Diet and Your Immune System- A COVID-19 Related Update

As we continue to progress through the COVID-19 crisis, we are starting to see the significant implications of this virus in cities across the US. The virus is starting to hit hard, and if you are following the events in New York, it is extremely concerning. There are things we can’t control- our age and preexisting conditions being two of these. But there are things we can control. And while social isolation, handwashing and respiratory protection are paramount, I wanted to say a few words on the role of diet and nutrition on the functioning of your immune system and perhaps helping to stave off the worst effects of the virus.

The severe effects of the COVID-19 infection typically relate to acute lung injury and ARDS (Acute Respiratory Distress Syndrome). While the infection itself is damaging, it is the excessive inflammatory response in the lungs (and the potential hyperinflammatory cytokine storm) that can be the factor that ends up requiring ICU care, mechanical ventilation and in some cases causing mortality. Is this excessive inflammatory response in any way preventable or modifiable? 

To be clear, I am not stating we can stop this virus in its tracks by improving the way we eat. What I am saying, is that what you eat affects the ability of your immune system and inflammatory response to function properlyIf there is even a chance that paying a little more attention to nutrition could keep some people out of the ICU, I am all in. 

There are 2 key components to this

  1. You need adequate but not excessive inflammation to mount an effective immune response to an infection. 
  2. You need a properly functioning immune system to fight off any infection. 

These two things are impacted by diet and proper nutrition. Unfortunately, medical efforts to treat the hyperinflammatory response in ARDS has, to date, not really helped to improve outcomes. Are there things we can do to try to prevent the excessive inflammation and keep your immune system operating to its full capacity? 

To give full disclosure, there are not massive double-blind placebo controlled trials confirming all these dietary links. However, there is a growing evidence base to back up the importance of certain nutrients in immune functioning and regulation of inflammatory response. Americans as a whole are eating a very calorie-rich nutrient-poor highly processed diet that may leave us with suboptimal intake of some important nutrients. We will briefly take a look at some of these key nutrients, vital for proper immune functioning, that may be inadequate in many people’s diets thereby putting them at increased risk: Vitamin D, Zinc, Magnesium and Omega-3 fatty acids.    

Vitamin D

Vitamin D is the biggest nutritional factor getting airplay right now. Vitamin D is a fat-soluble vitamin made by our bodies when skin is exposed to sunlight. It is not found in very many foods, and people in northern climates like Montana often have shockingly low values precipitated by the (very long) winter months predominated by less daylight hours and by cloud coverage. It is estimated that at least 40% of Americans and over 1 billion people worldwide are to some degree Vitamin D deficient. Vitamin D is as much a hormone as it is a vitamin, and has receptors on nearly every cell in the human body, especially immune cells. Vitamin D helps to regulate the immune response, so that adequate, but not excessive immune system activation and inflammation take place. Dancer et al reviewed the role of Vitamin D in ARDS in 2015 in the medical journal Thorax. They found significant increased risk of ARDS in patients who were Vitamin D deficient. In fact, the former head of the CDC just came out to state that vitamin D may have a role in curtailing the worst effects of COVID infection. 

Here’s my take: Vitamin D is crucial for immune function and regulation of inflammation. It is nontoxic at the recommended dosage range. Montanans and close to half the US population are low, and the older you are the more likely you are to be deficient. Get out in the sun when you can for at least 30 minutes a day (if it is peeking out from the clouds) without sunscreen. Take a daily vitamin D supplement of 1000 IU of vitamin D3 (the more active form) with a meal containing some fat to improve absorption. If you can get your levels tested, you want your 25-OH vitamin D levels above 40 optimally. (but not above 70). Some folks need higher doses to get their levels even above 30, and that may be safe and recommended with medical guidance and/or monitoring. 


Zinc is a mineral crucial to immune system functioning, and may also be inadequate in more people’s diets than we think. Zinc is involved with almost every aspect of immune function and is also involved with the maintenance and repair of the lining of your gut and lungs. The highest levels of zinc are found in seafood (oysters are highest), red meat, and nuts/seeds with pumpkin and sunflower seeds being the highest of the seeds. Suboptimal levels of zinc in the body are known to directly impair immune function and increase inflammation. Rough estimates out of Oregon State University show that overall, at least 12% of Americans are not getting enough zinc, but 40% of those 65 and older may be subclinically deficient.  Older people tend to both eat and absorb less zinc. Whether this is just an association or partially causative in age related immune decline is speculative. It is likely we are underestimating the number of people with inadequate zinc intake, because certain foods like grains and legumes contain anti-nutrients that block the absorption of dietary zinc in our intestines. People eating high amounts of grains and beans are at higher risk for zinc deficiency if they are not also eating decent amounts of seafood, meat, and/or pumpkin/sunflower seeds. There is not an adequate blood test for zinc levels, so make sure you are eating a steady intake of zinc rich foods. If you take a zinc supplement, 15-30 mg per day total zinc intake is sufficient but not excessive   ,and it should not be zinc oxide which is a form that is not well absorbed. Look for zinc aspartate, zinc gluconate, or zinc glycinate.

Omega-3 fatty acids

Omega-3 fatty acids which help to regulate inflammation are also rampantly deficient in the American diet. The biologically active omega-3’s are found mostly in cold water fish like salmon, cod, herring and sardines, foods that are not present in significant amounts in many people’s diets. Unfortunately, the omega-3 found in plants, like flaxseed, are not typically sufficient to get levels of the crucial EPA and DHA omega-3 fats to optimal levels in the human body. The anti-inflammatory omega-3 are also being crowded out in our bodies by the pro-inflammatory omega-6 fats like corn oil, vegetable oil, sunflower and safflower oil. The essential omega-3 fatty acids are important for healthy cell membranes, brain function, and for production of anti-inflammatory molecules called eicosanoids. Omega-3 are known to reduce inflammatory cytokines in the body. There have been trials using omega-3 fats to treat ARDS in the ICU, with some positive and some negative results, so this is still controversial. What I am suggesting is that by improving your body’s ratio of omega-3 to omega-6 fats before you get sick, you can shift away from the potential for a hyperinflammatory response to injury or infection. My take is this: most people are not getting enough EPA/DHA in their diet, and are getting WAY too much omega-6 fats pushing their body more toward excessive inflammation. Eating more salmon, sardines, herring or trout, or taking a high quality fish oil supplement of 500-1000 mg EPA/DHA daily will help boost your levels. If using a supplement, I recommend Carlson’s or Nordic Naturals. Using light olive oil instead of corn or vegetable oil to cook with is also an easy change to drop your intake of omega-6 fats, has no negative effects, and may help shift your body away from excessive inflammation. And BTW, the health benefits will also likely help to lower your risk for heart disease and Alzheimer’s. 


Magnesium Research is showing up to 50% of Americans are subclinically deficient in magnesium. This important mineral is involved with over 200 reactions in the body, and low levels are associated with increase in inflammation and impairments in immune functioning as well as increased risk of heart disease. Past research has shown direct association between magnesium deficiency and excessive inflammatory response to immune stress. Magnesium is found in green leafy vegetables, nuts, and fish but is depleted in the body by excessive alcohol, caffeine, sugar and salt in the diet. Make a real effort to increase the green leafy vegetables, fish and nuts in your diet for a host of reasons, not the least of which is the increased magnesium intake. Try to reduce sugar and salt in your diet, and keep alcohol and caffeine intake moderate. If you choose to take a magnesium supplement 200-400 mg is safe unless you have chronic kidney disease. Do not take magnesium oxide as it is poorly absorbed, but look for citrate or glycinate. With all nutrients, more is not better, but high doses usually result in diarrhea as the main side effect. 

Finally, there is an increasingly recognized link between the gut and inflammation, and taking measures to improve your gut health (such as reducing sugar/fructose and increasing fiber found in fruits and vegetables) is being shown to have positive effects on many inflammatory health conditions. The microbiome, which refers to the population of trillions of bacteria living in your intestines, is a master regulator of both your immune system as well as your intestinal permeability. A damaged or altered microbiome, known as dysbiosis, may be implicated in a number of modern inflammatory diseases and a pro-inflammatory state. What you eat directly impacts your microbiome, and a diet high in sugar (specifically fructose) and low in fiber (which feeds the microbiome) has been shown to cause increases in the potentially pathogenic gram-negative bacteria like Enterobacter in your intestines. In addition, a high fructose diet increases the permeability of the gut, allowing more bacteria and their by-products such as LPS (lipopolysaccharides) into the bloodstream, causing increased systemic inflammation and having the potential to dysregulate your immune system. 

    Recently it has been shown that there are increased amounts of pathogenic gut bacteria like Bacteroides and Enterobacter in the lungs of ARDS patients. These gram-negative bacteria are not normally present in the lung, and likely got there by a process called translocation where the intestine becomes more permeable (“leaky”) and allows gut bacteria into the bloodstream. This translocation causes a significant inflammatory response by the body. A current area of interest is the role that these bacteria are playing in the excessive inflammatory response and ongoing lung damage in ARDS. While this is a situation that may happen acutely in the critically ill patient, there is increasing evidence that this process is widespread and insidious in the non-ICU patient and is referred to as endotoxemia. Endotoxemia is known to be associated with increased systemic inflammation, and is now suspected to play a role in chronic liver disease, as well as cardiovascular disease. High amounts of fructose in the diet directly cause endotoxemia in recent research.


Take Home Points:

Remember that your diet not only feeds you, but also feeds either the good or bad bacteria living in your gut. Do yourself a favor, significantly cut back on sugar and fructose, and increase your fiber intake by choosing lots of vegetables, whole fruits (not juice) and opting for moderate servings of 100% whole grain products instead of the highly processed low fiber foods that are taking over our diets. Eating a low sugar, vegetable rich, whole food diet with a reduction in prepackaged processed food will help supply a variety of vital nutrients to support a functioning immune system, and a healthy microbiome. A healthy microbiome will help protect against increased permeability of the gut, decrease systemic inflammation as well as improve overall immune system functioning. The increased phytonutrient intake from fruits and vegetables also helps to reduce systemic inflammation through a variety of mechanisms. 

    Nutrients should always come from food first, but supplements can fill in the gaps. Make sure that you are getting adequate amounts of your omega-3 and zinc whether by food or supplement, and decrease your intake of the omega-6 rich oils. Eat nuts and green leafy vegetables for the magnesium. Take a vitamin D supplement but consider getting your levels tested if possible and safe to do so. The supplements discussed in this article at the doses recommended are safe and non-toxic : 1000 IU Vitamin D3, 15-30 mg zinc (the level found in a good multivitamin), 500-1000 mg EPA/DHA omega-3, 200-400 mg magnesium. Megadosing vitamin C is not likely to help, and taking high doses of certain vitamins and minerals may even be detrimental. Whether any of this will reduce your risk of severe infection or ARDS if you do contract a respiratory illness is purely speculative at this point. What I do know is that the average American diet is dangerous, damaging to the immune system, and hugely pro-inflammatory. Let’s do what we can to tip the scale in the other direction.  

Tom Flass MD, MS

This article was also featured on the Kalispell Regional Healthcare website on April 3, 2020

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