Originally Published: 16 NOV 21 03:44 ET

By Sandee LaMotte, CNN

(CNN) -- Want to fight off infectious diseases this winter? Consider boosting your immune system with nutritious food.

"What we eat is very important in terms of how our immune system responds to pathogens and how well it can defend itself against a pathogen," said Dr. Simin Meydani, senior scientist and leader of the nutritional immunology team at Tufts University's Jean Mayer USDA Human Nutrition Research Center on Aging.

Micronutrients such as vitamin C, vitamin D, vitamin B complex, zinc and selenium can help "pump up" the body's defenses against colds, flu and even Covid-19, Meydani said, but you won't boost immunity by "eating a large amount of one single nutrient or food component."

Forget focusing on "superfoods." A large variety of foods are needed to provide the micronutrients the body needs to mount a robust cellular immune response, said Stanford School of Medicine nutrition scientist Christopher Gardner.

"There isn't any one food or nutrient to rely on here, but rather it is the interplay of 'harmonious interactions' between the various micronutrients," Gardner said.

Plan your daily menus around a large variety of fresh and colorful red, yellow, orange, blue and green fruits and vegetables, along with some high-quality whole grains, a bit of lean protein and a splash of healthy oils.

Plants and grains are also the basis of the top-rated Mediterranean diet and DASH diet, which stands for "dietary approaches to stop hypertension," or high blood pressure. Both the Mediterranean and DASH diets avoid processed foods and focus on fruits, vegetables, beans, lentils, whole grains, nuts and seeds.

The Mediterranean diet can reduce the risk for high cholesterol, dementia, memory loss, depression and breast cancer, numerous studies found. Meals from the sunny Mediterranean region have also been linked to stronger bones, a healthier heart and longer life. Oh, and it helps with weight loss, too.

An overall picture

Just how much you can jump-start immunity with nutrients depends on your age, overall health and stress levels, according to Meydani.

In addition to eating well, it's important to stay at a healthy weight, reduce your stress, get quality sleep and regular exercise to keep those natural defenses in fighting shape. Without that healthy baseline, your body will have to work harder to knock out invaders -- and may even lose the match.

"The best defense against the acute threat of the coronavirus is chronically good health," said Dr. David Katz, founder and president of the True Health Initiative, a nonprofit dedicated to promoting evidence-based lifestyle medicine.

"If you encounter the virus a week from now, two weeks from now, three weeks from now, there is a chance you will do better because of what you do with your diet right now," he said.

Pump up the volume

If you want to maximize the impact of food on your immune system, you'll need to dramatically increase the amount of fruits and vegetables you eat each day, Meydani advised.

Her team investigated immune responses in animals fed two to three servings of fruits and veggies a day, and compared them to those who ate five to six servings a day or eight to nine servings a day.

"The eight to nine servings a day was where we were seeing the best effect," Meydani said. "So it's not just increasing the intake by a little bit, you've got to increase it substantially. People need to work at it in order to reach that level."

Finding ways to insert fruits and veggies into every meal and snack during the day may do more than pump up your immunity. A 2017 study found a significant reduction in the risk of heart attack, stroke, cancer and early death by eating 10 portions of fruit and vegetables each day.

Current dietary guidelines in the United States recommend at least 2 cups of fruit and 2.5 cups of vegetables a day. Yet surveys by the US Department of Agriculture found the average American eats only 0.9 cups of fruit and 1.4 cups of vegetables per day

It's not just Americans. One in 5 deaths globally -- that's about 11 million people -- occurred because of too much sodium and a lack of whole grains, fruit, nuts and seeds, a 2017 study found.

Anti-inflammatory foods

There is another reason to pack your plate with a variety of fruits and veggies -- the need to control your body's inflammatory response to bacteria and viruses.

"A certain amount of inflammatory response is needed to get rid of the pathogens and to help the the body's immune system perform its function," Meydani said. "But if you produce too many inflammatory components, it can be damaging to surrounding tissues. It can cause autoimmune diseases. It can cause chronic diseases."

Chronic inflammation has been linked in studies to cancer, heart disease, diabetes, arthritis, depression, Alzheimer's and many other diseases. In cases of Covid-19, extreme inflammatory reactions to the virus, called "cytokine storms," have been linked to more severe cases and death.

"In relation to Covid-19, the recommendation to increase fruits and vegetable consumption is even more important," Meydani said, "because of all the anti-inflammatory compounds such as flavonoids in them that can reduce the cytokine storm."

Unfortunately, today's Western diet is full of overly processed, fat-laden foods, sugary drinks and red and processed meats that can cause persistently high levels inflammation in the body.

Eating a lot of unhealthy, "ultraprocessed" foods like ice cream, cookies and pre-made consumer foods may shorten your life -- just a 10% increase in such foods was significantly associated with a 14% higher risk of death from all causes, studies have shown.

Instead, choose leafy greens, tomatoes, fruits, nuts, fatty fish and olive oil -- foods that can support a healthy inflammatory response without sending it into overdrive.

Studies are also finding that fermented foods such as kimchi, sauerkraut or kombucha may also help battle some types of inflammation by improving the microbiome in the digestive system,

"We observed a lowering of (approximately) 19 inflammatory markers in the study participants who consumed fermented foods for 10 weeks," said Stanford's Gardner, who co-authored a recent study.

Are supplements needed?

Just like any mammal, the human body is built to absorb nutrients from whole fruits, vegetables, nuts, grains and proteins more efficiently than processed foods or supplements.

But people with limited access to healthy food choices or who have certain medical conditions or anyone over the age of 65 may need to focus on adding specific micronutritents to their diet.

"I'm not talking about the frail elderly that are homebound," Meydani said. "I'm talking about older people who are healthy, active, but above the age of 65. For them, I think certain nutrient supplementation might be very helpful."

The role of zinc

Most people in the United States get enough zinc from the foods they eat. But a study by Meydani's team older adults with low serum zinc levels had twice as much pneumonia and a longer duration of pneumonia and antibiotic use than people with adequate levels.

Using zinc lozenges in the early stages of cold or flu, such as in the first 24 hours, may give the immune system a boost and reduce the duration of sickness by about a day, according to a review of 13 studies

The daily recommended dose is 8 milligrams for women and 11 milligrams for men for no longer than five days. Zinc can also interfere with the effectiveness of antibiotics or negatively interact with some blood pressure and rheumatoid arthritis medications, so check with your doctor before adding it to your diet.

The mineral selenium

A natural mineral found in the soil and absorbed by plants, selenium plays an important role in inflammation and immunity. We need the mineral to activate immune cells. It also appears to be able to reduce inflammatory response.

Most Americans get plenty of selenium from their diet -- it can be found in seafood, meat, poultry, eggs, dairy products, breads, cereals, and nuts, especially Brazil nuts, which can contain up to 91 micrograms.

It would only take five Brazil nuts to reach the daily upper limit of selenium for adults -- 400 micrograms a day. Taking too much can cause nausea, diarrhea, skin rashes, loss of hair or nails, discolored teeth, irritability and nervous system problems, according to the NIH's Office of Dietary Supplements.

Some benefits from vitamin C

Hundreds of studies over the years have looked at the benefits of vitamin C for the immune system -- and clinical trials are currently underway investigating the impact of vitamin C transfusions on the severity of Covid-19.

Most people believe that vitamin C boosts the immune system and protects against colds. However, reviews of studies to date find only a mild beneficial effect compared to a placebo, leaving some experts loathe to sing its praises.

Dual properties of vitamin E

Vitamin E can play a dual role in boosting the body's immune response. It acts as an antioxidant in the body, helping to protect cells from the damage caused by free radicals.

"Vitamin E can have an anti-inflammatory effect, but it can also boost your cell-mediated immunity in certain populations," Meydani said.

Some of the best sources for vitamin E are vegetable oils like sunflower and safflower; peanuts, hazelnuts and almonds; seeds; and wheat germ. Many people turn to supplements to boost their intake -- but be careful, high doses of vitamin E might increase the risk of bleeding.

"The optimal level from our studies was 200 international units per day," Meydani said. "This is very important, because people think that if something is good for your immune system, then the higher you go, the better it will be. And that's not the case."

The sunshine vitamin

Just like vitamin C, evidence on D's immune-boosting properties is mixed, Meydani said.

"We can't say that giving higher levels of vitamin D is going to improve your immune response and fight respiratory infections," she said. "Some studies have shown some benefit, but others have not confirmed them."

Your body makes vitamin D from the sun, so people with darker skin or who live in areas with little sunshine might be deficient, as are the elderly and exclusively breastfed babies, who require a daily supplement. Many foods in the Western diet are supplemented with vitamin D, and taking too much can be toxic.

"Vitamin D toxicity almost always occurs from overuse of supplements," according to the NIH's Office of Dietary Supplements. "Signs of toxicity include nausea, vomiting, poor appetite, constipation, weakness and weight loss."

It's still early days, but scientists are studying the connection between vitamin D and Covid-19. Most promising is the anti-inflammatory properties of vitamin D -- the hope is that supplementation will tamp down any hyperactive immune response to the virus.

"Cytokine storms," where the immune system goes haywire and overwhelms the body, are one of the leading causes of death from Covid-19.

A few studies have found low levels of zinc and vitamin D are associated with more severe Covid-19 symptoms, but more controlled research is needed "before we reach a conclusion," Meydani believes.

"So, I would say, people should make sure they do not have low levels of zinc, D or C, particulary those at risk of low consumption of these nutrients such as elderly," she said. "But at this point we don't know if giving higher level of these nutrients in those whose baseline levels are sufficient is going to provide additional protection."