The short answer to the question – how do you boost your immune system – is that you can’t. The very concept of “boosting” the immune system is not scientific and does not exist within mainstream medicine. That’s because it is based on a fundamental misunderstanding of immunity, and of biological systems. Having said that there are legitimate things you can do to optimize immune function, which all are simply ways of avoiding things that inhibit immune function. But first let’s cover some basic principles.

Basic Principles

Biological organisms are complex, dynamic, homeostatic systems. This may seem obvious when you think about it, but many dubious health claims violate this basic understanding of biology. The immune system itself is a highly complex system – so complex that even though we have a vast amount of knowledge about the immune system, we have a hard time predicting the net effect of specific changes to the system. We have studied drugs in auto immune diseases, for example, that have had the opposite of the predicted net effect. This means we need to be very wary of any study that purports to show a change in some measure of immune function, and then concludes that this is a good thing, a “boosting” of immunity.

But perhaps even more important is the homeostatic bit. Immunity is a delicate balance, and evolution has tinkered with this system for hundreds of millions of year. If there were a simple way to make this system function better, we would have evolved it already. Put another way, there is no simple way to hack this system with a supplement or other measure. Immunity is also a double-edged sword – it fights off invaders and damaged cells, while trying to minimize harm to our own tissue. Think of inflammation – this is a good thing when it is killing bacteria, but also causes a lot of harm. When this delicate balance is disturbed, the result can be an auto-immune disease.

Therefore, we should think twice or thrice before doing something meant to disturb this delicate balance. Chances are greater we will make things worse than better. We need carefully conducted clinical trials to determine the net effect of doing anything to immunity in specific clinical contexts. Also, there is nothing better than a healthy immune system – there is no “super” immunity you can get from your diet or taking a supplement.

What Works

To keep your immune system functioning optimally you need to be well nourished. The best way to do this is to have a good well-rounded diet. Taking supplements are generally not necessary, and they do not replace a healthful diet. Supplementation should be targeted and evidence-based – so essentially, if you are low in one or more nutrients as measured in your blood, or you have a medical condition that has a known increased demand for a particular nutrient, then supplementation is reasonable. There is no evidence for any benefit to routine supplementation. Just eat your fruits and veg and have a generally well-rounded diet.

Sleep deprivation has been shown in numerous studies to decrease immune function (although many authors are conservative about interpreting the clinical implications of their results). One hypothesis for what causes this effect is that during sleep our bodies reallocate resources not needed for wakeful activity to immune activity. So sleep is a good time to fight off infections.

Another way to prevent the inhibition of healthy immune function is to avoid stress. There are hundreds of studies showing changes to immune function as part of the stress response. However, the clinical implications of these changes is not clear. To quote one review article:

“…there is little or no evidence linking stress-related immune change in healthy humans to disease vulnerability. Even large stress-induced immune changes can have small clinical consequences because of the redundancy of the immune system’s components or because they do not persist for a sufficient duration to enhance disease susceptibility. In short, the immune system is remarkably flexible and capable of substantial change without compromising an otherwise healthy host.”

This brings us to perhaps the most important factor – being generally healthy in the first place. Disease states can compromise our physiological function, impairing the robustness of our immune response. It may also sap our resources, so fewer resources are available to fight off infection. We are seeing this in the current pandemic – the most susceptible people are older individuals with chronic illness. We can’t magically eliminate chronic disease, but it is important to make sure you are optimally treating any chronic conditions to minimize any negative effects on your immune protection.

Other acute illnesses are also a huge factor. If, for example, you get the flu, that will make you more susceptible to COVID-19 while your body is using up resources fighting off the initial infection. This is why it is so important to get your flu vaccine – and to keep up to date on all your vaccinations. Vaccines are the one legitimate way to actually enhance your immune function. It gives your immune system memory for a specific infecting agent, so that when it is encountered in the wild your immune system can mount a more rapid and robust response.

What Doesn’t Work

Pretty much anything else, but here are some specific popular examples. Perhaps the most common claim for a supplement that helps fight off infections is vitamin C. There are numerous studies looking at the protective effects of vitamin C, and they generally show no benefit. Specifically if we look at the common cold (which is the most studied) there is no evidence for a protective effect from routine vitamin C in the general population. There is possibly a tiny effect in terms of cold duration with taking vitamin C supplementation while sick. There may also be a benefit specifically for individuals who are undergoing short term extreme physical exertion. So basically, unles you are in that latter group, there is no reason to take vitamin C unless you are deficient or at risk for deficiency for some reason.

Vitamin D has a clear role in immune function, and vitamin D deficiency is now known to be associated with increased risk of auto-immune disease and increased susceptibility to infection. So again – your primary care doctor should check your vitamin D level and if you are low prescribe an appropriate supplement. This is a good time to reinforce the principle that there are optimal vitamin levels, and the more is not better. In fact you can have vitamin toxicity, including from vitamin D. So – targeted supplementation yes, routine or “mega” dosing no.

What about anti-oxidants? These remain  very popular supplement, despite the fact that there is no proven benefit to taking antioxidants routinely. Further still, if you take enough, if anything, they can have an inhibitory effect on immune function. Parts of the immune system use oxygen free-radicals as weapons against invading organisms. Suppressing those free-radicals partially disarms the immune system. This gets back to the homeostasis principle. We have already evolved an optimal balance (in the healthy state) of oxidative stress vs natural antioxidants. Don’t mess with that homeostasis.

Conclusion

As is often the case it is easy to get distracted by questionable and insignificant effects from ones that are huge and well-proven. If you want to avoid COVID-19 then avoid exposure – wash your hands, don’t touch your face, keep physically isolated, and consult your physician if you develop any possible symptoms. If you want to maximize your chances of fighting off exposure or minimizing illness, then stay generally healthy. Get plenty of sleep, eat well, treat your chronic illnesses, and stay up-to-date on your vaccinations. I would also generally avoid any product that claims to “boost” the immune system. These claims are not reviewed by the FDA, and they are medically meaningless.