ZOE Science & Nutrition - Most replayed moment: Can you really boost your immune system? | Professor Daniel Davis

Episode Date: August 25, 2026

Today we’re talking about the immune system. It’s one of the most important systems in our body, yet it’s also one of the hardest to understand. You can’t see it, you can’t feel it working, and there’s no simple way to measure whether it’s strong or weak. But when it fails, we certainly notice. Everywhere we look, supplements and superfoods promise to ‘boost’ our immune system. But how many of these claims are actually supported by science? Professor Daniel Davis joins me to explain how our immune system truly works, and what the evidence says is the best ways to support it. 🌱 Try our science-backed and tasty wholefood supplement Daily30 Get our brand-new app and Gut Health Test designed by world-leading gut health and nutrition scientists to build healthy eating habits 👉 Join ZOE Follow ZOE on Instagram. 📚Books by our ZOE Scientists The Food For Life Cookbook Every Body Should Know This by Dr Federica Amati Food For Life by Prof. Tim Spector Ferment by Prof. Tim Spector Good Mood Food (preorder) by Prof. Tim Spector Free resources from ZOE The Hormone Harmony Guide: Tuning Your Body’s Internal Orchestra Eating for Better Brain Health: Your brain-gut blueprint How to eat in 2026 - Discover ZOE’s 8 nutrition principles for long-term health Live Healthier: Top 10 Tips From ZOE Science & Nutrition Gut Guide - For a Healthier Microbiome in Weeks  Better Breakfast Guide Have feedback or a topic you'd like us to cover? Let us know here. Episode transcripts are available here.

Hello and welcome to Zoe Recap, where each week we find the best bits from one of our podcast episodes to help you improve your health. Today, we're talking about the immune system. It's one of the most important systems in our body, and it's also one of the hardest to understand. You can't see it, you can't feel it working, and there's no simple way to measure whether it's strong or weak. But when it fails, we certainly notice. Everywhere we look, supplements and superfoods promise to boost our immune system. But how many of these claims are actually supported by science? Professor Daniel Davis joins me to explain how our immune system truly works, where the evidence says is the best ways to support it. The immune system is very complicated.

I'm sure people are very aware of the importance of your immune system in fighting off all different kinds of infections. It's also important in preventing us from succumbing to other kinds of illnesses, cancer and all sorts of other problems that can arise in our health. it's inherently complicated because it has to fight off all different kinds of possible infections, including infections that have never before existed in the universe. A new type of virus could arise at some point, and our immune system has to be able to fight that. So the immune system is there to fight off things that are potentially dangerous,

but not attack any normal part of your healthy body. And to some extent, every single cell of you, is part of your immune system because almost all the different types of cells that make up the human body actually have some ability to know when they are infected with a virus, for example, or when they are turning cancer. So most cells of the body have already some inbuilt sensory mechanism to detect when they themselves have a problem and they could signify to other bona fide immune cells that they have a problem. So all of your body is part of the immune system. I mean, one thing I heard really clearly was that not only does it have to be incredibly good at attacking sort of these bad things outside, but it needs to be really smart at recognizing whether this is actually part of my body, so it mustn't be attacked.

I'm interested that you mentioned that just in this summary. Is that a very important part of what it needs to do in order to be healthy? Absolutely. Because you will know that there are. any number of products and ideas telling you how to boost your immune system. And on the face of it, that sounds quite a simple thing that we want to be doing. But having just told you the complexity of what the immune system has to do, a layer of depth in the idea of boosts of the immune system is immediately in power.

What does that actually mean? If you just somehow boost the ability of your immune system to be very active and start fighting things, then you don't want to do that because it would inadvertently start attacking things that are part of the normal healthy body and that would then cause you problems in itself. So it's important that we understand the complexity and the nuance in what the immune system has to be able to achieve because already we're at the point where some of the sound bites and clickbait you might come across for do this one thing to boost your immune system already now we could be questioning what does that actually mean? And I don't want to just boost my immune system in a very general way to make it more active.

I need it to be restrained in some situations because I don't want the immune system to react against my own body or things that are not particularly harmful to me like would happen if your immune system started reacting against food or something and gave you an allergic reaction. One of the things I thought of as you were describing this, like not reacting to things that are normal in the environment, is seasonal allergies or hay fever. And I'm probably thinking about that because I get this now regularly, which I never did as a kid. Is that an example of like my immune system? You know, I mean, sometimes we belittle allergies because they're not spoken about in the same way that cancer is, because allergies are not really a top 10 cause of death. But the number of people who get allergies, you just mentioned you get allergies, I get allergies, hay fever as well. And it's two billion people.

So allergies are incredibly important. And by and large, they are the outcome of our immune system reacting against something that is. not particularly harmful, like it could be pollen as an example, but there are many different types of allergies, of course. So just even that knowledge in itself hopefully means that thinking about the complexity of the immune system means it's all about having the immune system react to things that are truly dangerous, but not overreact against things that are not really a problem for the body. There's another side to that as well, which is that when you do get a genuine infection, a virus infection, let's say, and your immune system has to deal with that.

So that involves a heightened state of our immune system, the particular cells in your body that are good of fighting that virus have to multiply a number. You might experience a fever, depending on what type of infection is. But then, again, you don't want your body in that heightened state of immune activity because that would put it at risk of overreacting against something that's not harmful. So a lot of what we're talking about in good immune health. is keeping the immune system balanced rather than, you know, utterly boosted in some way. And I get the impression that maybe we don't come back to balance as much as perhaps we're designed to. I mean, you talked about the allergies, two billion people with allergies.

I just remember my grandparents sort of telling me that almost nobody had allergies when they were growing up. I don't know if that's true. Are we like coming back to normal in the same way that we are meant to? It's a very hard question to answer. I mean, the number of people diagnosed with allergies has been increasing, and some of that could be that we're better at recognising allergies than we used to be. It could be true that we are more susceptible to certain kinds of illnesses we used to be. And again, another reason for that might be that we're living longer on average. As we age, our immune system changes. Some autoimmune diseases do tend to arise more as we age. So there are many factors in the increasing prevalence of an immune system. problem. In terms of allergies, one of the things that you might be thinking about is the effect of our microbiome and there's this general idea that perhaps being exposed to something like a less hygienic environment or to microbes when we're younger trains our immune system to be better at dealing with allergies. So that comes from wonderful and pioneering research by

Gordon Strachan at St. George's Hospital in South London. We're not too far from where we're speaking right now. And he made the observation that large families, the younger siblings tend to develop less allergies or less hay fever. And he had the idea that perhaps in a larger family, especially a younger sibling was more exposed to infections when they were younger because the older, the other people in the family might get infections. So you had this idea that perhaps an exposure to germs when you're younger somehow protects you against allergies. And he called that the hygiene hypothesis, which probably many of their listeners will be somewhat familiar with. That does also correlate with studies of children growing up on farms.

One particularly careful study, or a group of studies, if you like, has looked at different communities in the USA, and one group essentially lives nearer to where animals live and the environment where animals are, and they are less likely to develop allergies than another farming community that tends to use mechanical farming tools and live further away from where the animals live. So that again fits with this idea of the hygiene hypothesis that more of an exposure to germs does somehow train your immune system to protect you against allergies when you're older. But as we've said right at the outset of this podcast, everything is very nuanced and complicated and there could be other explanations to those farming data. For example, unprocessed

milk also is used in farms, which may also have some protective benefit. We understand very little about what that is. From my perspective, as a scientist, I've kind of been trained to kind of hear something and then think of like 10 other explanations of why that might be true, rather than the one that I've just been told. And it's exactly the case in this, that there's lots of possible explanations for how growing up on a farm may influence the onset of allergies. And the bottom line is, we don't know enough about the details of the details of the

of how that works to give us an actionable outcome. And it's particularly difficult because I couldn't take dust from an animal house on a farm and somehow expose children to that to see what happens, because you would be exposing healthy children to something that we understand very little about and it is even potentially dangerous and that just isn't ethically possible to do.

So it's very hard to do the experiments to find out what is it. Is there some important ingredient to the environment of growing up in a farm that helps train our immune system. And right this very minute, we don't know enough detail to make this an actionable thing that leads to any advice. So we have to be very careful about explaining what the results are, what the experiment is. I'm pretty confident I speak for myself, but also for most of our listeners to say,

okay, I'd like to talk about some practical advice. Could we maybe start with whether there are any immune boosting supplements that would really support my immune health? The consensus view probably best comes from as obvious as it sounds, the sort of basic guidelines that you would get from, you know, medical establishment. As kind of unsatisfying as that sounds, that's probably where the consensus is. The best established example, or one of the good examples, would be vitamin D supplements during the winter.

For people living in the UK where it's certainly not as sunny as it might be in other parts of the world, that's the recommendation of the UK government, for example. And it is true that in a clinical trial, vitamin D supplements did reduce the chance that people with an average of 67 were susceptible to autoimmune disease that reduced the chance they get an autoimmune disease by 22%. So vitamin D levels are important. So that would be an example of a supplement that is recommended, for example, by the UK government. And are there any others that you would be recommending or are you, this is back to your like you don't want to really boost it and you've already thrown vitamin C out the window? There are established cases of vitamins being important. Vitamin A is very important, for example. But most people would be getting that level of vitamin A they need, for example, through a normal varied diet.

And so it wouldn't be the case that you need to supplement. So the one thing where I would say will affect your immune health and you can do something about it, perhaps. is long-term stress. A lot of the data that we've talked about, a lot of the experience we talked about are things coming from correlations between things. But with stress, we've taken that to another level. We've taken it to a kind of molecular level understanding

of what happens, so that we do understand in detail how stress very directly affects your immune health. When your body senses some threat, it goes into this fight-or-flight response, So a signal your hypothalamus will react to this perception of an issue and send the message to pituitary gland, send the message to adrenal gland, produces adrenaline and cortisol stress hormones. Those stress hormones get your body ready in a state of action to deal with the threat. And as part of that, they quieten down other body systems that aren't so important for dealing with that moment.

and that includes your immune system. So when cortisol levels are high, your immune system is quiet and down. So for a short period of time, this is entirely fine in a fight or flight response. So your example here is like, I've seen a lion, I need to run away, it's like forget about fighting a cold. All you need to do is escape this line. So it's all focused on that. We'll deal with the cold later. Let's run now.

But if that persists, Frick stress, of course, can be. long term if you're changing job, getting divorced, any number of reasons. Running Zoe. Running Zoe, yeah. Your immune system can be suppressed for a longer time and then that does correlate with more susceptible to infections. So that's true. That's not just one of these myths.

Like if you're under stress for a long period of time, you are more susceptible to infection. Yes. That has been proven in multitudes of ways. And what makes us really know that that's not just a correlation is because we can do very precise experiments. For example, if I just add cortisol to immune cells in a lab dish, they will be less good at killing cancer cells.

So that kind of molecular level analysis of how this plays out makes us confident that this is a process that is genuinely true and it's not a kind of something else going on that stress correlates with something else and immunism correlates something else. So that gives us confidence in that being an actual

thing. Now, here's the kicker in this, though. Everything I'm coming at you with is always more complicated. Right this minute, from everything we've said, stress, long-term stress can affect your immune health. It seems like I should do things to relax. Tai Chi, jigsaw puzzle, coloring, whatever it is you might do to relax to not be stressed in the long term. However, it hasn't actually been proven unequivocally that practices that reduce stress, improve your immune health. Everything would point to that being true and it is proven that, for example, practicing Tai Chi does lower your cortisol levels.

That is proven. But what's not proven is that those practices will protect you against infectious diseases. And that, again, comes back to this really important message that the experiments are really hard. On balance, it seems to me very clear that stress affects your long-term health and doing things to not get stressed

if you can is important. If you were going to give me and the listeners like a specific one or two things that we could just start doing straight after we stop listening to this call, what would they be? One, try to avoid where you can long-term chronic stress.

Two, every time you see something that feels like you've just been let in on a new secret to what's going to improve your immune health, be a little bit skeptical, and think about, well, what was the experiment they did?