and in cases of digestive disease, genuinely impairs immune response. Zinc as a remedy for the common cold, that's another matter, and it's this claim that the Cochrane review does not support in prevention. Our article zinc, immunity and cold deserves to be revisited in light of this 2024 synthesis, more recent than its writing.
9. Echinacea and winter plants
Twenty-four double-blind trials with 4,631 participants, including a total of 33 comparisons between echinacea preparations and placebo, met the inclusion criteria. None of the twelve prevention comparisons reporting the number of patients who experienced at least one cold episode found a statistically significant difference. Of the seven treatment trials reporting data on cold duration, only one showed a significant effect of echinacea compared to placebo. Echinacea-based products have not demonstrated benefit for treating colds, although a minor benefit of certain products remains possible. Most consumers and physicians are unaware that products available under the term echinacea differ considerably in their composition.
Karsch-Völk M, Barrett B, Kiefer D, et al. Cochrane Database Syst Rev 2014;(2):CD000530. DOI: 10.1002/14651858.CD000530.pub3
The last sentence of this review likely explains the disappointing results as much as it comments on them. Under the same brand name coexist different species, different plant parts, root or aerial parts, and different extraction processes. Aggregating trials conducted on such dissimilar products amounts to comparing things that have nothing in common but a label.
What to do with other winter plants? We propose several, and honesty demands saying that their level of evidence on respiratory infections is lower than that of vitamin D. The propolis, the shiitake, the reishi and the moringa each have data, detailed in their respective articles, but none benefits from a meta-analysis comparable to those cited here. This does not disqualify them; it places them.
10. The micronutrients whose deficiency matters
A review devoted to the relationships between nutrition and the immune system identifies several micronutrients whose insufficiency degrades immune response and whose correction improves it, emphasizing their role as cofactors in vaccine response (Munteanu and Schwartz, Front Nutr, 2022).
| Nutrient | Immune role | Who risks deficiency |
|---|---|---|
| Vitamin D | Modulation of innate and adaptive immunity, production of antimicrobial peptides | Very large portion of the French population in winter, elderly, dark skin tones, overweight |
| Zinc | Development and function of lymphocytes, integrity of barriers | Vegetarians and vegans, elderly, digestive diseases, alcoholism |
| Vitamin C | Function of phagocytes, protection against oxidative stress from immune response | Low fruit and vegetable consumption, smokers, isolated individuals |
| Vitamin A | Integrity of respiratory and digestive mucous membranes, first line of defense | Rare in France; malabsorption, highly restrictive diets |
| Iron | Proliferation of immune cells; both excess and deficiency are problematic | Menstruating women, vegetarians, regular blood donors |
| Selenium | Antioxidant defense of immune cells, antiviral response | Diets excluding fish and animal products, nutrient-poor European soils |
| Copper | Function of neutrophils and macrophages | Rare; prolonged supplementation with high-dose zinc, malabsorption |
The principle that runs through this entire picture is the same as that of vitamin D: it is the correction of a deficiency that produces the effect, not accumulation. This is also what we have documented for selenium, whose U-shaped curve shows that excess can be harmful. A multivitamin at nutritional dose makes sense as a safety net for someone with a restricted diet; it has no demonstrated benefit for someone who eats a varied diet.
11. What works best and costs almost nothing
- Hand washing. Transmission of respiratory viruses occurs largely through hands and surfaces. It is the most documented measure in community prevention, and it is free.
- Ventilation. Ten minutes twice a day, even in winter, reduces the concentration of viral particles in indoor air, the primary transmission site during cold season.
- Influenza vaccination for those concerned, and other vaccinations recommended based on age and risk factors. This is a medical decision, but it falls within the same subject and it would be odd to omit it.
- Smoking cessation. Smoking impairs respiratory tract defenses, increases the frequency and severity of respiratory infections, and quitting remains the most cost-effective intervention in healthcare.
- Management of chronic stress, which interacts with sleep and the baseline inflammation described in section 3.
We sell dietary supplements, and we place this section before the conclusion anyway, because priorities matter more than the catalog. A customer who buys acerola while sleeping five hours a night and never washing their hands has made a poor investment, and it is our job to tell them so.
12. When weakened immunity warrants a doctor's visit
Your infections are abnormally frequent or severe : multiple pneumonias, sinusitis, or otitis in one year, infections requiring repeated antibiotics, or infections that always recur in the same location.
An infection persists longer than expected, or does not resolve despite appropriate treatment.
You have prolonged fever without an identified cause, night sweats, unexplained weight loss, or persistent swollen lymph nodes.
You are taking immunosuppressive treatment, prolonged corticosteroid therapy, chemotherapy, or you are a transplant recipient: in this case, any supplement must be validated by your medical team, as some are contraindicated.
You have a chronic disease that is poorly controlled, particularly diabetes, which impairs your response to infections.
In all these situations, the approach is a medical examination to identify a specific cause: deficiency, diabetes, immune deficiency, or medication side effects. Taking supplements instead delays diagnosis without correcting anything.
13. Building your winter strategy
Our Organic acerola powder provides natural-source vitamin C with no additives. What studies support: a reduction in episode duration with regular intake, and infection risk cut in half for people under intense physical stress. Not universal prevention.
Discover organic acerola →Also discover our Plant-based vitamin D3, our Organic propolis and our complete immunity collection.
Choose the situation that matches yours best: the answer appears just below.
Three or four colds per season in an adult is ordinary and does not indicate failing immunity. In order of evidence: sleep first, seven to nine regular hours, then vitamin D, the only nutrient with high-quality evidence on respiratory infections, taken daily or weekly and after medical advice. Probiotics come next, with fewer episodes, shorter episodes and especially 42% fewer antibiotic prescriptions in the Cochrane review. And don't forget hand washing and ventilation, which do better than most supplements at zero cost.
Vitamin C does not prevent colds in the general population, this is established in over 10,000 participants. But in marathoners, skiers and soldiers in Arctic exercise, five converging trials show a risk cut in half. You are in this population, and regular intake therefore makes sense for you, which is not the case for your sedentary neighbor. Also pay attention to training load: intense and prolonged efforts transiently depress immunity, and periods of high load are accompanied by more frequent respiratory infections. Our article on fatigue and overtraining details the signals to watch for.
Once the cold has started, the validated arsenal is thin. Vitamin C started at this point has not shown a consistent effect in therapeutic trials. Zinc in lozenges could reduce duration by about two days, but with weak certainty of evidence, 97% heterogeneity between studies, and a likely increase in minor adverse effects, especially digestive and gustatory. In other words, it's doable with full knowledge, not an obvious choice. The rest falls under comfort: hydration, rest, nasal washes with saline solution. And if symptoms worsen, if fever lasts more than three days or if shortness of breath appears, consult a doctor.
Frequent, severe, prolonged infections or those requiring repeated antibiotics cannot be corrected with a course of supplements. They warrant a medical workup to search for a specific cause: deficiency, poorly controlled diabetes, medication side effect, underlying ENT pathology, or more rarely immune deficiency. Taking supplements instead delays this diagnosis without solving anything. Make an appointment, and if possible bring a list of your infectious episodes from the past twelve months and treatments received: this is exactly the information your doctor needs to guide their investigation.
This test provides guidance; it does not replace professional medical advice.
Frequently asked questions about immunity
What is the most effective lever for strengthening immunity?
Sleep, ahead of any dietary supplement. A review published in 2021 recalls that sleep deprivation impairs both innate and adaptive immunity, establishes a chronic inflammatory state and increases the risk of infections. Moderate and regular physical activity comes right after, with documented effects on vaccine response and immunosurveillance. No nutrient tested in a randomized trial reaches this level of effect. The honest hierarchy therefore places supplements after these two levers, never in their place.
Does vitamin D really strengthen immunity?
It is the nutrient with the best evidence in this area. A meta-analysis on individual data from 25 trials and 11,321 participants showed that supplementation reduces the risk of acute respiratory infection, with an odds ratio of 0.88. Two conditions stand out: the effect only appears with daily or weekly intake, not with occasional high-dose loading, and it is much more marked in people who are truly deficient, with an odds ratio of 0.30 below 25 nanomoles per liter versus 0.75 above. The quality of evidence was judged high.
Does zinc protect against colds?
Not for prevention, according to the most recent Cochrane review. Covering 34 trials and 8,526 participants, it concludes that zinc probably has little or no effect on the risk of catching a cold. However, taken at the onset of symptoms, it could reduce episode duration by about two days, with weak level of evidence and considerable heterogeneity between studies. The authors also note a likely increase in minor adverse effects during treatment, mainly digestive and gustatory.
Does vitamin C prevent catching cold?
Not in the general population. The Cochrane review covering 29 comparisons and 11,306 participants finds no reduction in the risk of catching a cold with regular supplementation. Two results are more interesting: in people subjected to intense physical effort, marathoners, skiers and soldiers in Arctic exercise, the risk is reduced by half; and with regular intake, the duration of colds decreases by 8% in adults and 14% in children. However, starting vitamin C once the cold has begun has not shown a consistent effect.
Do probiotics really help against winter infections?
It is the second best case after vitamin D. The 2022 Cochrane review, covering 23 trials and 6,950 participants, finds fewer people experiencing at least one upper respiratory infection episode, episode duration reduced by approximately 1.2 days, and especially fewer antibiotic prescriptions. The level of evidence remains low to moderate according to criteria, strains and doses vary greatly from study to study, and adverse effects are minor and digestive. This is encouraging without being definitive.
Is echinacea effective against colds?
The evidence does not support it. The Cochrane review devoted to this plant, covering 24 double-blind trials and 4,631 participants, concludes that echinacea-based products have not demonstrated benefit for treating colds. A weak preventive effect remains possible, with prevention trials showing consistent but non-significant positive trends of questionable clinical relevance. The authors also highlight a major difficulty: products sold under the name echinacea differ considerably depending on species, plant part and extraction method.
Should I do a course of vitamin D in winter?
At our latitudes, cutaneous synthesis is zero from October to March, and deficiency is frequent in France at the end of winter. Data on respiratory infections argue for daily or weekly supplementation rather than for high-dose ampoules spaced out, which have not shown protective effect in the reference meta-analysis. The dose should be discussed with a doctor, particularly in case of kidney disease, sarcoidosis or ongoing treatment, and a blood test allows you to know where you stand rather than supplementing blindly.
How do I know if my immune system is weakened?
There is no simple test measuring immune strength, and this is a frequent disappointment in consultation. Certain signals do warrant medical advice: unusually frequent, severe or prolonged infections, repeated infections affecting the same organ, prolonged fever without cause, unexplained weight loss, or deep and lasting fatigue. These situations warrant searching for a specific cause—diabetes, deficiency, immunosuppressive treatment or immune deficiency—rather than justifying blind supplementation with supplements.
Does sports strengthen or weaken immunity?
Both, depending on the dose. Reference work in exercise immunology describes a beneficial effect of moderate and regular physical activity, with less inflammation, better maintenance of thymic mass and improved vaccine response, even in older people. Conversely, prolonged and intense efforts can transiently depress T lymphocyte, NK cell and neutrophil function, and athletes frequently report upper airway infections during periods of high load. Daily brisk walking therefore does better for immunity than a marathon.
What role does sleep play in immunity?
A central and underestimated role. Sleep supports the body's defense against infections and inflammatory attacks; its deprivation is accompanied by alterations in innate and adaptive immunity parameters, chronic inflammatory state, and increased risk of infectious and inflammatory diseases. In other words, getting enough sleep is not an added comfort tip at the end of an article: it is probably the most powerful immune lever at your disposal, and it is free.
What to think of products that promise to boost immunity?
The verb "boost" does not correspond to anything measurable, and there is no authorized health claim in Europe formulated in these terms. An immune system cannot be stimulated like pressing an accelerator: overactivated immunity is not good news, it is the mechanism of autoimmune diseases and allergies. What science supports is more modest and more useful: correcting deficiencies that impair immune response, particularly vitamin D deficiency, and supporting the general conditions for proper functioning—sleep, physical activity, and varied diet.
Which nutrients really matter for immunity?
A review dedicated to the links between nutrition and immunity identifies several micronutrients whose deficiency impairs immune response, notably vitamins A, C and D, zinc, selenium, iron and copper. The important point is that the benefit of supplementation depends on baseline status: correcting a deficiency improves response, adding it to someone whose intake is adequate has no demonstrated effect. This is exactly what the meta-analysis on vitamin D showed, where the effect is four times stronger in severely deficient individuals.
What to concretely start with before winter?
With sleep, aiming for seven to nine regular hours, and with moderate daily physical activity—brisk walking is sufficient to produce the effect described in studies. Next, assess your vitamin D status with your doctor, since it is the only nutrient with high-quality evidence on respiratory infections, and prefer daily or weekly dosing. Then, possibly probiotics during the season. Hand washing, room ventilation, and flu vaccination for those concerned are among the most effective measures, and cost almost nothing.
- Innate immunity
- First line of defense, immediate and non-specific: skin and mucous barriers, phagocytic cells, NK cells, inflammation.
- Adaptive immunity
- Specific and memorized response, carried by T and B lymphocytes and antibodies. Slower to develop, but lasting: this is what vaccination solicits.
- Upper respiratory tract infection
- Infection of the nose, throat, sinuses or ears: common cold, rhinopharyngitis, sinusitis, sore throat, otitis. Primary criterion in most trials cited here.
- Individual participant data meta-analysis
- Synthesis that combines data from each participant rather than published results alone. This is the most reliable format, used in the reference study on vitamin D.
- Odds ratio
- Measure of the effect of an intervention. A value below 1 indicates risk reduction: 0.88 corresponds to approximately 12% less risk.
- Heterogeneity
- Degree of divergence of results among studies included in a meta-analysis. Heterogeneity of 97%, as with zinc treatment, means that the calculated average has limited value.
- Immunosenescence
- Aging of the immune system, marked by lower vaccine response and higher baseline inflammation. Regular physical activity attenuates certain aspects of it.
- 25-hydroxyvitamin D
- Circulating form of vitamin D, measured in blood to assess status. Below 25 nanomoles per liter, the deficiency is considered severe.
- Martineau AR, Jolliffe DA, Hooper RL, et al. Vitamin D supplementation to prevent acute respiratory tract infections: systematic review and meta-analysis of individual participant data. BMJ. 2017;356:i6583. doi:10.1136/bmj.i6583
- Zhao Y, Dong BR, Hao Q. Probiotics for preventing acute upper respiratory tract infections. Cochrane Database Syst Rev. 2022;8(8):CD006895. doi:10.1002/14651858.CD006895.pub4
- Nault D, Machingo TA, Shipper AG, et al. Zinc for prevention and treatment of the common cold. Cochrane Database Syst Rev. 2024;5(5):CD014914. doi:10.1002/14651858.CD014914.pub2
- Hemilä H, Chalker E. Vitamin C for preventing and treating the common cold. Cochrane Database Syst Rev. 2013;(1):CD000980. doi:10.1002/14651858.CD000980.pub4
- Karsch-Völk M, Barrett B, Kiefer D, Bauer R, Ardjomand-Woelkart K, Linde K. Echinacea for preventing and treating the common cold. Cochrane Database Syst Rev. 2014;(2):CD000530. doi:10.1002/14651858.CD000530.pub3
- Garbarino S, Lanteri P, Bragazzi NL, Magnavita N, Scoditti E. Role of sleep deprivation in immune-related disease risk and outcomes. Commun Biol. 2021;4(1):1304. doi:10.1038/s42003-021-02825-4
- Nieman DC, Wentz LM. The compelling link between physical activity and the body's defense system. J Sport Health Sci. 2019;8(3):201-217. doi:10.1016/j.jshs.2018.09.009
- Simpson RJ, Kunz H, Agha N, Graff R. Exercise and the regulation of immune functions. Prog Mol Biol Transl Sci. 2015;135:355-380. doi:10.1016/bs.pmbts.2015.08.001
- Munteanu C, Schwartz B. The relationship between nutrition and the immune system. Front Nutr. 2022;9:1082500. doi:10.3389/fnut.2022.1082500
