The immune system is one of the most awe-inspiring parts of the human body. Beautifully designed, artful, and complex, our immune system protects us from infection, removes cellular debris and waste, seeks out and destroys abnormal cells, and allows us to live in relative harmony with our environment.

If we learned anything from the COVID-19 pandemic, it’s the importance of a healthy and robust immune system capable of protecting us in times of illness. Fortunately, you can do much at home to bolster your immunity—starting with your plate.

Understanding the role of nutrition in immune health can help you optimize your immunity safely and naturally. Here are some essential nutrients for immune support.

Protein

Made up of chemical “building blocks” called amino acids, protein plays a crucial role in immune health. Amino acids activate natural killer cells—macrophages and T and B lymphocytes—which create antibodies that fight off bacterial and viral infections. Protein also promotes healthy gut-associated lymphoid tissue (GALT), helping the immune system located in the gut to fight off infection.

Having low intake of protein can make it easier for an infection to take hold and make it harder to fight off. For instance, research suggests that inadequate protein can increase our susceptibility to influenza and Zika viruses.1 How much protein do you need? The average adult needs 0.8 grams per kg (0.36 grams per pound) of body weight. Many experts recommend that those over the age of 65 should consume 1–1.2 grams per kg of body weight per day, spread across the day, to maintain a healthy immune system, healthy bones, and maintain muscle mass.

Depending upon your dietary preferences, good sources of protein can include eggs, beans, nuts, lentils, quinoa, soy foods, dairy products, and lean meats.

Dietary fiber

Dietary fiber, or the parts of plant foods your body can’t digest or absorb, helps to modulate the immune system, including GALT. Fiber has been shown to reduce illness-causing inflammation in the body significantly and is inversely linked to the risk of death from infectious and respiratory diseases.

One study found that for every 10-gram per day increase in dietary fiber, the risk of dying from infectious and respiratory diseases decreased by 34% and 18% in men and 39% and 34% in women, respectively.2 A diet high in plant fiber, including fruits, vegetables, legumes, and whole grains supports the growth of beneficial gut microbes. Gut microbes break down the fibers into short-chain fatty acids, which optimize the immune response. These fibers are called prebiotics, which act as food for the microbes.

Good sources of dietary fiber include whole grains, beans, peas, chia seeds, apples, berries, and nuts. I also recommend probiotic-rich foods such as sauerkraut, yogurt, fermented veggies, kombucha, and miso.

Vitamin A

Once dubbed “the anti-infective vitamin” by scientists, vitamin A stimulates the production and activity of virus-fighting white blood cells. It is key to maintaining healthy barrier function in the skin, lungs, gut, and bladder. This means it prevents illness-causing organisms from crossing these barriers and invading the bloodstream. Vitamin A also plays a key role in helping train the immune system to recognize dangerous pathogens from harmless antigens. Low vitamin A levels can increase the risk of lung problems and respiratory disease.3 Yet studies show that up to 45% of people in the United States don’t get enough vitamin A from food alone.4

Vitamin A comes in two primary forms—preformed retinol (animal foods) and provitamin A carotenoids (such as beta-carotene), which can be converted to the active retinol form. Good sources of vitamin A include beef liver, cod liver oil, eggs, butter (preformed retinol), as well as sweet potatoes, canned pumpkin, carrots, cantaloupe, cooked spinach, broccoli, and kale (rich in carotenoids).

When considering supplements, it is important not to overdo the “preformed” vitamin A, generally listed as retinyl palmitate or retinyl acetate. Retinol intake of greater than 5,000 IU (1,500 mcg RAE) per day has been associated with an increased risk of osteoporosis in adults. However, studies have also shown that how well carotenoids are absorbed and converted to the active form of vitamin A is also highly variable between individuals. If supplementing the diet, a supplement providing 2,500–5,000 IU per day of vitamin A, with at least 50% being beta-carotene and/or mixed carotenoids, is probably the best approach.

Vitamin C

This nutrient is critical to the body’s immune response. It stimulates the activation and function of key immune cells, including natural killer cells—neutrophils, T cells, and B cells—that produce antibodies. Vitamin C is essential in maintaining the skin’s barrier function, which is vital to preventing infection via this route. Some studies suggest that vitamin C can modestly shorten the severity and duration of minor respiratory infections.

Around 16 million Americans are seriously deficient in vitamin C, and 66 million have a marginal deficiency.5 This may be due to insufficient fresh fruits and vegetables and sustained high-stress levels. Vitamin C is vitamin synthesizing norepinephrine, a primary stress hormone. Infections deplete vitamin C levels, so it’s important to ensure you’re getting enough before and during times of stress.6

Good sources of vitamin C include kiwi, strawberries, citrus, broccoli, and sweet red peppers. Aim for 5–7 servings of fruits and vegetables every day. While the RDA of vitamin C for adults is 75–90 mg per day, most experts believe optimal body levels are achieved at 200–400 mg daily.

Zinc

This mineral affects multiple aspects of the immune system and is necessary for developing and maturing different immune cells. With adequate zinc on board, the risk of pneumonia can be reduced by improving mucociliary clearance and barrier function of the cells that line the lungs, as well as having direct antibacterial effects against Streptococcus pneumoniae, a major cause of pneumonia and middle ear infections. Zinc also has antiviral and anti-inflammatory activity, the latter of which helps to modulate the immune response. So, it’s unsurprising that less-than-optimal zinc levels increase a person’s risk of infectious diseases or that researchers have noticed a link between low zinc and COVID-19.

One study of patients hospitalized with COVID-19 found that they had significantly lower zinc levels than controls and that 57% were zinc deficient.7 Alarmingly, the odds of severe complications from COVID-19 were five times greater in these zinc-deficient patients.7

Since zinc deficiency affects 30% of the global population, we should all ensure we get enough of this immune-boosting mineral in our diets.8 Good sources of zinc include red meat, shellfish, lentils, nuts, seeds, and whole grains.

Though zinc is a vital nutrient, excessive intake is not safe as it can interfere with the absorption of copper, leading to anemia, an impaired immune response, and neurological problems. The upper tolerable limit for adults is 40 mg per day. Most of us benefit from taking 10–20 mg per day, and I generally recommend 30 mg per day for those over the age of 65.

Vitamin D

Vitamin D is crucial to a healthy immune system. It has anti-inflammatory and immunoregulatory properties and supports the body’s immune response. It aids the function of immune cells, such as macrophages and T-cells, which protect against illness-causing pathogens. Interestingly, evidence suggests vitamin D deficiency may increase the risk of developing autoimmune conditions.

Low vitamin D levels are linked to an increased risk of infection and disease.9 Similar to zinc, vitamin D deficiency has also been shown to worsen outcomes for people with COVID-19.10 One study that included 780 cases of COVID-19 infections found that patients who were D-deficient were 13 times more likely to die compared to those with sufficient levels of vitamin D.11

Vitamin D deficiency is a serious problem in the United States; a report by the Centers for Disease Control and Prevention (CDC) found that 66.8 million Americans had levels that classified them as deficient, while 23 million Americans were severely deficient.12

The “sunshine vitamin” is best acquired by skin exposure to the sun’s ultraviolet rays. However, many people either spend much of their day indoors, use sunscreen, or live in areas with limited sunlight in winter. Though foods like salmon, egg yolks, cod liver oil, mushrooms, and fortified yogurt or cereals can provide some vitamin D, most people benefit from taking a vitamin D supplement.

The RDA for adults 19–70 years old is 600 IU (15 mcg) per day and 800 IU (20 mcg) per day for those over 70 years of age. However, the upper tolerable limit, the maximum amount unlikely to cause harm, is 4,000 IU per day, and many people benefit from taking 1,000-2,000 IU per day, especially during the winter.

My Recommendations

I’m a big believer in food as medicine, and it is imperative to have a nutrient-dense diet. Limit sugary and processed foods and load up on fruits, veggies, legumes, whole grains, and lean meats (if not vegetarian or vegan). Drink plenty of water and herbal teas to stay hydrated. I know getting the nutrients we need from diet alone is not always possible, so taking supplements can be necessary for maintaining a thriving immune system. When considering supplements:

Look for a multivitamin that can provide the B vitamins, vitamin E, and selenium in addition to the following:

• Vitamin A: 2,000–5,000 IU per day (with 50% as beta carotene/mixed carotenoids)

• Vitamin C: 250–500 mg per day

• Zinc: 10–20 mg per day

• Vitamin D: 1,000–2,000 IU per day (best to have vitamin D tested in late fall/early winter and correct any deficiencies)

If 65 and older and/or have risk factors for infection, look for a multivitamin that provides the following:

• Zinc 20–30 mg per day

• Vitamin C 400–500 mg per day

• Vitamin D3 2,000–4,000 IU per day

Keep zinc lozenges on hand and take 5–10 mg every 3–4 hours at the first sign of illness.

If you’re about how nutrient deficiencies may impact your immune system, talk to your healthcare provider about getting tested. You can also visit www.requestatest.com to order tests yourself.

Sources

1. Ah O. Protein Energy Malnutrition and Susceptibility to Viral Infections as Zika and Influenza Viruses. J. Nutr. Food Sci. 2016;6:2. doi: 10.4172/2155-9600.1000489.

2. Park Y, et al. Dietary fiber intake and mortality in the NIH-AARP diet and health study. Arch. Intern. Med. 2011;171:1061–1068.

3. Timoneda J et al. Vitamin A Deficiency and the Lung. Nutrients. 2018;10:1132.

4. Reider CA, et al. Nutrients 2020: 12, 1735.

5. https://www.cdc.gov/nutritionreport/pdf/Nutrition_Book_complete508_final.pdf

6. Hemila H, et al. Vitamin C Can Shorten the Length of Stay in the ICU: A Meta-Analysis. Nutrients 2019 Apr; 11(4): 708.

7. Jothimani D, et al. COVID-19: Poor outcomes in patients with zinc deficiency. Int J Infect Dis 2020 Nov;100:343-349.

8. Wu, D, et al. Front Immunol 2019; doi: 10.3389/fimmu.2018.03160

9. https://pubmed.ncbi.nlm.nih.gov/21527855/

10. Alipio, M. Vitamin D Supplementation Could Possibly Improve Clinical Outcomes of Patients Infected with Coronavirus-2019 (COVID-2019). SSRN Electron. J. 2020, doi:10.2139/ssrn.3571484.

11. Raharusun P., et al. Patterns of COVID-19 Mortality and Vitamin D: An Indonesian Study. SSRN. 2020 doi: 10.2139/ssrn.3585561

12. CDC 2nd National Report on Biochemical Indicators of Diet and Nutrition in the U.S. Population Holick MF, et al. J Clin Endocrinol Metab 2011; 96(7):1911-30