Exercise, Nutrition, and Viral Infections – What You Need to Know

If you’ve been wondering what to believe about exercise, nutrition, and your own immunity, we would like to clarify all the confusion for you. Here are some facts from your friendly gym-going physician.

Written by Dr. Louis Minders

Why is the immune system a problem in viral infections?

The usual immune response to viral or other infections is the destruction of cells infected with virus. The body recognises these cells in two ways:

1.Viral particles pushed to the surface of the infected cell – the cellular equivalent of waving a flag to let your allies know you are in trouble.

2.Your immune system detects that the cells are unusually silent and that none of the usual flags are flying and assumes that that cell must be in trouble.

What results from this declaration of war is a system wide call to action accompanied by cellular learning (antibody productions) and widespread destruction of cells that are thought to contain the virus. Each person responds in a different way to the virus and this leads to some variation.

A good way to understand this is to consider two viruses we know a lot about:

1. The poliovirus that infected many children and resulted in significant disability in some of them. Since the widespread use of the polio vaccine this virus has essentially been eradicated. 90-95% of poliovirus infections were asymptomatic. In the 5-10% that did develop symptoms this resulted in very vague symptoms of headache, sore throat, fever, nausea etc. Paralysis only occurred in 0.1% of cases but resulted from the destruction of motor nerves by the virus and the immune system. 

2. Influenza has significant variation in its presentation – from some individuals feeling slightly unwell to others who become severely ill and die. In the case of a viral pneumonia, damage to lung tissues is mediated by direct viral destruction as well as the damage caused by the immune system’s response to the virus being there. The inflammatory substances result in fluid leakage from the small blood vessels as well as the destruction of infected cells.

So, what we can deduce from this is that the immune system is a bit of a “Goldilocks” situation – you need the right amount of immune response to keep you safe but not too much of an immune response that will destroy your cells and kill you. Most of the mortality related to COVID-19 is as a result of an intense inflammatory storm that results in organ dysfunction.

Why the health of the host matters?

You may have heard that in the case of COVID-19, obese individuals, and those with other chronic diseases, are disproportionately affected by the virus. Why is this?

Obesity is a proinflammatory condition. There is an increase in the amount of circulating pro-inflammatory substances as well as a reduction in anti-inflammatory substances. It is this chronic inflammation that is understood to lead to insulin resistance and ultimately diabetes, as well as the narrowing of blood vessels through the deposition of cholesterol on the vessel walls. In addition to the deposition of cholesterol on the vessel walls the rupture of atherosclerotic plaque and tendency to form blood clots (resulting in heart attacks and strokes) are also understood to be causally related to inflammation and ultimately obesity. 

Chronic disease – whether it be diabetes or hypertension – is also a pro-inflammatory state in much the same way. I realise there is a lot of negative press about statin drugs but the mechanism by which these reduce cardiovascular events in high risk patients is more related to their anti-inflammatory effect than their improvement of the numbers. But I digress…

The problem in these individuals is not only that they have a reduced functional reserve and maybe don’t breathe so well at baseline but it also has a lot to do with the fact that their systems are already in this super-inflammatory state and therefore more likely to cause harm when exposed to a pathogen.

So why does exercise matter?

So, the reason for my lengthy pre-amble comes down to the simple fact that exercise is anti-inflammatory and essentially supresses the immune system. Had I not briefed you on why the immune system is sometimes not your friend we would have seen a mass exodus and an increase in Netflix subscriptions instead.

Major disclaimer: Intense, prolonged exercise, defined as 60% of heart rate reserve for more than 60 minutes (such as running a marathon, or completing that fiendish workout concocted at the 2020 CrossFit Games) does reduce immune function to the point that you are more susceptible to Upper Respiratory Tract Infections (all things cold and flu). Proof once and for all that the Fran cough may actually become something more sinister, assuming you do it several times over. Below this threshold something truly incredible happens:

-Increased antipathogen activity of your tissue macrophages – the cells that identify and eat disease for breakfast.

-Increased circulation of immunoglobulins and anti-inflammatory substances

-Increased circulation of various other immune cells that make up the immune system

These changes are known to enhance your body’s ability to find and destroy infections but lower overall inflammation. Studies have shown that those who engage in moderate intensity exercise had a reduction in the duration, incidence and severity of Upper Respiratory Tract Infections by around 40-50% which is more than any supplement/medication. This moderate exercise explosion also reduced stress hormones that upregulate our inflammatory response. This response is even thought to enhance vaccine effectiveness, assuming you and your tender deltoid can make it to class.

In our older population, exercise restored some of the immune function that is lost with aging, and enhanced vaccine effectiveness. More data are needed in this area to make specific recommendations.

Now what of nutrition?

What you take in after exercise can further enhance the immunomodulation. Our love-hate relationship with carbohydrates actually take on another dimension when it comes to inflammation. Taking in 30-60g of Carbohydrate during intense endurance exercise (>90min) counteracted some of the inflammatory response with about 30-40% reduction in the inflammatory response. Ensuring that the carbohydrate intake came from fruits with higher polyphenol levels, like bananas, raisins, and dates further enhanced the anti-inflammatory effect.

In those not killing themselves with endurance events there are indeed some important considerations when it comes to nutrition and reduced susceptibility to disease:

-Vitamin D supplementation is warranted, especially during our winter months. Health Canada recommends taking 400U/day but one can take up to 2000U/day if there is no sun exposure or intake with foods is low. Realistically, most adult Canadians will require between 1000-2000U/day. Please be careful about reading supplement labels as too much Vitamin D is also dangerous.

-Omega 3 fatty acids. Ideally you should be taking these in with your diet but if you are unable to do so get some of these anti-inflammatory fatty acids in capsule form. 250mg/day is recommended. Flax, chia, walnuts, and salmon are the champions of the Omega 3 world and can get you 10x above the needed intake in a single ounce! Tofu, brussels sprouts, beans and avocado will also get you to your daily needs. For those taking anticoagulants or antiplatelet medications caution is advised.

-Everyone likes to talk about vitamin C but realistically most supplements overdo this and can actually put you at risk of kidney stones. You do not see benefit above 200mg in the general population but intake of 1-2g/day can be useful if you are sick. For your daily intake you will usually be able to get what you need from the food you eat but reaching 1-2g with food will be a challenge. Guavas, kiwifruit, bell peppers, and strawberries are the champions, but kale, oranges, tomato, broccoli, and peas are also great choices. 

Imagine a salad with kale, bell peppers, tomato and strawberries topped with walnuts and salmon as the ultimate disease fighting combination. You can swallow your vitamin D supplement with some water. 


Fuelling your body correctly and engaging in moderate intensity exercise is a useful tool to modulate your immune system and get to that “not too hot”, “not too cold” state. I realise that a lot of the focus here was in respiratory infections and getting sick but please keep in mind that all chronic diseases are inflammatory processes, and these interventions work for that too. There is a war going on inside you and you owe it to your system to provide the most conducive environment for victory.


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