ZOE Science & Nutrition - 5 threats to your immune system–and how to fight them | Prof. Daniel Davis MBE

Episode Date: September 11, 2025

Many supplements claim to “boost” your immune system. Now, we all want to avoid getting sick during the winter months, but do any of these products really work? And is trying to “boost” your immune system even a good idea? Jonathan speaks with Professor Daniel M. Davis, MBE — a leading expert on immunology and Head of Life Sciences at Imperial College London. Daniel has published 145 scientific papers, authored four best-selling science books, and spent 25 years researching how our immune system works. He even helped discover the immune synapse, a breakthrough that changed our understanding of immunity. Daniel explains how your immune system really works, why it’s connected to mental health, and how it can even detect cancer cells. He also clears up common myths and shares what genuinely helps keep your immune system strong through the winter months — and what won’t make a difference. By the end of the episode, you’ll know the practical, science-backed steps you can take to reduce your risk of colds and flu this winter without wasting money on products that don’t deliver. Unwrap the truth about your food 👉 ⁠Get the ZOE app  🌱 Try our new plant based wholefood supplement - Daily 30+ *Naturally high in copper which contributes to normal energy yielding metabolism and the normal function of the immune system Follow ZOE on Instagram. Timecodes 00:00 Your body’s surprising, built-in cancer defense 02:07 What is the immune system? 03:55 Why ‘boosting’ your immune system can be dangerous 05:28 Are allergies a sign of an overactive immune system? 07:53 The 'hygiene hypothesis': Does being too clean make us sick? 11:05 The unintended global experiment of the COVID pandemic 12:40 How your immune system is constantly fighting cancer 14:20 The revolutionary new therapies that unleash your immune system on cancer 16:05 Is your immune system killing cancerous cells right now? 17:12 Your immune system is the most unique thing about you 19:25 Why you shouldn't blame yourself for getting sick 21:18 The myth of vitamin C and the Nobel prize winner who started it 23:55 The truth about vitamin C and colds 25:06 Does being cold actually give you a cold? 26:15 The powerful link between your immune health and mental health 28:06 Cytokines: The secret messengers you’re going to hear more about 29:00 Could inflammation predict teenage depression? 33:40 What is inflammation, really? 40:33 The double-edged sword of personal health data 42:06 Are there any immune supplements that actually work? 43:35 The one thing that directly harms your immune health 45:05 Why 'fight or flight' is bad for your immune system 47:08 How much exercise is best for your immune system? 48:16 The two most practical pieces of advice for a healthy immune system 📚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 SpectorFree resources from ZOE Live Healthier: Top 10 Tips From ZOE Science & Nutrition Gut Guide - For a Healthier Microbiome in Weeks  Have feedback or a topic you'd like us to cover? Let us know here.Episode transcripts are available here.

Welcome to Zoe Science and Nutrition, where world-leading scientists explain how their research can improve your health. Walk into any store and you'll see rows of supplements claiming to boost your immune system. We all want to avoid being sick during the winter months, so these products can seem tempting. But do any of them really work? And it's trying to boost your immune system really a good idea. To help us understand, we're joined today by Daniel Davis, Professor of Immunology and Head of Life Sciences at Imperial College London. He's published 145 scientific papers and authored four best-selling science books.

Daniel has researched immune health for 25 years and helped discover the immune synapse which revolutionized our understanding of this system. Today, he'll help us bust us. myths about the immune system and explain how immune health is linked with mental health. Most importantly, he'll tell you how to support your immune system. By the end of today's episode, you'll know what you need to do to reduce your risk of catching cold or flu this winter. And just as importantly, you'll know what won't help. Dan, thank you so much for joining me today. Thanks, Jonathan. It's wonderful to be here.

I'm aware that our immune system helps to fight off, like these microbes that are out there in the world trying to infect me and make me sick. And honestly, every time a scientist starts talking about the immune system to me, I end up feeling more confused at the end of the conversation than at the beginning. So I think it's brilliant that we have one of the world's leading experts on the immune system here to hopefully clear this up for me. And I imagine from many of our listeners too. So can we just start at the beginning really, which is like, what is the immune system

and why is it so challenging to study? 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 could 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,

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 the 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 impairment. 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 allergy, 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 it 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 recognizing 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 have 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 at a farm may influence the onset of allergies. And the bottom line is we don't know enough about the details 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 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 listening to this in thinking that much of the world has sort of gone through an unintended experiment about this sort of hygiene hypothesis and so many of us live through COVID. There was sort of social distancing. We were definitely much more conscious many of us about wash our hands. We weren't with other people. I know from speaking to doctors that there was

like this really big fall of many other infectious diseases through this period. And then obviously we've all gone back into life. So I imagine that you're going to tell me like over the the next decade or something. There's this sort of fascinating, almost worldwide intervention to see whether or not this has had any impact on our immune systems. Yeah, so that's a very interesting point, and that's exactly true. The pandemic has opened up a lot of what our immune system is, does, how it changes. Obviously, everyone experienced different symptoms and outcomes from being infected with COVID-19, and that correlated very precisely with things happening in their immune system. So we did learn a lot about that, and the

learning is continuing. You talked about cancer, and I guess when I was thinking about immune health, it's like it's all fighting off viruses and infections. How does cancer fit into this? Many decades ago, people did think that cancer wasn't really anything to do with the immune system because, I mean, there are some rare occasions when cancer can be caused by virus, but most of the time cancer is just your own cells that are dividing in an out-of-control way, and so it's your own body's cells.

gone wrong. And so your immune system doesn't have anything as obvious to look at as, you know, the spike protein of the COVID virus that is definitely alien to the body. But we have dramatically changed our view of that. The immune system can definitely see cancer developing in many ways, in fact. Two big ways. For a cell to turn cancerous, mutations arise that make the cell start to divide, multiply it in an uncontrolled way. And then that leads to some changes in the protein molecules that that cell makes and your immune system can directly detect those changes in protein molecules. That's one way.

Another way is that when a cell becomes cancerous, very often that cell itself has an ability to know that it's dividing in an uncontrolled way. For example, its DNA might be damaged in some way or if it was exposed to UV light, for example. that cell itself can start to display molecules at its surface that tell the immune system, look, I'm damaged, kill me. So cancer can be seen by the immune system. It couldn't be more important because this does lead to all kinds of new medicines. One of the major new ways of us treating cancer in some patients is to use immune therapies that are called checkpoint inhibitors. We spoke earlier in the podcast of the importance of the immune system coming back down to its normal resting state after it's fought off a virus, for example.

That's a very important process. The immune system has to be able to bring your body back down to its normal state once you've cleared out a virus. But if that process happens while your immune system is trying to fight off something like cancer, that would inevitably be bad. So in a long-term situation where the immune system is fighting off a cancer developing, you don't want the immune system to put the brakes on and come back down to its normal resting state. But that does happen. And so checkpoint inhibitors are a type of therapy where you block the brakes on the immune system from coming on and you keep up the immune cells being able to fight off a particular type of cancer.

And for some patients, I mean, it doesn't work for all patients, doesn't work for all types of cancer, but for some patients who have been told they have a short time to live, live now live for a much, much, much, for a longer time on account of these types of therapies checkpoint inhibitors. So we go from basic science of what is the immune system, you just asked me, does it fight cancer? Yes, there was a revelation in understanding that immune system can fight cancer. And then not only is that important in its own right, but that kind of insight very directly leads to developing new medicines that allow the immune system fight off cancer. So sort of right now and all the time, one of the important jobs that my immune system is doing,

is effectively sort of scanning my body for any like new cancer and like hitting it before it becomes some sort of like bigger cancer that I might end up being aware of and a doctor being aware of? Absolutely true. So all the time your cells are becoming damaged through exposure, for example, to UV light, sunlight or other things are going on in the body. And one of the jobs of your immune system is to kill off cells where cancer might be developing and a lot of the time you would not be aware of that.

In fact, it's quite hard to know how often in your body is a cell kind of on the way to turning cancerous and your immune system has killed it off. Again, the experiments are actually really hard to do. How would you know that if you're not aware of it? So the museum is one of its jobs is to stop cancer developing in the body and how often that happens. How important is it's quite hard to know, but it's definitely happening in your body in mind all the time, I'm sure. So there's probably something right now there's like a cell in my body that might be going wrong and my immune system is zapping that. Absolutely killing it off. And this is the kind of research that my own lab does, actually. So we are watching these processes happen down a microscope to understand the details of what's happening. I invite you back probably to go deep into that. I actually want to ask another question, though, because you mentioned before, which I'm fascinated by, this idea that my immune system is completely different from my wife's immune system and your immune system. Why?

One reason why your immune is different to mine is simply because of the genes we have. You and I have the same set of genes, the sort of 22,000 genes that make up the human genome. But we all have slightly different versions of some of those genes. A small fraction of all the genome is a bit different between you and me. So that gives us our differences. And so people might think that the genes that are most different between people, the genes that vary the most between me, you and everyone listening to this, podcast would be genes that control things that are just obviously different, like our hair

colour, our eye colour, our skin colour. But actually, the genes that vary the most between every person listening to this podcast are genes in our immune system. So right off the bat, at that level of how our genes vary, your immune system is pretty much the most unique thing about you. This couldn't be more important. So if you got infected with a virus, And you said your wife got infected with the same virus and she recovered quicker than you. That might well be. One reason for that would be that she might have a different version of immune system genes. I feel like she would argue it's just because of like my moral failings and like just complaining too much and not getting on with life.

Jonathan, right. That flippant jokey comment couldn't be more important to me because one of the things that I personally take away from the fact that we all have different immune system. some genes, is that if you get a virus and I get a virus and you recover in two days and I'm flat out ill for a week, because we're bombarded with a lot of messaging about health, exercise, you know, do this stuff. I'll be thinking, oh no, Jonathan's probably going to the gym every day and he's not stressed and look, his whole life is wonderful and look at my life and I'm ill for a week. But that is not true. It could just be. that by chance you've inherited genes that are make you better at detecting that type of virus.

So we mustn't blame ourselves. There's another really important take-home message from that, which is that this is a fundamental story about human diversity. There's no inherent hierarchy to our immune system genes. No one has a better or worse set of genes, really. Our diversity in immune system genes is what makes all of us stronger. and there are very clear examples of this. So if you have a particular immune system gene

called HLAB 27, you are more likely to develop a particular type of autoimmune disease and closing spondylitis. It doesn't mean that if you inherit that gene, you're likely to get that autoimmune disease, but it means that people with that autoimmune disease are more likely to have that gene. On the face of it, then that means

that that particular immune system gene would be bad to inherit. But actually, if you're infected with HIV, there's a big difference amongst people of how long it takes to develop AIDS. And people with that particular immune system genetic inheritance are more likely to spend a longer, much longer time from infected with HIV to when they would develop AIDS if they weren't treated.

So it means that that particular immune system gene correlates with them being better at fighting off the virus HIV. So immune system gene, but on the one hand seems to have a negative consequence, it's more likely correlated with some autoimmune disease developing, is good for you in another sense that it correlates with you helping fighting off HIV.

So this is really important because it means that the worst things that ever happens to humanity have happened because of our misunderstanding of what our diversity is. It is fundamentally important to how we live and how we survive different kinds of diseases.

I'd love to switch on to talk about some of these myths that we understand about the immune system and you've already talked about the idea that boosting it is bad I'd love to talk about vitamin C because I wasn't feeling very well last week and my daughter's babysit immediately said, oh, you should go and take lots of vitamin C. And I was like, I'm pretty sure I've heard from people that maybe this doesn't really work anymore. What are the facts?

So vitamin C is actually a really fascinating story. I mean, vitamin C is important for health, no question, but the thing that we're talking about here is about her immune health. I've been brought up thinking if I have a cold, right, drink some orange juice, then you'll be fine. And that somehow made sense to me, but it's really just a prejudice. It's really just something I've been brought up with. And now I'm trained to be more skeptical about these things and think about what is the actual evidence, what are the experiments? I started to look into that more. And actually it turns out that the way we think about vitamin C has been hugely influenced by one individual, Linus Pauling, who won two Nobel prizes. He wrote a scientific paper

on the nature of chemical bond and the journal it was published in, the scientific magazine, if you like, that published it, said, normally we send this to be looked at by other scientists, but in this case, there's no one in the world that is good enough to judge this particular piece of work. We'll publish it anyway. It was that level of scientist. Later in life, he campaigned. against nuclear bombs and he won a second Nobel Prize this time for peace. So he was someone that we are listening to and he was

very easily seen on the TV and radio. In 1970 he published a book on vitamin C in the common cold and boom. New factories had to be built to keep up with demand. Everyone is listening to him we need higher doses of

vitamin C. His argument was that the level of vitamin C that we were told to have comes from a level that was known to stop you getting scurvy, which is something that happens if you're very deficient in vitamin C. And his argument was that, well, that's not the same as knowing how much is actually healthy for you to have, just that it avoids a particular illness. He cherry-picked the data that was out there, and basically, through anecdotes, came to conclusion that we need more vitamin C and it would help us fight off colds. Him and his

wife, Ava Helen, they both took about 10 times the amount of vitamin C that's recommended and felt it gave them a lot more energy fought off colds. He actually also said that high dose of vitamin C will stop you getting cancer, all sorts of other things. Now, the truth is vitamin C will not stop you catching a cold. That is unequivocally proven in clinical trials and tests. The people who have a regular supplementation of a high dose of vitamin C, on average, get over a cold about 8% quicker, which would mean that if you're ill for a few days, you might get better a few hours earlier

if you supplement with a high dose of vitamin C. Even that information alone is very difficult to act on because people that take a high dose of vitamin C are probably doing other things in their lives as well, which might make them get over a cold a bit quicker. So it's very hard to know, but by and large, vitamin C is not going to help you with a cold. So for me, the take-home message is not only that orange juice isn't going to help me with my cold,

it's also that we need to be very, very careful about any one person's opinion. Well, I think you've just demolished taking vitamin C in order to stop getting a cold. I'd love to come on to one other myth, and we got this a lot, but it made me actually think of my grandmother, who used to always say, now we're getting into the wind to wrap up warm, because otherwise you're going to catch a cold. Is that true? Wrapping up warm won't stop you catch a cold,

but it is true that there is a seasonal variation to the types of viruses that are multiplying well in a particular season, and it does make us more susceptible to the cold in a certain time of year. Help me to unpack that. So you're saying that I'm more likely to catch flu and a cold in the winter, but it's not because I am physically exposed to the cold? Yes. There's some limit to that,

because if you were so cold that your health was affected in a general way, then you would maybe succumb to the symptoms of the cold more. If you looked after yourself in a general way and kept warm, your body would be in a stronger position to be able to deal with an infection. Dan, I'd love to switch to links between immune health and mental health, because we've talked a lot about protection against diseases and against cancer, but I know in your book you also discussed that it's not just physical health that the immune health. system is somehow engaged with. Is that right? When your immune system is fighting off a virus infection and you might experience a fever, you know that a fever is affecting you mentally as well

as physically. Obviously, your temperature is raised. I mean, I feel rubbish whenever I'm sick. I complain all the time. Everyone at Zoe knows that. Everyone at home is like, I'm like a terrible invalid. I'm incredibly frustrated because I want to be doing all this stuff, but I just feel tired and I feel like my brain is like somehow, like everything is like cotton wool. In hindsight, it feels obvious that our mental health is somehow connected to our immune health. Perhaps some of that has evolved deliberately. Like if you have an infection, then you feeling weaker, tired, staying in bed is good for you in terms of helping your healing process, but also it's good for humanity because you're not

wandering around and affecting everyone else. So some of that process is very likely to have evolved in a very deliberate way. Now, there are some very interesting experiments looking at whether more generally than that, our immune health is connected to our mental health. It's very hard to assess anybody's immune health in any way because it's so complicated. So, Dan, there's not just like a simple blood test that just gives me a scale on my immune health in the way that I might get a sort of cholesterol check? I think the answer has to be no for the moment. But one of the ways we can get some sense of what's happening inside a person is to look at the particular types of

molecules that immune cells use to communicate with each other. So during an immune response, as we've mentioned, there are also different kinds of immune cells in the body. And they have to communicate with each other. And one of the ways they do that is that an individual cell will produce a protein molecule that comes out and then goes and touches another immune cell. And those kinds of molecules are called cytokines. And there are over a hundred different types of cytokines. and they do all kinds of things in the body. In fact, it's a bit unfortunate to me that cytokines aren't more famous. They deserve more public recognition.

They're easily as important as antibodies, which people have probably heard of. I would predict that over the next decade, cytokines are something that everyone listening to the Zoe podcast is going to have heard more about. Cytokines, over 100 of them, they're how immune cells communicate with each other. Now, cytokine levels flux up and down in the body, in your blood, anyway during the day. Different ones are rising in the morning, in the evening, etc. But you can measure cytokine levels to give some proxy of how much the immune system is in a state of

activation in a person. Studies have shown that if you measure the levels of one particular cytokine that happens to be called IL6 in children age nine, those who have a higher than average level of cytokine are more likely to develop depression when they're age 18, which isn't entirely proof of anything in itself, but is in line with the idea that a higher activation of the immune system at some age correlated with the later development of some mental condition later in life. It's quite powerful, although it's only a correlation. And so again, with any correlation, there could be any number of other things. It could be that the higher cytokine level is an outcome of something else that then linked to mental health later. And it's certainly not true that all

cases of depression are linked to higher levels of cytokines. But there is something in that. Another version of that type of experiment that's more reductionist requires scientists to use animals. And of course, you know, the mental health of a mouse is an impossible thing to fathom. But it turns out that if you inject a mouse with a cytokine like IL-6 that mimics some sense of inflammation or some level of an immune response happening, that mouse is more likely to then not explore their cage going to a darker part, not be in the light, not interact with other mice, which is somehow reminiscent of behaviours that could relate to depression.

So again, it's another level of evidence that an immune molecule used by immune to communicate each other can elicit behaviors that look a bit like what might happen in a mental health condition. It's really interesting. I know that, you know, from myself as an individual, that my mental health is definitely affected when I'm unwell. Like, I very rapidly go from being sick to then, like, feeling quite low about it,

feeling like we'd be sick for ages, you know. Again, as a sort of suggestion, I might be a bit of a hypochondri. Maybe that's part of why I co-founded Zoe. So it's really interesting that you're saying there is some evidence that this sort of immune system been on for too long or too much could have some effect in a way that's measurable about how you feel in this sort of more mental health way?

Another very important line of evidence comes from when people are taking a particular medicine that blocks the action of a cytokine. So stopping immune cells communicate with each other. And one of those is called anti-TNF, anti-TNF, TNF being a particular cytokine that means that immune cells communicate with each other, and that's often taken by people with rheumatoid arthritis, for example, because it dampens the inflammation that they have. And there is evidence that people on an anti-TNF medicine very quickly feel in some way rejuvenated even before that type of medicine has had a clear physical impact on their actual autoimmune problem that they were taking that kind of medicine for. So there's several lines of evidence that our immune health and mental

health are connected. There are some correlations between cytokine levels and mental health developing. There are examples where people are taking medicines that are in the immune system, but it seems to also affect their mental health. And there are experiments in animals where eliciting immune reactions seems to affect the behaviours of mice that would be normally thought of as a mental condition. But like a lot of the things that we're discussing today, it's fascinating. It's extremely important. It might make some people think about their mental health in a different way. Every time you get a new window into how the body works, it might make you think about where some mental health condition might be originating from. But what's really important

is firstly, this wouldn't apply to all mental health conditions. But paramount is that this cannot be taken yet to indicate what you do about it. There are hints of something really important happening between our immune system and our mental health. But it isn't yet at the point where there's an actionable thing for you to do. And Dan, a lot of scientists that I've interviewed on this podcast over the last few years are looking at interventions with food or the microbiome and talking about impacts on inflammation as one of the things that they are measured. and believe is involved, we've been mainly talking about immune system, but I've heard you use the word inflammation once or twice. Is inflammation linked to what you're describing? Okay, yes, that's a good

point. So we should just unpack what these terms mean. So inflammation is essentially, yes, a sort of level of activation of immune system. So the most acute example of inflammation would be if you get a cut and the cells are swarming into that place where you've experienced a cut and it goes red, it gets bigger, it gets slightly hot. And that's, so your immune system is there to fight off any opportunistic germs that might enter the body through that cut. That's a kind of localized inflammation. But equally, rheumato arthritis where you have a sore joint is also due to inflammation in the sense that the immune system is active and it's started to attack parts of the body or become overactive in a joint where it shouldn't be and it's causing you a

problem, that is also inflammation. In a sense, it's any state of the immune system that is active is essentially what inflammation is. But that can also be on a whole body level. You can have a sort of higher level of background inflammation. And that tends to be the idea that many of your immune cells are sort of slightly being tickled in some way. So they're producing, for example, more cytokines that would be inflammatory, causing immune cells to be slightly active and that would be a sort of background level of whole body inflammation, which can happen, for example, as we age. And that's one reason why it's thought that older people are more susceptible to autoimmune disease is because they have a higher level of this background inflammation in their body. Fascinating. So if we can sort of, I think what you're saying is great to have this turned on when I'm sick and I want to deal with it.

But if this is sort of higher than sort of normal for long periods of time and you can potentially measure that through. these cytokines you're talking about, that may be having some of the negative impacts. And here you're saying one of the potential impacts that it might have is actually on your mental health over time. Yes, that is all possible, but it's also quite speculative. Every part of what you said is the best sort of scientific consensus opinion of how we're thinking about things, but a lot of it isn't really proven. Yes, there's this idea that as we age, there's more of a background inflammation going on in the body, and that might link to one of the reasons why problems might arise and could also affect our mental health.

But equally, we don't really know if that's actually totally true. It's like, it's like that's where our thinking is. We haven't really proven that, yes, now we can measure five cytok in your blood and tell you you are more likely to get depression. We need to counteract that immediately. We're not at that level. We're on a path to understanding. And so the answer is, yes, you and I are talking about things that are sort of known, but not entirely clearly established. And science is always on a trajectory. Hopefully we saw that during the pandemic. Do you remember early on we would be talking about what we need to do to counteract the COVID virus?

Should we be wearing masks? If an envelope comes in your door, should you not be touching it for a week? And the science evolved and told you that as we went along, we did start to learn more about it. And that is true of all of these big issues. If you want to help your friends and family support their immune system and stay healthy this winter, why not share this episode with them now? I'm sure they'll thank you. One thing that I know my colleagues, Professor Tim Spectre and Sarah Berry have talked about quite a lot as we've been doing these really big nutrition and microbiome studies here at Zoe is how little scientists have tended to measure anything about how people feel. And so sleep, energy, mood, all of these sorts of things that historically, they've said, well, if you can't measure it with like something in your blood or something like that, then it's almost sort of being discounted.

So it's sort of somehow not sufficiently scientific. And so one of the things we've done with a lot of these studies at very large scales, right, hundreds of thousands of people is look at what happens to things like energy and mood. And I think in a number of these interventions seen amazing shifts. And what they've said is, like, just basically this hasn't tended to be measured before. And so I'm just listening to what you're describing about not knowing the impact on, like, your mental health. And you talk about depression. But obviously, for many of us, it's more just sort of how do I feel each day. And part of what I understood is it just hasn't really been measured very much.

So, of course, you can't know what happens until you start to measure it. My own personal view to add to what you just said is it's not. so much that we haven't tried or thought about measuring things, it's actually that it's really, really hard. Let's take sleep. Immune health, as we've already said, it's actually quite a hard thing to do any measurement about. When you look at cytical levels,

you could look at antibodies produced in response to a vaccine, but there's not an easy way of saying, how was anyone's immune health doing? There's an easier way of looking at how well people do in a cognitive test. So a very obvious thing is you just look at how much people sleeping for and then see how they perform in some kind of cognitive test afterwards. And the bottom line from that kind of experiment is that sleeping seven hours is best. If you sleep less than that, you perform less well on average in a cognitive test. If you sleep more than that,

which is a bit more surprising, you also do less one in a cognitive test. Okay, great. So now we know seven hours is the amount of sleep. But let's take another view of that. Seven hours might be true on average. I have no idea what's best for you or me individual. Some of us are going to sleep later. Some of us are waking up earlier. Some of us are napping in the daytime. Some of us are sleeping with lights on, lights off. Some of us are sleeping in a noisy environment, a quiet environment. So you start off with something very simple. But once you think, hold on a minute, what was the experiment? And now you start to think, okay, I've already told you things are very different individually for all these things. There's a nuance in seven hours sleep isn't enough information. Would it matter if I did three hours sleep and then four hours sleep, later, what about the different phases of sleep, the depth of sleep, the environment I'm sleeping in, are other people in the room? All kinds of nuances relate to sleep. So I think it's not that these things haven't been measured. It's that it's really, really hard. So in that example of sleep, we are also on a trajectory of improving these measurements. I'm sure that

many commercial companies are thinking about ways to monitor your sleep in a more complicated way, a wearable device, something in your mattress. More and more and more, we will get more and more complicated measurements that do help us navigate things. That then leads us to a whole other issue. As all of these things become more and more possible for you to understand your own body individually with more and more measurements, more metrics, sleep, the levels of a thousand billion things in your blood, how much of that do you really want to know? How exactly is that going to help you live happier? These are very difficult issues to navigate.

Already now you can have a genetic test that might say you've got a one in 10 charts of developing a certain type of cancer in the next 10 years. And what if that was a one in five charts? What if that was in five years? These are very, very hard things to think about personally. So we're entering a world where, first of all, these measurements are hard. interpreting what they mean for you individually is extremely hard as well what does it mean for you personally when you get this information

all of these things are very very difficult to deal with so dan i think you've drawn a picture for me of just how important like my immune system is my immune health how much is affecting everything from fighting cancer to like my mental health potentially i'm pretty confident i speak for myself but also from most of our listeners to say, okay, I'd like to talk about some practical advice, therefore, about what I could do to set myself up as well as possible. I'm going into the winter. You know, I know that I've got

more risk of getting cold and flu and all of these sorts of things. Like what actually might I be able to do to set myself up as well as possible? 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 focus 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 all 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 the immune system correlates something else. So that gives us a process. 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 practically, that practically, 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. We had a lot of questions about exercise, and I guess at both ends. Like, does regular exercise help my immune system, but also, what about, for example, when you're not feeling very well, or what about if you're doing something really intense, actually, is that bad?

And I feel, it's funny, so often as pop-house, I sort of come back to, like, what my grandmother used to say, and how often I feel like it's hard to say that something sort of sensible. Just like, oh, doing something regular is good, but, like, if you really push yourself, they might actually be bad. Yeah, so roughly speaking, there's a lot of truth. Obviously, there's always going to be a lot of truth in whatever your grandmother said to you, apart from maybe vitamin C.

The best benefit for exercise always comes to people that are not doing much exercise and they start doing some exercise, they get a huge benefit from that. A benefit to their immune system? Yes, there is a benefit to your immune system, and that comes from lots of different things happening in the body. Muscle, for example, produces the cytokines we spoke about in terms of immune cells, communicate with each other. 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? I love it. Dan, we've covered so many things on this show. So I always try and do a wrap up, but I feel more like it's a set of fascinating things I've learned. So I discover that my immune system is fighting cancer inside me right now while I'm doing this show. The vitamin C is not going to stop me catching a cold. That the story I've been told about long-term stress affecting my immune system and my risk of getting infections is actually true, but it's months, not ours. So it's

okay to do something stressful like a podcast, but like constant anxiety and worry really can affect my health, that we all have a completely different immune system. So it's not just that I'm feeble when I get sick and the rest of my family don't. It could just be it's that particular virus, that there's some real evidence about this hygiene hypothesis that like living in like a completely hygienic environment when we're little might not actually be good for us. Exercise does help my immune system. And you said, sort of wherever you are, like, particularly if you're not doing very much, like stepping up to more can really not only help with everything else that I've heard on this show, but actually can literally improve my immune system.

There seems like to be good reason to believe that my mental health can be linked to my immune system. And finally, I heard it here for the first time, cytokine. I don't know what they are. Most people listening to the show don't know what they are, but in the next few years, they're going to be famous. You said there's about a hundred of them. It's somehow how the immune system is communicating with each other. And it's great when it goes up when we're sick, but actually generally we would like this to be low because we don't want our immune system boosted all the time.

We want it actually to be like sort of low and sort of not doing too much most of the time and then able to sort of shoot in and fight something when required. Okay, that's a good summary. Everything we said was as truth as we can make it. But when you go to university to study immunology, let's say, the textbook is a thousand pages. And even that is just the beginning. I'll end this episode with something I think you'll like,

a free Zoe gut health guide. If you're a regular listener, you know just how important. it is to take care of your gut. Your gut microbiome is the gateway to better health, better sleep, energy and mood. The list just goes on. But many of us aren't sure how to best support our gut. I wasn't sure before doing Zoe, which is why we've developed an easy-to-follow gut health guide. It's completely free and offers five simple steps to improve your gut health. You'll get tips from Professor Tim Specter, Zoe's scientific co-founder and one of the world's

most cited scientists, plus recipes and shopping lists straight to your inbox. We'll also send you ongoing gut health and nutrition insights, including how Zoe can help. To get your free Zoe gut health guide, head on over to zoey.com slash gut guide. Thanks for tuning in and see you next time.