How to Strengthen Your Immune System Through Diet
You’re not eating for this pandemic—you’re eating for the next one
This story is part of How to Eat in the New Normal, a weeklong series about how the Covid-19 pandemic is changing the way we eat, with expert advice for making food choices that help you stay healthy and happy
Let’s get something out of the way: It’s impossible to immediately boost your immune system. Vitamin C supplements, turmeric juices, green smoothies — none of them will give you virus-fighting superpowers.
That doesn’t mean vitamins and minerals aren’t critically important for immune health, but the benefits take time to build up. The salad you eat today is a long-term investment, not a quick fix or an insurance policy against an immediate threat.
“There are a lot of factors that go into enhancing the immune system. It does not happen overnight, and it’s not all about diet,” says Kristin Kirkpatrick, MS, RD, lead dietitian and manager of Wellness Nutrition Services at the Cleveland Clinic. “Starting today is a good first step, but the key is keeping up these better habits.”
Vitamins and minerals are essential for cellular health in every organ system in the body. For immune cells, vitamins C, D, and E, and zinc appear to be particularly important. These nutrients “are needed for the immune system to maintain its normal function and have the ability to produce the arsenal they need to combat pathogens when they face them,” says Simin Nikbin Meydani, PhD, professor of nutritional immunology at Tufts University. “When you have deficiency of these nutrients, the cells of the immune system cannot function normally and their ability to fight pathogens will be impaired.”
Below is a quick rundown of how each of these nutrients benefits the immune system and how to work them into your diet. But first, an important caveat: None of these vitamins and minerals have been tested on the body’s ability to fight off the novel coronavirus. There is no evidence that vitamin C, vitamin D, zinc, or any other nutrient is an effective prevention method or treatment for Covid-19. Still, it’s a good time to prioritize taking care of your body and staying as healthy as you can.
Vitamin C is the micronutrient most famously associated with a robust immune system. It is an antioxidant that protects cells from toxic byproducts that immune cells produce when destroying pathogens. Vitamin C also helps with the production and function of immune cells that attack viruses and bacteria. According to the Oregon State University Micronutrient Information Center, “Immune cells accumulate and concentrate vitamin C and then quickly use it up during an active immune response.”
Citrus fruits, strawberries, tomatoes, bell peppers, broccoli, and kiwi are all excellent sources of vitamin C. The National Academy of Medicine recommends 90 milligrams of vitamin C per day for men and 75 milligrams for women. Those amounts can easily be achieved with a cup of strawberries, half a cup of red pepper, or one large orange.
Vitamin D stimulates the body’s first line of defense by increasing the number and activity of cells that gobble up invading pathogens.
The nutrient is unique in that it is primarily synthesized in the skin from sunlight. Vitamin D is harder to obtain through diet, although it can be found in fatty fishes like salmon, mackerel, and sardines. Milk, orange juice, and many cereals are also fortified with vitamin D to aid with calcium absorption. Adults should consume 15 micrograms of vitamin D, also measured as 600 international units (IU).
Vitamin E is another type of antioxidant that shields immune cells from damage and hormones that suppress the cells’ activity. Through its protective effects, vitamin E enhances immune cell function.
A fat-soluble nutrient, vitamin E is found in vegetable oil, seeds and nuts, and avocados. Adults need an average of 15 milligrams of vitamin E a day, which can be achieved through three tablespoons of vegetable oil or two ounces of almonds.
Zinc is essential for immune cell maintenance and development. Deficiency of this micronutrient can cause cells to break down and malfunction. Too little zinc is known to suppress the immune system, particularly in adults over the age of 60.
Shellfish, red meat, and beans are all good sources of zinc. Women should consume eight milligrams of zinc a day, and men should aim for 11 milligrams, which can be accomplished with six ounces of steak or just two oysters. Meeting the requirement with beans is a little trickier — it would take almost eight cups of black beans to get 11 milligrams of zinc.
“There are a lot of factors that go into enhancing the immune system. It does not happen overnight, and it’s not all about diet.”
The best way to get these vitamins is through diet, the one exception being vitamin D, which an estimated 30% to 50% of people are deficient in. Tom Frieden, MD, former director of the U.S. Centers for Disease Control and Prevention, recently advised people to take vitamin D supplements based on a review paper from 2017 that showed the nutrient could help protect against respiratory illnesses in people who were deficient.
For the rest of the vitamins and minerals, dietitians advise eating a variety of fruits, vegetables, lean meats, and whole grains throughout the week rather than focusing on eating specific foods and amounts every single day. Keeping that as your goal will take some of the stress out of meal planning while still ensuring you get enough of the important nutrients.
“The easiest thing is to eat balanced meals that contain a variety of foods throughout the week, [with] half of your plate being fruits and vegetables,” says Academy of Nutrition and Dietetics spokesperson Kristen Smith, MS, RD. “Make sure that each day you are eating different types of fruits and vegetables throughout the week. Eating foods with a variety of color will also help ensure that you’re meeting your vitamin needs.”
If you’re unable to achieve the recommended amount every day, you aren’t going to become deficient in a nutrient overnight. “Usually it takes longer for fat-soluble nutrients such as vitamin E to be depleted compared to water-soluble nutrients like vitamin C,” Meyrani says. “Regardless, you are not going to become deficient in nutrients if you do not consume the recommended level for one day, as the body has a store of them.”
There also appears to be a ceiling effect where consuming more than the recommended daily amounts of a specific vitamin probably won’t help you.
“There’s very little clinical trial data to support what a lot of people consider to be conventional wisdom, saying you should have higher levels of [vitamins] D, C, and zinc,” says Howard Sesso, ScD, MPH, an associate professor of medicine at Harvard Medical School.
Studies of various vitamin supplements have shown that only people who are deficient in a specific nutrient see improvements in their immune function after taking them, whereas people with no deficiencies see no benefit.
Another reason to be wary of supplements is that if you get too much of a specific nutrient, it can actually cause irritation in the gut and even toxicity. “We do know that for many vitamins and minerals — not just for vitamin D and for vitamin C, but also for zinc — that sometimes when you consume too much of a vitamin, [there are] concerns about potential toxicity,” Sesso says. “I would recommend against any type of megadosing. I don’t see much benefit to that.”
What is important for both overall health and potentially limiting the severity of Covid-19 is maintaining healthy cardiovascular and insulin systems. The risk for hospitalizations, ventilation, and death from Covid-19 are all elevated in people with preexisting conditions, especially high blood pressure and diabetes.
One of the easiest, most important ways to lower your risk for these chronic diseases is limiting sugar and red meat consumption — changes that can also lower levels of inflammation in the body over the course of four to six weeks. Multiple studies have shown that eating a diet high in processed foods can increase a person’s risk for chronic disease and early death.
Now that the motivation to eat well is top of mind for many people, it’s a great time to make healthy changes in your diet that can be sustainable for the long term. Think of it this way: You’re not eating for the current pandemic—you’re eating for the next one.