6 immune-boosting nutrients for cold and flu season

The 2023-2024 flu season is currently underway in the United States, with all parts of the country reporting elevated levels of flu activity. So far, there have been at least 2.6 million flu illnesses, 23,000 hospitalizations, and 1,300 deaths from the flu, according to CDC estimates.^[]

Many of us have already taken preventive measures in the form of this year’s flu shot, which is designed to protect against multiple flu viruses. The composition of this season's shot contains both egg-based vaccines and cell- or recombinant-based vaccines.^[] Because our ability to combat viruses and infections wanes as we get older, taking further steps to bolster our immune defenses is imperative.

Probiotics

The Food and Agriculture Organization of the United Nations and WHO defines probiotics as “live microorganisms that, when administered in adequate amounts, confer a health benefit on the host.”^[]

Essentially, probiotics are foods or supplements comprised of live bacterial or yeast cultures that help to reduce the number of “bad” gut bacteria that cause illness or inflammation, while also replacing those bad bacteria with “good” or “friendly” bacteria.

The beneficial health effects of probiotics on gastrointestinal issues—such as inflammatory bowel disease, diarrhea, and irritable bowel syndrome—are well known and have been extensively studied in the literature. Perhaps lesser known are the health benefits that probiotics may confer on the immune system, which may be of particular import during the cold and flu season.  

Further support for these findings comes from a Current Opinion in Gastroenterology review, in which researchers found that probiotics can protect against viral infections, reduce the risk of getting the common cold, and lessen the number of symptomatic days from common colds in healthy people.^[]

In yet another review, wherein researchers described the immunological mechanisms of probiotics and their health benefits on the host, they concluded the following: “Probiotic bacteria, their cell walls or probiotic fermented milk have significant effects on the functionality of the mucosal and systemic immune systems through the activation of multiple immune mechanisms.”^[]

It’s important to note, however, that probiotics have been linked to serious and opportunistic infections in postoperative patients, the immunocompromised, and those who are critically ill. Probiotics should, therefore, not be taken by these individuals, and should never be given to premature infants. In pregnant women, infants, and young children, probiotics should be used with caution.

Zinc

Zinc is a powerful micronutrient most commonly found in proteins. Lean meats, poultry, and shellfish are particularly rich in zinc, but milk, whole grains, beans, seeds, and nuts are good sources, too. Zinc has been shown to be essential in neurocognitive health,^[] wound and scar healing,^[] and immune system defense^[] against invading bacteria and viruses.

According to the NIH, “The body’s immune system needs zinc to do its job. Older people and children in developing countries who have low levels of zinc might have a higher risk of getting pneumonia and other infections.”^[]

The agency added: “Some studies suggest that zinc lozenges or syrup (but not zinc dietary supplements in pill form) help speed recovery from the common cold and reduce its symptoms if taken within 24 hours of coming down with a cold. However, more study is needed to determine the best dose and form of zinc, as well as how long it should be taken before zinc can be recommended as a treatment for the common cold.”

However, a word of caution: Although zinc is vital for strong immunity, excess consumption can actually lead to decreased immunity.

Protein

Protein deficiency can lead to poor immunity because antibodies and immune system cells not only rely on protein to help facilitate the repair of body tissue, but to fight off viral and bacterial infections as well.

“It is well accepted that protein calorie malnutrition impairs host immunity with particular detrimental effects on the T-cell system, resulting in increased opportunistic infection and increased morbidity and mortality in hospitalized patients,” wrote University of Pennsylvania School of Medicine researchers in one Critical Care Medicine publication.^[]

In addition to eggs, fish, poultry, and meat, the following foods have high-protein content: milk, yogurt, cheese, soy foods like tofu, beans such as lentils, chickpeas, peanuts, quinoa, and chia seeds. While dark, leafy green vegetables, such as kale, contain protein, they are not sufficient to fulfill daily protein requirements when eaten alone. However, eating these vegetables in combination with other protein-rich foods can boost protein intake to meet requirements.

Vitamin A

Vitamin A is one of the most important nutrients for a healthy immune system.

The authors of a Journal of Clinical Medicine article put it best: “Vitamin A (VitA) is a micronutrient that is crucial for maintaining vision, promoting growth and development, and protecting epithelium and mucus integrity in the body. VitA is known as an anti-inflammation vitamin because of its critical role in enhancing immune function. VitA is involved in the development of the immune system and plays regulatory roles in cellular immune responses and humoral immune processes. VitA has demonstrated a therapeutic effect in the treatment of various infectious diseases.”^[] 

Vitamin A can be found in many natural food sources, particularly sweet potatoes, carrots, kale, spinach, red peppers, apricots, and egg yolks. And some foods, such as cereal and milk, are often fortified with vitamin A.

The downside to this is that, because many foods contain vitamin A—which can accumulate in the body over time—it can put people at risk for toxicity if they are also taking supplements or medications that contain vitamin A. Getting too much of this nutrient has been shown to result in headaches, nausea, and dizziness, and even coma and death. Pregnant women should be particularly cautious, as large amounts of vitamin A consumption can lead to birth defects.

Vitamin C

When people think of nutrients that play a role in immune health, vitamin C is probably one of the first—if not the first—to come to mind. This probably goes back to our early school days when we learned that sailors cured scurvy by eating vitamin-C rich citrus fruits, namely oranges, lemons, and grapefruit.^[]

Looking at this on a deeper, more scientific level, vitamin C promotes the formation of antibodies that bind to and destroy dangerous toxins, bacteria, and viruses in our bodies before they can wreak havoc. Vitamin C also promotes wound healing by stimulating the production of collagen.

“Vitamin C contributes to immune defense by supporting various cellular functions of both the innate and adaptive immune system. Vitamin C supports epithelial barrier function against pathogens," wrote the authors of an article published in Nutrients.^[] "Vitamin C accumulates in phagocytic cells, such as neutrophils, and can enhance chemotaxis, phagocytosis, generation of reactive oxygen species, and ultimately microbial killing. It is also needed for apoptosis and clearance of the spent neutrophils from sites of infection by macrophages. […] Vitamin C deficiency results in impaired immunity and higher susceptibility to infections," they continued.

Furthermore, with respect to the common cold, the NIH notes, “Although vitamin C has long been a popular remedy for the common cold, research shows that for most people, vitamin C supplements do not reduce the risk of getting the common cold. However, people who take vitamin C supplements regularly might have slightly shorter colds or somewhat milder symptoms.”^[]

Because our bodies don’t produce or store vitamin C, we need to eat foods rich in this nutrient daily. Aside from citrus fruits, other vitamin-C rich foods include strawberries, papayas, red peppers, and tomatoes.

Vitamin E

Vitamin E is generally found in whole grains, seeds like sunflower seeds, nuts, and vegetable oils, and works in conjunction with vitamin C to fortify healthy cells for increased pathogen resistance.

Concluded the authors of one study: “Vitamin E has been shown to enhance immune responses in animal and human models and to confer protection against several infectious diseases. […] Several considerations are warranted for the advancement in our understanding of vitamin E’s role in immunity. For in vitro studies to support implications for the regulation of immunological diseases, the physiological relevance of vitamin E levels used for treatment should be considered. Different forms of vitamin E exert differential effects on immune cells. Cell-specific effects of vitamin E provide valuable evidence regarding the immunomodulatory mechanisms of vitamin E, but the interplay between immune cells should not be ignored, because interactions between immune cells are critical in the regulation of immune function.”^[]

According to the NIH, although most American diets provide less than the recommended amounts of vitamin E, deficiency in this nutrient is rare in healthy people.^[] Additionally, high doses of vitamin A in supplement form can increase the risk of hemorrhagic stroke.