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Seasonal Immune Support: What the Evidence Says on Vitamin D, C, Zinc & More

Well, it’s that time of year again. School has started, days are shorter, and winter jackets are coming out. These seasonal fluctuations in life may or may not be appreciated depending on hobbies and geographic location, but one thing stands the same despite these changes: no one is excited about catching a cold or seasonal influenza.

Whether you were affected by illness or not last year may impact your motivation on avoiding the situation this year, and strategies you elect to prevent illness. For some, there may be masking, now normalized after the pandemic, increased hand washing, and avoidance of crowded indoor spaces. For others, particularly those with children in schools or jobs that require social interaction and contact, implementing a dietary supplement strategy, particularly one that is evidence-based, may be at the top of their list. Regardless of which bucket you fall into, the primary objective is the same: let’s just not get sick.

Why Does the Incidence of Colds and Flu Increase in Winter?

There are a lot of factors at play here. Although there is a surge in respiratory infection rates in the fall,1 when kids go back to school, the highest incidence in influenza rates are later in the year, typically between December and March, with February being the peak month in nearly half of the 38 years surveyed.2

One of these factors is that cold, dry winter air can increase the stability and transmission efficiency of both respiratory viruses and influenza.3,4 Not only that, dry air also weakens the respiratory tract’s normal defenses, impairing mucociliary clearance and airway epithelial repair mechanisms, as well as innate antiviral defenses.5 Addressing the humidity variable alone may give one a leg up on infections: a study found that introducing a humidifier that brought the air to ~45% relative humidity in preschool rooms in Minnesota in the dead of winter (January-February) significantly decreased both the number of influenza A virus positive samples (air and fomites) and actual influenza-like illnesses leading to school absences.6

One might also wonder, does less daylight weaken the immune system in winter? Exposure to sunlight, particularly bright light that includes wavelengths between ~450-480 nm (blue light), is a major driver of our circadian rhythm, which impacts melatonin, cortisol, and countless other things.7 However, such may not be the case as shorter days alone in human test subjects actually were found to upregulate facets of immune function including natural killer cell cytotoxicity, peripheral blood mononuclear cell proliferation (in both absence of and response to stimulation), and plasma levels of interleukin-6.8 So, it appears that less daylight increases the body’s drive to protect us more from infection. But a bigger part of the story may be the big D – that is, vitamin D.

The skin produces vitamin D3 (cholecalciferol) upon exposure to UVB radiation from the sun. However, when the sun becomes lower in the sky, UVB radiation is forced to travel through a greater distance of the atmosphere and ozone which absorbs much of its intensity, so much that it starts to have a negligible impact on the skin’s vitamin D production in the northern parts of the US beginning in the fall, with no amounts of vitamin D3 production (if one lives at the latitude of Boston) between November and February.9 So, that sunny day in the depths of winter may seem to help improve your mood, but it does nothing for vitamin D.

But, how fast do vitamin D levels fall during winter? Well, human data has shown that vitamin D levels may even drop exponentially when sun UVB exposure ceases, with most dramatic drops seen in those having higher 25(OH)D levels initially.10 In larger Northern populations (Toronto, Canada), the average decline in mean 25(OH)D levels was 30% from fall to winter,11 while a very large (>3.4 million people) population study of people from all over the US found that the trough (lowest) 25(OH)D levels are in February, with approximately 1/3 of the population having 25(OH)D levels less than 25 ng/mL at and around this time.12

→ Find more articles on all things immunity on our Immune Health Resource Center.

The Best Vitamins and Minerals for Immune Support

Clearly, diminishing levels of vitamin D, with its well-known importance for immune system function, are important to address. Essential nutrients with similar importance for immune health are vitamin C and zinc. Some of the specific infectious disease risks related to low vitamin D, vitamin C, and zinc levels have already been discussed elsewhere. [CD1] Although vitamin C and zinc are not primarily affected by seasonal changes in sunlight exposure, their metabolism changes rapidly during infection, with increased vitamin C utilization and redistribution of zinc from circulation into tissues and immune cells as part of the acute immune response.13,14

But do you need more vitamin C, vitamin D, and zinc when you’re sick? Should you take more to prevent an infection? How much is too much? And how should these nutrients be taken to optimize their benefits? Should you take them all at once, or divide the doses during the day? All of these are common questions people have as they embark upon trying to prevent or resolve seasonal infections. And, many of these questions have evidence-based answers.

Vitamin C

The question of how much vitamin C the body can absorb at once has been addressed. In otherwise healthy individuals who were depleted of vitamin C, oral vitamin C absorption is dose-dependent and saturable, with rapidly diminishing absorption above 100 mg: at 100 mg, 80% is absorbed, at 200 mg, 72%, and at 500 mg, only 63%.15 At 1,000 mg, oxalate levels in the urine were increased, although they remained within the normal reference range. Although this landmark study was performed in 1996, it helped inform subsequent recommendations for vitamin C intake.

The body appears to utilize more vitamin C during the common cold: in a small human study, at a dose of 200 mg, infection-associated leukocyte depletion was not reversed/prevented, but at doses of 6 g it was.16 Most subjects experienced abdominal colic and loose stool at the 3 g/day dose. This data led the researchers to suggest the dose of 1 g/day for infection prophylaxis; however, this 1973 study predated the landmark absorption study in 1996.

From these key studies we can surmise a lot about vitamin C, including how vitamin C should be taken in sickness and in health, how to dose it, and how much might be too much. For healthy individuals, single doses of up to 200 mg will be mostly absorbed; if more is taken, dividing doses during the day may improve fractional absorption and maintain bioavailability. For individuals with seasonal infections, the evidence supports increased utilization/requirements during infection, not increased intestinal absorption.

How much vitamin C is too much? Well, at the higher end of dosing, both gastrointestinal disturbances, including abdominal pain and diarrhea, may occur, while increased urinary oxalates may pose a problem for individuals prone to calcium oxalate kidney stones. Not discussed within either of these studies, individuals with G6PD deficiency should use caution with very high doses of vitamin C because rare cases of hemolysis have been reported, particularly with high-dose intravenous vitamin C.17 Additionally, individuals with iron-overload disorders should approach higher doses of vitamin C with caution as it can increase non-heme iron absorption.18

Zinc

The same questions that exist for vitamin C are common question for zinc: how much zinc should be taken to prevent infection, should more zinc be taken during a cold, how much zinc is too much, and should zinc all be taken at once or divided during the day?

For individuals using supplemental zinc to maintain adequate zinc status, approximately 10–15 mg/day is a reasonable daily dose. Zinc deficiency is well established to impair immune function and resistance to infection.19,20 Zinc deficiency is not uncommon, with a recent survey of Western countries finding this mineral to be deficient in 31% of women and 49% of men living independently, with even higher percentages of institutionalized individuals having deficiencies.21

In the setting of infection, particularly the common cold, there is data to suggest that increasing the total daily intake of zinc short-term can be beneficial in reducing respiratory tract infection symptoms and shortening the duration.22 One study looked at zinc supplementation in children for the prevention of and treatment of the common cold, finding that 15 mg/day helped both prevent colds and reduce cold-related school absences, and that the higher dose of 30 mg/day shortened the duration of cold symptoms and decreased their severity significantly.23

However, taking doses of zinc substantially higher than the RDA for an ongoing period doesn’t pan out to be necessarily better, for multiple reasons. Firstly, chronic higher zinc intake leads to homeostatic adaptation that reduces fractional absorption. This was shown in a study of postmenopausal women eating a controlled diet supplemented to include 14 to 47 mg of zinc per day. It was found that at day 0, significantly less zinc was absorbed in those with less total intake, however, at week 8 there was no significant difference in the amount of zinc absorbed with all individuals being shown to absorb between 5.4-6.4 mg/day.24 Additionally, plasma zinc concentrations were unaffected.

Another thing that can become an issue with chronic high levels of zinc intake is that it can impair copper status. In daily multivitamins, we see smaller amounts of copper included alongside zinc; however, this is not common in immune-focused zinc products. One therefore may question, is it okay to take additional zinc in the setting of an infection without extra copper? And if so, for how long?

Two studies have attempted to answer this question; both were in healthy adult men. In one study, men were supplemented 30 mg of zinc/day alongside dietary intake of 10 mg for 14 weeks, finding no negative impact on markers of copper status.25 In the other study, men were supplemented with 25 mg of zinc, given twice daily, for a total dose of 50 mg (alongside any dietary intake) for 6 weeks. At week 6, it was found that erythrocyte Cu,Zn-superoxide dismutase (an antioxidant enzyme) was significantly decreased in the supplemented group versus placebo, suggesting this dose did impair copper status.26 These studies suggest that higher zinc intake of 30 mg/day during winter cold and flu season is safe for healthy individuals, however, taking some additional copper (such as within a multivitamin) should be considered.

Finally, to answer the dosing question, is it better to take zinc all at once, or as divided doses during the day: a study looked at zinc absorption from aqueous zinc sulfate providing zinc ranging from 2.2 to 30.1 mg/dose, finding that doses of zinc in excess of 20 mg had relatively small and progressively diminishing increases in zinc absorption.27 From this, we can conclude that higher seasonal zinc supplementation should be divided during the day.

In otherwise healthy individuals who were depleted of vitamin C, oral vitamin C absorption is dose-dependent and saturable, with rapidly diminishing absorption above 100 mg: at 100 mg, 80% is absorbed, at 200 mg, 72%, and at 500 mg, only 63%.

Vitamin D

Unlike vitamin C and zinc, vitamin D is a fat-soluble nutrient that is stored in the body, and maintaining adequate vitamin D status is more relevant than acutely increasing intake when an infection develops. Although vitamin D plays an important role in immune function, there is not strong evidence that increasing the dose acutely will prevent or resolve seasonal infections. Maintaining blood 25(OH)D levels above 30 ng/mL is recommended for reduced infection risk and general health and well-being.28 Because levels can decrease fairly rapidly, especially if sun exposure abruptly drops, vitamin D supplementation beginning in the fall is suggested, although year-round intake of 1,000–2,000 IU/day is generally safe.29 Higher doses of 4,000 IU/day during the winter for up to 5 months have also been shown to be safe.30

Often, the question about vitamin D dosing that arises is should vitamin D be taken daily, or can a bolus dose be taken weekly or even monthly and achieve the same therapeutic effect? A systemic review and meta-analysis of randomized controlled trials has addressed this, finding that no significant protective effects were seen if vitamin D was given weekly or monthly to every three months, while significant protective effects against respiratory infections were seen if it was given daily.31

Immune-Support Supplements Beyond Vitamins and Minerals

Astragalus (Astragalus membranaceus) is a botanical that comes from Chinese medicine traditions and has been researched for its immune-supportive and adaptogenic qualities. Adaptogenic botanicals are used to promote stress resilience, and some, such as astragalus, enhance immune function under stress. Prolonged stress is well recognized to impair aspects of immune surveillance, including both number and function of our protective natural killer cells,32 and can contribute to broader immune dysregulation. Strenuous physical exercise is also a stress that negatively impacts immune function and happens to be a setting in which astragalus has been studied and found to have protective effects. Specifically, daily supplementation of astragalus to elite rowers during a 6-week training camp was found to prevent declines in natural killer cell and T regulatory cells that were observed in the placebo group who were not given the supplement.33 Additionally, it promoted a healthier Th1 response in the athletes, rather than the Th2 response that strenuous exercise induced.

Multiple studies have investigated the possible benefits of astragalus in individuals with cancer, a setting in which therapeutic goals include both reducing fatigue and promoting a healthy protective immune response. In one study of individuals with advanced cancer and moderate to severe cancer-related fatigue, the infused astragalus-derived drug significantly reduced fatigue symptoms for a prolonged period even though the injections were only given every 4 weeks.34 A systemic review and meta-analysis of 8 studies also found astragalus was effective in reducing cancer-related fatigue and improving quality of life.35

The impact of astragalus on immune function in different types of cancer has been investigated in numerous trials, in fact, a 2025 systemic review and meta-analysis included 31 clinical trials investigating just this. Overall, the findings from this large review found that supplementation with astragalus improved treatment outcomes, enhanced immune function, and demonstrated a high level of safety.36 Earlier systemic reviews and meta-analysis looking at the addition of astragalus to chemotherapy in the setting of non-small cell lung cancer and gastric cancer found that it significantly increased NK cells, CD3+ cells, CD4+ cells, and the CD4+/CD8+ ratio, additionally improving both objective response rate and 1-year survival rate compared with chemotherapy alone.37,38

Studies suggest that higher zinc intake of 30 mg/day during winter cold and flu season is safe for healthy individuals, however, taking some additional copper (such as within a multivitamin) should be considered.

Herbs and Autoimmune Conditions

A question that comes up with botanicals like astragalus is the safety for individuals with autoimmune disease. Historically conservative opinions have suggested against this as the possibility of stimulating an immune response can be problematic when the immune system attacks self-antigens. A 2026 review took a look at 112 studies published from December 2020 to December 2025, finding that rather than having a purely stimulatory function, astragalus and its derivatives help to restore immune tolerance via multiple mechanisms including modulating core signaling pathways (including both Th1/Th2 balance and the Treg/Th17 axis), regulating immune cell function, and altering gut microbiota composition, collectively having an effect of systematically reinstating immune homeostasis.39

Specific ways astragalus may help protect against seasonal infections such as respiratory and flu virus may be related to its direct antiviral effects and impact on mucosal defenses. Astragalus polysaccharides have been shown to inhibit viral replication in vitro in multiple studies,40,41 while whole astragalus preparation has been shown to inhibit viral-induced cellular damage and death.42 Respiratory immune defenses may also be improved by astragalus as it has been shown to induce expression of human cathelicidin antimicrobial peptide LL-37, a key molecule that helps protect bronchial epithelial cells from infection.43

Given its broad safety data in special settings such as cancer and more recent findings related to autoimmune disease, it is a botanical worth consideration for seasonal immune protection.

Practical Take Homes

Hopefully, the information presented here helps clear up common questions that even practitioners may have about the key nutrients most often recommended for immune support. Basic infection preventative strategies should be implemented before illness starts: consider vitamin D as sun exposure declines, maintain adequate vitamin C and zinc intake, address dry indoor air, and support the body’s broader defenses with adequate sleep, nutrition, stress management, and other evidence-supported approaches. Botanicals such as astragalus may offer additional immune support, particularly during periods of physical or psychological stress, but supplementation should complement—not replace—the fundamentals. Ultimately, there is no single nutrient or botanical that guarantees protection from seasonal infections; but, a consistent, evidence-based approach that supports multiple aspects of immune function may help lower your infection risk as you navigate cold and flu season.

References

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