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Keeping Your Immune System On Guard

Keeping Your Immune System On Guard

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Six supplements that support healthy immune system function

Let’s face it: we are surrounded by a veritable sea of viruses, bacteria, and other infectious agents. What keeps these nasties at bay, in addition to physical barriers such as the skin and mucus secretions, is the immune system. If pathogens happen to gain entry, the immune system (hopefully) goes to work, attacking the pathogens before they set up a full-blown infection.

The first line of defense is innate immunity, which combines physical and biochemical barriers with a cellular response mediated by several different types of white blood cells. Receptors on the various white blood cells are designed to sense the presence of viruses, activate signaling cascades that eliminate viral components from cells, and induce apoptosis (programmed cell death) of cells that become infected.[1] If the virus manages to avoid our innate defenses, a more adaptive and specific response is triggered, mediated by lymphocytes. Lymphocytes produce cytokines including interferon (IFN)-γ and induce the production of antibodies that help alert the other immune responders of the pathogen’s presence, further activating their response as well.[2]

A good diet,[3] regular exercise,[4] sufficient sleep,[5] and a healthy microbiome are all important for a robust immune response.[6],[7],[8] Supplementing the diet with key nutrients and botanicals may also help reduce the risk and severity of infections and support a faster recovery at all ages.[3],[9],[10] Based on the latest scientific evidence, we’ll discuss six nutrients that support the immune response to infections.

Vitamin C

Taking supplemental Vitamin C may reduce the incidence of colds in individuals with suboptimal vitamin C levels.

Adequate vitamin C (ascorbate) is indispensable for a healthy immune response.[11],[12],[13],[14] In fact, vitamin C has a pivotal impact on both innate and adaptive immune responses against viral and bacterial infections.[12] Among its many functions, vitamin C stimulates the production of IFN at the initial stage of viral infection, thereby heightening the body’s defenses.[15],[16]

Multiple clinical trials have shown that vitamin C supplementation may not only shorten the duration and severity of common cold symptoms, but also reduce the incidence of colds in individuals with suboptimal vitamin C levels.[13],[14],[17]

Vitamin C is quickly depleted during the common cold, influenza, or pneumonia, meaning that vitamin C requirements go up dramatically when one is ill.[13],[18] Ascorbate levels in white blood cells were shown to decline by 50% when subjects contracted a cold, and only returned to the original levels one week after recovery.[19] In healthy individuals, a daily vitamin C intake of 100 to 200 mg provides an adequate amount of vitamin C to saturate the plasma.[20],[21] However, doses that are 10 times higher may be needed once an infection has taken hold, due to increased requirements for the vitamin to fight the infection.[14],[22],[23] Consuming 1 gram per day of vitamin C was shown to shorten the duration of colds in adults by 6% on average, and consuming ≥2 grams per day shortened the duration of colds by 21%.[24] A meta-analysis also showed that extra doses of vitamin C could benefit patients who contract the common cold, even if they were already supplementing vitamin C regularly.[22]

Vitamin C is also important for those who suffer from allergies and asthma, which you can read more about here.

Vitamin D

Vitamin D deficiency is widespread among all ages, especially in the winter months and those with limited sun exposure. Low levels of vitamin D are associated with an increased risk of upper respiratory infections.

Vitamin D is mainly thought of in terms of bone health, but this fat-soluble vitamin has emerged as a central regulator of the body’s defense against infections.[25],[26],[27] Among its numerous immune-supportive functions, vitamin D induces the expression of peptides known as defensins and cathelicidins,[28],[29] which can inhibit many different viruses and bacteria.[30],[31],[32],[33]

Vitamin D deficiency is widespread among children, adults, and the elderly.[34] Low levels of vitamin D are associated with an increased risk of upper respiratory infections (influenza and pneumonia), as well as otitis media (ear infections), urinary tract infections, infectious hepatitis, and other infectious diseases.[26],[35],[36] The data suggest that vitamin D supplementation may cut the risk of acute respiratory tract infections by an average of one-third or more,[26],[37] and that individuals with low starting levels of 25(OH)D (<40 nmol/l) are likely to see the most benefit.[38],[39] A recent systematic review concluded that vitamin D supplementation was safe and that it protected against acute respiratory tract infections overall.[40]

To reach a 25(OH)D blood level of at least 75 nmol/L (30 ng/mL), which is the level advised by many experts, requires 1,000 to 2,000 IU per day of supplemental vitamin D3 (cholecalciferol), or even more in some individuals.[34],[41],[42],[43],[44] There is a good rationale to combine vitamin D with vitamin C and zinc (discussed below), in light of their synergistic effects on respiratory tract infections.[45] Vitamin D also plays an important role in the setting of allergies and asthma, much like vitamin C.


Even moderate zinc deficiencies can increase the risk of opportunistic infections.

Zinc is a trace mineral that is crucial for the proper function of immune cells and for resistance against infections.[46] As with many nutrients, plasma zinc levels tend to be low in older adults.[47],[48],[49] Zinc deficiency reduces the activity of serum thymulin (a thymic hormone that is required for maturation of T-helper cells), resulting in cell-mediated immune dysfunction.[50] Even moderate zinc deficiencies can increase the risk of opportunistic infections, including pneumonia.[46],[49],[51],[52],[53]

Zinc exerts its antiviral effect by interfering with multiple stages of the viral life cycle, which may include inhibiting the virus entry into the host and reducing its ability to replicate.[54],[55],[56],[57],[58] Zinc supplementation can help reduce the duration of common cold symptoms if it is administered within 24 hours of the onset of symptoms.[59],[60] A meta-analysis of three randomized placebo-controlled trials, using zinc lozenges, estimated a threefold increase in the rate of recovery from the common cold in the groups receiving zinc.[61] A combination of zinc (10 to 30 mg daily) with vitamin C (1 gram daily) has also been suggested to shorten the duration of respiratory tract infections, including the common cold.[11],[62]


A recent review concluded that “Supplements containing selenium up to 200 mcg per day have potential as safe, inexpensive, and widely available adjuvant therapy in viral infections.”

Selenium is considered a cornerstone of the body’s antioxidant defense system. It serves as an essential cofactor for glutathione peroxidase (GPx), an enzyme that utilizes glutathione to clean up reactive oxygen species that damage immune cells.[63],[64],[65] Consequently, selenium supports the acute cellular immune response toward infectious agents.[66],[67],[68] In humans, supplementation of selenium to adults (50 or 100 mcg per day for 15 weeks) increased the production of IFN-γ,[69] a cytokine that is critical for innate and adaptive immunity.[70] Elderly participants of a randomized controlled trial who had low plasma selenium concentrations at baseline, and who received a daily supplement of 100 mcg selenium and 20 mg zinc for 15 to 17 months, showed a better antibody response after influenza vaccination than individuals in the placebo group.[71]

When selenium levels are insufficient, the amount of GPx can drop to a mere 10% of normal levels.[72] This impairs the immune response and allows viruses to replicate freely.[73],[74],[75],[76] Selenium deficiency has been shown to increase one’s susceptibility to viral infections, especially to RNA viruses such as human immunodeficiency virus (HIV), hepatitis C, and influenza A.[66] In a mouse model of influenza, the mortality of the selenium-deficient mice was 75%, whereas the mortality of mice supplemented with selenium was 25%.[77] A recent review concluded that “Supplements containing selenium up to 200 mcg per day have potential as safe, inexpensive, and widely available adjuvant therapy in viral infections.”[66] Higher doses (over 300 mcg per day) should be avoided, as there is a narrow window of safety for selenium.

Green tea catechins

Epigallocatechin-3-gallate (EGCG), the most abundant catechin in green tea, can inhibit many of the viruses that afflict humans.

Green tea (Camellia sinensis) has been one of the most commonly consumed health-promoting beverages in many parts of the world for thousands of years.[78] Several studies have reported that green tea polyphenols, including catechins, enhance cellular immune responses.[79],[80],[81] Among healthy adults aged 18 to 70 years who took green tea capsules twice daily for 3 months, there were 32% fewer subjects with symptoms of a cold or flu compared to subjects taking a placebo.[82]

Epigallocatechin-3-gallate (EGCG), the most abundant catechin in green tea, can inhibit many of the viruses that afflict humans, with activity shown in the laboratory against adenovirus, enterovirus, hepatitis B and C, HIV, herpes, influenza, and rotavirus.[83],[84],[85],[86],[87] A study of the mechanism of action against influenza showed that EGCG reduced the viral membrane integrity, thus making the virus less capable of invading host cells.[88]

A clinical trial of 124 adults aged 65 and older showed that gargling with a green tea catechin extract three times daily for three months lowered the rate of influenza infection, from 10% in the control group to only 1.3% in the catechin group.[89] In a randomized controlled trial of 200 health care workers, supplementation with green tea catechins (378 mg) and theanine (210 mg) daily for five months lowered the risk of influenza from 13% in the placebo group to 4% in the catechin group.[90] (Theanine is another ingredient of Camellia sinensis, often used to promote relaxation,[91] which also primes immune cells to respond to infections).[92]

A recent randomized controlled trial evaluated the effect of consuming a catechin-containing beverage on the incidence of acute upper respiratory tract infections (URTIs) in 270 health care workers.[93] Participants consumed either three daily doses of 57 mg catechins, one daily dose of 57 mg catechins, or a placebo for two months. The URTI incidence was 27 to 28% in the placebo and low-dose catechin groups compared with only 13% in the higher-dose group, indicating that the risk of URTI was reduced to about half by ingesting a catechin-containing beverage three times daily.


Dysbiosis (an imbalance in the gut microbiota) can increase one’s risk not only for gastrointestinal infections but also for respiratory tract infections.

The microbiome plays an important role in the regulation of immune responses and the defense against pathogens. Dysbiosis (an imbalance in the gut microbiota) can increase one’s risk not only for gastrointestinal infections but also for respiratory tract infections.[94] Compared with a placebo, probiotics were shown to help reduce the number of participants experiencing episodes of acute URTIs, shorten the mean duration of an episode of acute URTI, and reduce the number of days absent from work.[95],[96],[97],[98]

In a study of influenza virus in mice, oral administration of Lactobacillus species was shown to stimulate the gut and respiratory immune responses, alleviate clinical symptoms, and reduce virus-associated lung damage.[99],[100] The daily intake of fermented milk containing a strain of Lactobacillus casei was shown to reduce the incidence and duration of URTIs in healthy middle-aged office workers.[101] The participants consumed 100 billion viable probiotic cells, or a control, once daily for 12 weeks during the winter season. The incidence of URTIs during the intervention period was significantly lower in the probiotic group (22.4%) than in the control group (53.2%).

Another major class of probiotics, Bifidobacterium, has an especially important role in the immune response against viral and bacterial infections.[102],[103],[104] Bifidobacterium is particularly important for adults who are middle-aged and beyond, because gut populations of bifidobacteria decline with age (and with antibiotic use),[105],[106],[107] which contributes to a lowered resistance to infections. Four different clinical trials have shown that Bifidobacterium lactis supplementation was highly efficacious in boosting immune cell activity in elderly individuals, who are most susceptible to respiratory infections.[108]


The complex, integrated immune system needs multiple specific nutrients, which play vital and often synergistic roles at every stage of the immune response.[9] Even mild nutritional deficiencies can render us more susceptible to infectious microbes looking for a cozy home.[109],[110] For targeted immune support, vitamins C and D, zinc, selenium, green tea catechins, and probiotics may be just what your body needs.

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The information provided is for educational purposes only. Consult your physician or healthcare provider if you have specific questions before instituting any changes in your daily lifestyle including changes in diet, exercise, and supplement use.

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