Fasting is a great tool for so many things. You can use it to regulate food intake and lose body fat. Fasting can help you shift body composition, normalize your appetite, and gain control over your relationship to food. Many people report cognitive enhancements from fasting, and it’s a surefire way to speed up the transition into ketosis and full-blown fat adaptation. There’s strong evidence that we look, feel, and perform best skipping the occasional meal—that it’s the evolutionary norm for humans not to have constant, unceasing access to food. After all, we didn’t always have 24 hour grocery stores and fast food restaurants. But what about fasting with a cold?
And what about intermittent fasting and the immune system? Should you fast at all when you’re sick? What about fasting with the flu? Or how about bacterial infections—can fasting help with those? These are actually some of the most common questions I receive. Because intermittent fasting seems to help with so many other conditions, it makes sense to wonder about its relationship to the immune response.
There are two main types of infections that most people worry over: viral infections and bacterial infections.
Viral infections include influenza (flu), the common cold, viral gastroenteritis, and the various coronaviruses. There are also things like measles, chickenpox, and viral meningitis, but most people aren’t very worried about catching those these days.
Bacterial infections include pneumonia (most pneumonias are bacterial in origin, though some can be viral), bacterial gastroenteritis, food poisoning, and bacterial sinus infections.
Fasting With a Cold – Viral Infections
In general, fasting doesn’t look like a great idea if you’re dealing with a viral infection like the flu or common cold. Why?
Studies on Fasting and Viruses
Animal studies show that mice who fast have a worse response to subsequent viral infections. In one paper, mice were either fasted every other day or placed a normal diet, then exposed to a “viral mimetic” (a type of chemical that replicates a viral infection). The mice who fasted ended up with higher cortisol, a more inflammatory immune response, more severe symptoms, and acted sicker than the mice who ate.1
Another mouse study found that in animals exposed to an infectious virus, a fat-based (fasting) metabolism was detrimental to survival and a glucose-based (fed) metabolism was beneficial.2
Viruses Deplete Nutrients
Viruses are much smaller than bacteria and generally cause trouble by hijacking cells and using your body’s machinery to replicate. To do so, they often steal nutrients from the host. One example is selenium, a crucial nutrient for viral replication. Studies show that viral infections can induce selenium deficiencies and that correcting those deficiencies by, well, eating selenium-rich foods can improve the outcome of infections.3
Most viruses will deplete nutrients and you need to eat to replenish them.
Fasting Inhibits MTOR, Which is Good and Bad
Blocking MTOR (mammalian target of rapamycin) through fasting reduces expression of another major immune component: the interferon-inducible transmembrane protein (IFITM). 4 Think of the IFITM as a barrier preventing some viruses from gaining a foothold in your body, including influenza, Ebola, SARS, MERS, Marburg, Dengue, and hepatitis C. These are IFITM-sensitive viruses, but there are also IFITM-resistant viruses, like the common cold.
In fact, one study found that increasing IFITM levels increased vulnerability to infection by the common cold virus.5 Using fasting to reduce mTOR and lower IFITM expression could conceivably increase resistance to the common cold while increasing vulnerability to flu (and the others like Ebola and so on). Double edged-blade.
Or perhaps it’s triple-edged.
Part of dealing with an infection is learning from it. Our immune systems have to seroconvert antibodies so that when we encounter the infection again, our immune system is better equipped to head it off at the pass. This is the concept behind vaccination—a measured dose of the infective agent that trains our immune system to defeat the real thing in the future. As it turns out, inhibiting mTOR through fasting could affect our ability to seroconvert antibodies in response to viral infections.
In studies of older adults, higher levels of mTOR predict lower rates of seroconversion, and giving them an mTOR inhibitor improves seroconversion after a flu vaccine. If fasting reduces mTOR (and it does), it should in theory improve the antibody response to a vaccine or infection.
Weird, right? Fasting reduces mTOR, which could impair your short term response to an infectious insult (or improve it if it’s the common cold) while improving your long term response. You might still get sick but at least your chances of developing longterm immunity should increase.
Fasting With a Cold – Bacterial Infections
In general, fasting looks like a better idea when you’re sick with a bacterial infection.
Research on Fasting and Bacteria
Animal models of bacterial infections find that mice tolerate them much better in a fasted, ketogenic state. In fact, the ketone body beta-hydroxybutyrate is able to directly nullify some of the oxidative stress associated with bacterial infections;6 while “ketogenesis was required for survival in bacterial inflammation, it was dispensable in the case of viral inflammation.”
Bacterial Infections Reduce Appetite Naturally
One indicator is that your appetite often falls off a cliff when you have a bacterial infection. The last you want to do when you’re dealing a bout of food poisoning is eat. This doesn’t usually happen with viral infections, and I believe that’s important. In those “base” states of survival, what you crave is a good indicator of what you need.
A key component of the innate immune system’s ability to deal with bacterial infections is phagocytosis: When a class of immune cells called neutrophils surrounds, engulfs, and destroys individual bacteria. The phagocytic index describes the number of bacterial cells a neutrophil is able to engulf and nullify in a set time. Generally speaking, higher phagocytic activity means you have a better response to bacterial infections.
Luckily, we know pretty well how to increase and decrease phagocytic activity in people. To decrease phagocytic activity (make neutrophils worse at engulfing and destroying pathogenic bacterial cells), you simply feed a person sugar.7 It could be orange juice, honey, sucrose, fructose, or glucose—any simple source of sugars—and if you give 100 grams to a person, their phagocytic capacity drops for at least five hours. Give the person nothing—let them fast—and their phagocytic capacity climbs. Even at 5 hours after eating the sugar, the phagocytosis still hasn’t caught up to that of the fasting person.
Later studies confirm that low fasting blood glucose is a strong predictor of a strong neutrophil:leukocyte ratio8. Lower glucose, more neutrophils available to take on bacterial pathogens. Fasting is a reliable way to drop your blood glucose.
Phagocytosis doesn’t work so well against viral infections because a virus sequesters itself in the host’s cells. Neutrophils can’t engulf and kill the host cells; that’d just be counterproductive and probably quite dangerous. However, there’s evidence both ways: that neutrophils can enhance the immune response to viral infections but they can also exacerbate the damage done to host tissues. It isn’t clear what role they play so I’d advise against consciously manipulating them through fasting.
Intermittent Fasting While Sick: Making Sense of it All
There are no easy, straightforward rules governing the optimal fasting strategy for infections, whether viral or bacterial. Each virus is different. Every bacteria is separate. Nothing in biology is simple. What we do know:
If fasting stresses you out, it will be bad for your immunity. Cortisol depresses the immune system.
If fasting ruins your sleep, it will be bad for your immunity. Proper sleep is absolutely essential for an optimal immune response.
If you’re hungry, let that be your guide. Eat. Don’t force the issue.
If you’re not hungry, skip the meal. Again, let your body’s signals be your guide.
When faced with an immune insult, or if something’s “going around,” cut back on the fasting or at least keep it shorter than normal. 16 hours instead of 30. 20 instead of 48.
Realize that fasting is not a panacea. It’s not the answer to everything.
Understand that bacterial and viral infections often tag along with each other. A virus will weaken the host enough to allow bacterial pathogens to flourish. You’ll often be dealing with both at once. I’d imagine that something that allows you to stay fed while also enjoying a fat-based metabolism—like a lazy ketogenic diet—could work well here.
Fasting can prune damaged parts of your immune system and replace them with renewed components.9 This is good for long term immune health, but if an infectious agent happens to catch you in the middle of an extended fast while you’re doing the pruning, your risk of infection probably goes up. There’s always a give and a take.
There are no magic bullets, but it cuts both ways. You are resilient. While most of the humans throughout history didn’t make it through hundreds of thousands of years of death, destruction, famine, and disease, your gene line did. So don’t think you have to pick one or the other—fasting or feeding—in response to illness. Go with what feels best, don’t get dogmatic, and just take it easy.
What’s your go-to feeding strategy for dealing with sickness? Do you differentiate between viral and bacterial illnesses? Let me know down below!
Mark Sisson is the founder of Mark’s Daily Apple, godfather to the Primal food and lifestyle movement, and the New York Times bestselling author of The Keto Reset Diet. His latest book is Keto for Life, where he discusses how he combines the keto diet with a Primal lifestyle for optimal health and longevity. Mark is the author of numerous other books as well, including The Primal Blueprint, which was credited with turbocharging the growth of the primal/paleo movement back in 2009. After spending three decades researching and educating folks on why food is the key component to achieving and maintaining optimal wellness, Mark launched Primal Kitchen, a real-food company that creates Primal/paleo, keto, and Whole30-friendly kitchen staples.