Can Foods Really Bolster Your Immune System?

healthy foods — Many foods you like to eat are good for your immune system. Tom Werner/Getty Images

Having an immune system that runs like a top matters all the time. But it's top of mind in cold and flu season, or when people are worried about a deadly virus. You may have read that certain foods are good for bolstering your immune system but is that true?

First off, there's no way to "boost" your immune system. "It's better to say enhancing immunity," says Kristin Kirkpatrick, a registered dietitian with Cleveland Clinic.

Here's why: The human immune system has two facets: innate and acquired (or adaptive). The innate immune system is your first line of defense that starts working the moment you're born, based on your mother's lifestyle while you were in the womb. These are cells that go to work immediately fighting off infections. If the innate system can't do the job, after four to seven days, white blood cells and proteins — antibodies — targeted to a specific pathogen will be mobilized to fight the infection. This immune respone may lead to body inflammation, fever and other uncomfortable reactions.

Still, eating the right foods can maintain or improve your body's immune function. "A healthy diet, along with other things like sleep and stress management, all those things help contribute to that robust immune system," Kirkpatrick says. Eat a diet of 80 percent real food, and avoid excess sugar, which slows down immune function. "Good food diversity and colorful plants are going to be a key factor to a healthy gut," Kirkpatrick says. "But don't kill yourself if you have a cookie."

Nutritionists compare the body and immune system to a luxury imported car. Unfortunately, some of us treat our automobiles better than our bodies, says Alison Brown, a nutritional researcher and chair of the National Organization of Blacks in Dietetics and Nutrition. "If your car is a luxury car, you can't just put regular gas in a luxury car. You want to put premium fuel in it," Brown says. Same goes for your body. Here's what you need to fuel it right.

Vitamin A

Evidence shows vitamin A enhances immune response in ways that researchers are still unpacking. Retinoic acid supports the innate immune system, including natural killer cells. Plus, it's needed to form the epithelial cells and mucous layer that act as defensive barriers in the lungs and intestines.

Best sources: "Carotenoids are found in plant-based sources of vitamin A and retinoids are in food like liver or egg yolks," says Brown. Cod liver oil is a potent source of the latter. Orange or golden foods like sweet potatoes, carrots, egg yolks, cantaloupe melon are rich in vitamin A, but so are spinach and broccoli.

Folate

Folate, also known as B9, helps send signals to the natural killer cells, which are critical for immune response, says Kirkpatrick. It also helps with forming and repairing DNA.

Best sources: Lentils and other legumes, like asparagus are good sources, so are leafy greens like broccoli sprouts, Brussels sprouts and beets, along with beef and chicken liver.

Vitamin C

Vitamin C is a powerful antioxidant that does many beneficial things for the body. A 2017 research paper that explored vitamin C's impact on immunity says that it supports protective epithelial cells and killer cells' work. It's also critical for helping immune cells called macrophages to clean up infected areas. "Vitamin C deficiency results in impaired immunity and higher susceptibility to infections," wrote Anitra Carr, a professor at the University of Otago in New Zealand, who researches vitamin C. The study notes eating a diet high in vitamin C is the best protection.

Best sources: All citrus fruits from lemons and tangerines to oranges and grapefruits offer an ample amount of vitamin C. So do kiwi fruits and spinach, whether fresh, canned or frozen.

Vitamin D

Vitamin D, aka the sunshine vitamin, is critical to optimal immune function, says Kirkpatrick. In the body, it acts as a hormone that signals actions in cells and helps fight off viruses and infections. Low vitamin D levels are associated with higher rates of sepsis, MRSA and hepatitis C. But, when people have at least 30 ng/ml of vitamin D in their bloodstream, it can prevent or blunt the severity of pneumonia and respiratory infections.

Researchers at the Medical University of South Carolina currently are studying whether supplementing with vitamin D can lessen the severity of COVID-19 infections. They're hoping D3 supplementation could better the survival odds for African Americans and the elderly in care homes, who are often vitamin D deficient, and have been especially vulnerable.

Best sources: Salmon, other fatty fish and egg yolks offer small amounts of vitamin D. Sun exposure is an easy way to synthesize vitamin D, unless you have dark skin, so D3 supplements are key. Nutritional epidemiologist Walter Willett and colleagues at Harvard University's T.H. Chan School of Public Health recommend taking a supplement of 1000 or 2000 IU per day if you believe your levels might be low — for instance, if you have darker skin or limited sun exposure.

Vitamin E

This nutrient acts as an antioxidant, protecting cells from damage and supporting the creation and action of T cells, a type of white blood cell that's important to the immune system. A 2019 paper from Tufts University said that "Although deficiency is rare, vitamin E supplementation above current dietary recommendations has been shown to enhance the function of the immune system and reduce risk of infection, particularly in older individuals."

Best sources: Almonds, sunflower seeds, and most seeds are abundant in vitamin E. You'll also find it in avocados, spinach, mangoes, butternut squash, and cooking oils like safflower and olive.

Zinc

Zinc is a micronutrient that winds up in fizzy vitamin drink powders and throat sprays because there's evidence that it helps people recover faster from colds. Two different meta-analyses found that people who took zinc lozenges (75-100 mg) significantly reduced their cold durations as long as the zinc was taken early in the cold's lifecycle.

Best sources: You'll find zinc naturally in beans, legumes like lentils, and a range of nuts from almonds and peanuts to pumpkin seeds. Fatty fish like salmon and mackerel are good sources of zinc, too, Brown says.

Selenium

Selenium is a trace mineral, like zinc but it's less well-known. Selenium is required by proteins called selenoproteins that help kick off the immune response to fight invading germs or viruses. But it also makes sure the response isn't too vigorous, which can cause inflammation and cell damage.

Best sources: "The main source of selenium in any decent amount is Brazil nuts," says Kirkpatrick. "The nut everyone throws out of their mix; that's selenium." Fish, chicken and ham also have small amounts of selenium.