Boosting your immune system: what you need to know
This is the question on everyone’s mind as we face new and old infectious diseases.
We see online claims that we can “boost” our immune system with just about everything from Vitamin C or Vitamin D to essential oils and silver nanoparticles. But how do we know which of these recommendations, if any, really work?
That’s a tough question to answer, mainly because the immune system is not “one thing” that we can easily measure. It’s an intricate and delicate system with many different components. Helping one section might impair another, or boosting a certain section may have nothing to do with fighting viruses.
That makes knowing what to do challenging.
Finally, if you are older or have an underlying health condition (including obesity, diabetes, or high blood pressure), you have an increased risk of just about any infections, so prevention is always a top priority.
If you’re young and otherwise healthy, your risk of complications is much lower.
Low-carb nutrition and immune function
As you may have read at Diet Doctor, we believe the evidence is strong to support the many health benefits of eating low-carb diets.
In fact, it may be more important now than ever to maintain a healthy lifestyle, including a long-term healthy eating pattern. That means an eating pattern that provides essential nourishment and also keeps your blood sugar and weight in check.
It appears that individuals with type 2 diabetes and other metabolic conditions are at higher risk of complications from infections. The evidence is clear that low-carb and ketogenic diets can be effective tools for treating and reversing these metabolic conditions.
While we can’t prove that low-carb nutrition boosts immune function per se, it makes sense to limit the conditions (e.g. high blood pressure or high blood sugar) that might make things worse.
In addition, one study showed that a keto diet reduces the risk for mice infected with influenza. One question we should ask is, if we follow a diet that is proven to help with weight loss and metabolic health and may also beneficially affect immune function, what do we have to lose?
With that in mind, here are some of our top tips to decrease your risk of catching viral infections.
These are things that you can do today that don’t cost anything. We recommend you prioritize these basic steps for protecting and improving your health — and potentially your immune system.
While these actions are always important aspects of maintaining good health, they may be crucial during times of increased risk.
- Proper handwashing: most viruses are killed by proper handwashing for 20 seconds with soap or using hand sanitizer that is greater than 60% alcohol.
- Don’t smoke: Smokers have an increased risk of catching infections and suffering severe complications from respiratory infections.1 We shouldn’t need more reasons not to smoke, but this highlights the importance even more.
- Get adequate sleep: Sleep is important for health in general, and as a bonus it may also benefit our immune function. For instance, one study showed those with insomnia had, on average, less immune response to the influenza vaccine, while another study in twins showed those with worse sleep had altered expression of genes related to immune function.2
Last — and we can file this one under “How in the world did they convince people to do this study?” — 153 volunteers were inoculated with the rhinovirus (the virus that can cause the common cold). They found those who slept less than seven hours were three times more likely to develop symptoms than those who slept more than eight hours.3
Again, the science in this area may not be robust, but when it comes to overall health, proper sleep helps. In times like these, you should prioritize sleep hygiene.
If you’re spending more time on electronics like tablets, phones, and TVs, it may be a good time to invest in blue-light blocking glasses and to look for non-tech related activities to do in the evening, like puzzles, crosswords, or reading an actual book (not an ebook!).
- The right amount of exercise: Observational studies show that those who exercise tend to suffer fewer infections than those who do not.4
While those studies have confounding variables, the general consensus is that exercise overall is likely beneficial, with some caveats.5
Some studies show bouts of strenuous exertion (>1.5 hours with an average heart rate >75% maximum) may temporarily decrease immune function. In addition, elite athletes who “overtrain” tend to suffer from infections more frequently than others.6
Our advice? Stay active, but remember: high-risk infection season may not be the best time to start a new high-intensity exercise routine. If you already enjoy strenuous exercise, consider decreasing the frequency or intensity by 10-20% during an outbreak (this is not scientifically backed but is recommended by some experts). Also, try to focus on home or outside exercise. Indoor workouts and shared gym equipment could potentially increase the risk of viral transmission.For more guidance, see our extensive exercise guide.DD+ MEMBERSHIP
- Stress management: While acute stressors may temporarily enhance immune functions, chronic stressors likely diminish immune function.7 Worrying about the stock market, stressing about politics, and focusing on the uncertainties of the future can raise cortisol levels, which may negatively impact our immune function.
While data are difficult to interpret in this area, one study resported students with increasing stress levels before an exam had decreased number of natural killer cells, the cells that are the “first responders” of our immune system.8
We can’t make all stressful situations disappear. But we can all take measures to control our response to stress. Meditation, mindfulness exercises, and getting outside and going for walks are all examples of activities that are free and relatively easy to do.
Whether stress management techniques help your immune system or not, they can potentially help blood pressure, blood sugar, and make your days much more pleasant.
- If you drink alcohol, drink in moderation: In times of stress, some people turn to alcohol as a coping mechanism. While meditation, nature walks, and mindfulness exercises are likely healthier ways of coping, for some they aren’t enough, and alcohol adds a little something extra. There’s no judging here. We all have to do what we can to get through tough times.
However, studies show a relationship between chronic heavy alcohol consumption and increased susceptibility to infections.9 Some of these studies showed an increased risk among heavy drinkers of acute respiratory distress syndrome (ARDS), the lung complication responsible for viral related deaths.
The trick is knowing where to draw the line. While there is little science, most experts suggest that a reasonable daily limit is two drinks for men and one drink for women. Keep in mind that following a low-carb lifestyle may decrease your tolerance to alcohol, so you may need to adjust your intake. You can read more about alcohol and low-carb lifestyles in our evidence-based guide.
Could taking vitamins, minerals, or other supplements help protect you from infections? Contrary to what you might read on the internet, this is a question that can’t be answered definitively. Here’s what we do know about certain supplements that reportedly have immune-boosting properties.
For decades, Vitamin C has been used to help prevent the common cold. Among other functions, this vitamin can help maintain healthy skin that provides a barrier to germs and other harmful invaders. In addition, some — but not all — studies suggest it may improve the function of certain white blood cells that fight infection.10 In addition, there is conflicting evidence about the potential mortality benefits of high dose Vitamin C for patients with sepsis, the most severe form of systemic infections.11
While it’s unclear whether taking a Vitamin C supplement is beneficial for viral infections, for most people there’s no harm in taking up to 2,000 mg per day (the upper limit set by the National Academy of Medicine).
For smokers and high-risk individuals, it’s definitely worth considering. Vitamin C is water-soluble, so your body will excrete whatever you don’t need into your urine. However, at very high doses, Vitamin C may cause diarrhea or increase the risk of kidney stones (especially in men), so be sure not to exceed 2,000 mg daily.12
As both a hormone and a vitamin, Vitamin D plays a number of important roles in health.
In recent years, people have taken very high doses of Vitamin D with the intention of boosting immunity. But is this an effective tactic? A 2017 systematic review of 25 randomized trials found that taking a Vitamin D supplement seemed to have a mild protective effect against respiratory-tract infections in most people, but provided much greater protection in those who were very deficient in Vitamin D.13
Observational studies suggest that low Vitamin D levels correlate with a higher risk of complications and death from some viral infections.14
If your Vitamin D levels are low, you may have a better chance of staying well if you supplement with 2,000 IU per day (or more, with medical supervision). Many — perhaps even most — people are deficient in vitamin D.15 So it’s probably wise to take a Vitamin D supplement during the flu season and at times of high risk from other infections.
Of course, your body can make Vitamin D on its own when your skin is exposed to sunlight, so try to get some sun whenever you can. How much sun depends on the time of year and your location. A good starting point is 15 minutes of exposure to a large body part (such as the torso or back). Just remember to avoid sunburns, as excess sun exposure carries its own risks.16
Zinc is a mineral involved in the white blood cell response to infection. Because of this, people who are deficient in zinc are more susceptible to cold, flu, and other viruses. One meta-analysis of seven trials found that supplementing with zinc reduced the length of the common cold by an average of 33%.17
Taking supplementary zinc may be a good strategy for older people and others at increased risk from infections. If you decide to take zinc, make sure to stay below the upper limit of 40 mg per day, and avoid administering nasally, due to the risk of olfactory complications.18
Turmeric is a spice commonly used in Indian and Asian cuisine, including curries. It contains a bright-yellow compound known as curcumin, which emerging research suggests might enhance immune function.19 However, there isn’t any convincing evidence showing that it helps fight viral infections yet.
On the other hand, adding turmeric to your food adds flavor, and taking a curcumin supplement is unlikely to cause any harm in otherwise healthy people. If you have any medical conditions — especially if you take blood thinners — check with your doctor before supplementing with curcumin.
Echinacea is an herb that can reportedly help prevent the common cold. But is this reputation well-deserved? A recent systematic review of randomized trials found that echinacea may possibly have a mild protective effect against upper-respiratory infections but doesn’t appear to reduce the length or severity of illness.20 It appears to be safe to take on a short-term basis. If you’re at high risk for infectious complications, you may consider taking it for the next several weeks.
Garlic, a popular and pungent herb with a characteristic aroma, is widely believed to have antibacterial and antiviral effects, including helping to fight the common cold.
A 2014 randomized controlled trial found that people who took a garlic supplement had fewer colds and recovered more quickly from colds than people who didn’t take garlic.21 Although this is encouraging, this is just one study. Other high-quality trials are needed to confirm whether garlic is truly beneficial for the common cold or other upper-respiratory infections. For now, enjoy garlic for its zesty flavor and unmistakable aroma rather than counting on it to boost your immunity.
Fruits, veggies and seeds
Getting plenty of fruits, vegetables, and seeds is a common recommendation seen on many sites, but the evidence is inconclusive if it truly helps fight or prevent infections.
In one often-quoted study, elderly volunteers were randomized to less than two or greater than five daily servings of fruits and vegetables.22 They found the group with the higher fruit consumption had a better immune response to the pneumonia vaccine but not to the tetanus vaccine.
Another claim is that “eating the rainbow” and getting “adequate phytonutrients” improves immune function and reduces infection risk. Unfortunately, “eating the rainbow” and getting “adequate phytonutrients” are poorly defined terms, and such messages are usually compromised by being based on nutritional epidemiology studies heavily impacted by the underlying diet (i.e mostly in high-carb diets) and healthy-user bias.23
Therefore, we cannot conclude that any one specific food will improve your immune function. However, as with many other potential health benefits, it makes sense to stick to a diet that provides adequate essential nutrition and is rich in minimally processed natural foods. It may not be more complicated than that.
Refined carbs and sugars
Laboratory evidence suggests sugar may impair white blood cell function, but no credible evidence shows eating it makes you get more infections.24 However, other evidence suggests acute rises in blood sugar may increase risk of infections and complications.25
Some data suggest that elevated blood sugar levels portend a higher risk of complications from viral infections.26 Therefore, it would make sense that we want to limit these blood sugar elevations. Refined carbohydrates and simple sugars are two of the biggest offenders for elevated blood sugar and therefore should probably be avoided.
This is different than saying studies show avoiding these foods results in fewer infections. (We don’t have that evidence.) Plus, as we have mentioned many times, it’s difficult to isolate the effect of one food since any food’s effects have to be studied within the context of the underlying diet (i.e. standard American diet vs. a low-carb diet).
However, one simple solution is to use the measurement of your own blood sugar as a guide. If higher blood sugar is associated with more complications, it makes sense we want to limit that.
We suggest measuring your blood sugar either with a regular glucometer or, even better, with a continuous glucometer (CGM) if you have access to one. If the foods you eat cause your blood sugar to rise above 140mg/dl (7.8mmol/L), consider eating something different.
Studies show that a low-carb, moderate protein, higher fat diet effectively reduces blood sugar and can even reverse type 2 diabetes.27 We don’t have proof that this will “boost your immune system,” but it may help keep blood sugars in check which may be associated with decreased infectious risk.
Chicken soup/bone broth
Treating colds and the flu with chicken soup may be the most popular urban myth of all time. Surprisingly, it may not be 100% a myth.
One study showed chicken soup “inhibited neutrophil migration,” which the authors suggest could improve our ability to recover from infections.28 However, this is one of those instances where laboratory findings may not translate to clinical improvements such as fewer or less serious infections.
But it’s hard to argue with a tasty homemade soup with chicken, a few low-carb veggies, and plenty of real salt. Immune booster or not, it sounds like a great meal for a wintery day. We chalk that one up to good self-care.
With all the focus on how certain foods affect your immune system, you may also wonder, what about fasting?
One study in mice showed that fasting, or more specifically refeeding after a fast, restored immune function that had been suppressed by chemotherapy.29 During the fasting period itself, however, it appeared to impair the immune system. In addition, the beneficial response to refeeding may be lessened in the elderly.30
These are very important caveats. Over the long term, intermittent fasting and refeeding may boost the immune system. However, during an acutely increased risk of infection, it may not be a good time to try prolonged fasting, given the potential for a temporary decrease in immunity.
This may sound surprising for those who have heard the phrase, “starve a fever.” The theory is that humans have evolved to not feel hungry and purposely avoid food during an acute illness as a protective mechanism, which may in turn limit nutrients the virus needs to replicate. To be clear, this is all conjecture without any quality supporting evidence.
Other evidence suggests that ketones are beneficial for immune function, and perhaps that could be why some recommend fasting.31 But considering all of the data together, if that were the case, you would likely be better off eating a keto diet and not fasting.
Based on the limited data available, we suggest not fasting longer than 36 hours during an acute infection outbreak, especially if you are older than 60 years old. It’s logical that you can continue with shorter-duration time-restricted eating, although there is no data on this either.
Over-the-counter pain and fever medications
Should you take one of the common over-the counter (OTC) medications for the headaches and fever that are often the symptoms of viral illnesses?
In particular, what about taking non-steroidal anti-inflammatory drugs (NSAIDs)? These include drugs like aspirin, ibuprofen (Advil, Motrin, Midol) and naproxen (Aleve, Naprosyn).
The data are unclear.32
Some older observational studies support more severe disease complications arising from colds and flu in those who are taking NSAIDs. But this could be because the more severe diseases cause more symptoms, and that is why more people take NSAIDS to combat those symptoms.
A Danish study reported no increased risk of death or complications in those who took anti-inflammatory medications.33 And another observational study in England likewise found no association between NSAID use and risk of death from certain viral infections.34
However, it may help to remember that a fever is part of your body’s response to combat the virus. By itself a fever is not dangerous for most people unless it is sustained above 104 degrees Fahrenheit (40 Celsius). 35 It is uncomfortable, but not dangerous.
One approach is instead of taking an OTC pill for a headache or fever, consider starting with cool sponge baths, damp washcloths, and remember to rest and drink plenty of fluids with electrolytes, such as using Diet Doctor’s electrolyte elixir drink.
If you still need more aggressive care, one strategy could be to use acetaminophen first for symptoms like fever or body aches, and only if that doesn’t control your symptoms, then consider taking NSAIDs for the shortest time possible to help you feel better. But we should be clear that this recommendation is not based on strong science, so consult with your healthcare provider for more specific guidance.
If you are already taking daily NSAIDS for arthritis or other chronic pain issues, it does not appear that there is an increased risk of complications, but you should discuss with your physician if an alternative exists.
In short, your immune system plays an important role in decreasing your health risk from viral infections.
The more actions we take to keep ourselves generally healthy, the better.
Good hand hygiene can help prevent catching viruses. Doing what you can to reduce specific risk factors may help your body recover quickly if you do get an infection. Plus, your overall health may benefit from the following:
- Eating a nutritious diet that minimizes high blood sugar
- Prioritizing restful sleep
- Managing your stress
- Stopping smoking
- Participating in moderate exercise that you enjoy
- Getting sunshine and fresh air where possible
While the scientific evidence is spotty, taking some specific supplements may improve your overall health and are likely to not be harmful if taken as directed.
If nothing else, consider taking Vitamin D because you may be deficient in the months with shorter daylight availability.
Likewise, a low-carb diet that minimizes high blood sugar and is rich in nutrients and whole, minimally processed foods may contribute to better overall health.
Remember, however, that a low-carb diet does not have some magical quality that will keep you safe from infection. But it may be a good start!
Boosting your immune system: what you need to know - the evidence
The guide contains scientific references. You can find these in the notes throughout the text, and click the links to read the peer-reviewed scientific papers. When appropriate we include a grading of the strength of the evidence, with a link to our policy on this. Our evidence-based guides are updated at least once per year to reflect and reference the latest science on the topic.
All our evidence-based health guides are written or reviewed by medical doctors who are experts on the topic. To stay unbiased we show no ads, sell no physical products, and take no money from the industry. We're fully funded by the people, via an optional membership. Most information at Diet Doctor is free forever.
Should you find any inaccuracy in this guide, please email firstname.lastname@example.org.
International Journal of Environmental Research and Public Health 2018: Tobacco smoke induces and alters immune responses in the lung triggering inflammation, allergy, asthma and other lung diseases: A mechanistic review [overview article; ungraded] ↩
Frontiers in Immunology 2018: Debunking the myth of exercise-induced immune suppression: Redefining the impact of exercise on immunological health across the lifespan.
[overview article; ungraded] ↩
Allergy, Asthma and Clinical Immunology 2008: Enhancing versus suppressive effects of stress on immune function: Implications for immunoprotection versus immunopathology [overview article; ungraded] ↩
British Medical Journal 2017: Vitamin D supplementation to prevent acute respiratory tract infections: systematic review and meta-analysis of individual participant data [systematic review of randomized trials; strong evidence] ↩
Nutrients 2020: Low 25-Hydroxyvitamin D Levels on Admission to the Intensive Care Unit May Predispose COVID-19 Pneumonia Patients to a Higher 28-Day Mortality Risk: A Pilot Study on a Greek ICU Cohort [observational study, weak evidence] ↩
JRSM Open 2017: Zinc lozenges and the common cold: a meta-analysis comparing zinc acetate and zinc gluconate, and the role of zinc dosage [systematic review of randomized trials; strong evidence] ↩
Complementary Therapies in Medicine 2019: Echinacea for the prevention and treatment of upper respiratory tract infections: A systematic review and meta-analysis [systematic review of randomized trials; strong evidence] ↩
Annals of Medicine 2021: Admission hyperglycaemia as a predictor of mortality in patients hospitalized with COVID-19 regardless of diabetes status: data from the Spanish SEMI-COVID-19 Registry [observational study, weak evidence] ↩
Diabetes Research in Clinical practice 2018: Effect of dietary carbohydrate restriction on glycemic control in adults with diabetes: A systematic review and meta-analysis. [systematic review of randomized trials; strong evidence] ↩
PLoS medicine 2020: Adverse outcomes and mortality in users of non-steroidal anti-inflammatory drugs who tested positive for SARS-CoV-2: A Danish nationwide cohort study [nonrandomized study, weak evidence] ↩
Annals of Rheumatic Disease 2021: Use of non-steroidal anti-inflammatory drugs and risk of death from COVID-19: an OpenSAFELY cohort analysis based on two cohorts [nonrandomized study, weak evidence] ↩