How to not get sick this winter: 3 foods to fight colds, beat flu season, and protect your immune system

Why do you catch every cold going, while others rarely get sick?

Many of the cold and flu tips you grew up with are myths, but in today’s episode, world-leading immunity expert Prof. Daniel Davis and Prof. Tim Spector explain the five everyday habits that can protect your immune system.

You’ll learn why some people get sick more often than others and test common beliefs about how to boost immunity and beat flu season: Does getting cold and wet really give you a cold, and can vitamin C or immune-boosting supplements actually help?

We explore the five habits that science links to immune health and share three food tips to help you stay well this winter.

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Actionable takeaways

Does stress affect immune health?

Yes. Short bursts of stress are a normal part of life, but chronic stress can affect how well your immune cells work. Try a walk in nature, mindfulness, tai chi, or another activity that helps you feel calmer.

How can you get better sleep?

Sleep supports your immune response. If you’re struggling to get enough, try giving yourself more time in bed, making your bedroom dark and quiet, and switching off devices before bedtime. Find a routine that works for you instead of worrying about getting it perfect.

What should you eat for a healthy immune system?

Eat a variety of fiber-rich plants, such as vegetables, fruit, beans, whole grains, nuts, and seeds. Fiber feeds gut microbes that interact with your immune system. Include fermented foods like yogurt, kefir, kimchi, or sauerkraut, and limit ultra-processed foods.

How much exercise do you need to support your immune health?

There’s no specific amount of exercise that works for everyone. If you’re not very active, starting with some regular movement, such as walking, is a useful step. It’s never too late to start, whatever your age. Build up gradually from where you are now.

Are immune-support supplements worth buying?

Usually not. Be skeptical of supplements marketed as immune boosters, as a label claiming to support immunity doesn’t necessarily mean a product will stop you getting sick. Supplements can be useful if you have a deficiency or have been advised to take one.

Watch the episode here:

Jonathan: Daniel, should we aim to boost our immune system to make it as reactive as possible?

Daniel: No.

Jonathan: Can simple lifestyle changes improve my immune health?

Daniel: Up to a point.

Jonathan: Tim, if two people get the same virus, will they have the same symptoms?

Tim: No

Jonathan: Do identical twins have the same response to the same germ?

Tim: No, not usually.

Jonathan: Does anger impact our immune system?

Daniel: Up to a point.

Jonathan: Tim, are gut bacteria important for our immune health?

Tim: Absolutely. They're crucial.

Jonathan: Daniel, can vitamin C prevent a cold?

Daniel: No.

Jonathan: If I get a fever or a runny nose, is that a sign that my immune system has failed me?

Daniel: No.

Jonathan: Thank you for all of those. I think there's some, like, fascinating and surprising answers. I'm still shocked that you're telling me that my nan was wrong, and if I get cold and wet outside, I won't go and get a cold, because that is just like the recurrent story of my childhood. Like, wrap up properly, you know, put on your jacket. If you get cold and wet, you'll get sick. And you're telling me that actually doesn't follow.

Daniel: That's not really true. I was brought up thinking the same thing as you just said, and I was brought up thinking orange juice. You know, if I have a cold, I should have a high dose of vitamin C, it would help me get over it. But these things are myths. You do need to be very careful about what is actually a prejudice you've brought up with and what the truth is, what the scientific evidence is.

Jonathan: It's amazing. Well, I think it's a great sign of why I'm really excited about this podcast, because clearly the topic is something that we're all, you know, generally familiar with, right? We know we have an immune system that protects us from germs, and it's really important. But I think most of us don't understand it very well, and I think it's really interesting that most of us have these assumptions often like from our parents and our grandparents, and you're saying, well, a lot of them are wrong. And I'm particularly interested, you know, as we're going into sort of flu season, to understand this question around, you know, why is it that some people get sick and others seem completely impervious? And for me personally, that's a very familiar idea because my whole childhood, my dad basically going around saying his immune system is great, he would never get sick. Whereas my mom was labeled as having a terrible immune system 'cause she was always getting sick. So I really love this idea of understanding a bit more why do we respond differently? And then practically, what can we all do to bolster our immune health to try and make sure that we don't get sick? We're constantly surrounded by microbes that want to infect us, so how come we aren't all sick all of the time?

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Daniel: Well, that is down to your immune system that helps your body fight off all kinds of different germs. And at one level, your immune system is a collection of different kinds of cells flowing through your blood or your tissues and organs, and that those cells have the job of fighting off germs like viruses, bacteria, or fungi. At another level, every single cell in your body is pretty much part of your immune system because Every cell in your body pretty much has some ability to know when it's been damaged or infected by a germ, and it can send out a signal to the specialist cells of your immune system to try and deal with that problem. So a lot of your body is geared up to protecting you from these kinds of infectious agents, different kinds of germs. It's not an easy job because there are all different kinds of germs that cause all different issues inside the human body, and over millennia, germs have evolved their own sneaky ways to avoid exactly this being got at by your immune system. So it's a very complicated situation of your body trying to fight off all these different kinds of germs.

Jonathan: And so does that mean that in a perfect world, we want a really reactive immune system that immediately jumps on all of this? 'Cause, you know, it sounds like we're in a dangerous neighborhood and, you know, there's somebody behind every corner waiting to jump out and get us.

Daniel: Your immune system has to be in some state of balance. Your immune system has to not attack any part of your normal self. The normal cells of your body, the normal tissues and organs, your healthy state of body has to be protected from this ability of the immune system to literally kill off things. So that means boosting your immune system in any which way is not actually gonna help you. If your immune system is overly reactive and starts fighting off things it shouldn't, your own body tissues, that would lead to an autoimmune disease or an allergy or some kind of problem where your immune system itself is causing you that problem. So you have to have your immune system in balance, not just boosted.

Jonathan: And I'm really interested by this answer you gave to one of the quick-fires at the beginning where I said, you know, "If I've got a fever or a runny nose, doesn't that mean that my immune system hasn't done its job?" Which is what I was assuming, but you just said to me, "No." No. Could you help me to understand?

Daniel: It's absolutely part of your body's response to trying to deal with a threat. So certain types of viruses or infections will cause your body to go into this fever state. The way in which that happens is immune cells sense the presence of the virus. They produce molecules that act on the brain, that then increase essentially your body's temperature in lots of different ways, and that is important for many reasons. Firstly, most kinds of viruses find it quite hard to multiply in number at a higher body temperature, and many of your immune cells work more effectively at a slightly increased body temperature. But it's also very true that we do have different responses to different kinds of infections, so there is an enormous diversity, and that's fundamentally really important to how people are so different in the way in which they handle different kinds of illnesses.

Tim: There's also differences between males and females, because females are designed to have babies, and therefore they have to have a different immune system so that it doesn't reject,, the pregnancy, doesn't treat it as a foreign agent. And that's why lots of studies do show there are differences between males and females, and some studies showing that males actually suffer worse symptoms from colds and flu,, because they have a different immune system. So it's the opposite of perhaps what your parents are showing. And that's why we get much more autoimmune disease in females than males.

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Jonathan: Well, I think this has brought us actually onto the heart of what we wanted to cover. Because in your previous episode with us, Daniel, I remember being amazed at sort of how our immune system manages to keep us safe most of the time. And today I think we really wanted to understand some more about why it is that some people get sick more often than others and what they can do about it, and have the two of you help us to understand that, what's real and what's not, and then talk about, well, what are the things that you could do to try and make sure you don't get flu this winter? Could we maybe therefore start with genes? Because, like, when I was growing up, everything was about our genes, and that was going to explain, you know, all about it. So again, I think my dad probably would have said something like, "Oh, well, you know, your mom hadn't have as good genes as I do for immune health." Is it all about our genes, and is there a sort of ideal set of genes that we could have, but only some of us are lucky enough to have them?

Daniel: So if we think about our genes, we know that all of us roughly have the same set of genes, but there's small differences that we've each inherited. And if you ask people what genes vary the most between each person, they probably think of things that affect us physically, like our hair color, eye color, our skin color. But by far, the biggest difference in our genes are in our immune system genes. That affects us quite profoundly in how each person responds to any particular type of infection or other illnesses, even how they respond to different kinds of medicines. And we do understand quite a lot about that. When you ask me, but is there like an ideal genetic inheritance, there is no optimal, no perfect, no best genetic inheritance in your immune system. In fact, it's incredibly diverse, and that just means that each one of us is slightly better and slightly worse at almost every kind of illness. But as a population, that diversity gives us an enormous strength as humanity dealing with any kind of illness or infectious germ that might arise.

Jonathan: So- Can you help me to understand that?

Daniel: So the truth is that we've all inherited these different genes. Genes make protein molecules in cells. That's how they do their stuff. Now, this is how the immune system works in a textbook level immunology way, right? This is what happens. So the cell is continuously making lots of protein molecules inside it, and those protein molecules are being chopped up into small pieces and then put up for show at the surface of your cells. So all the cells in your body pretty much are displaying up at their surface a sample of what is being made inside that cell. And then your immune system can look at those samples to see if there looks like there's something that shouldn't be there, like something being made by a virus. But those samples of things being made inside your cells are presented up at the surface of a cell with a sort of cup-like molecule from your own body And that molecule that's presenting a sample of what is being made inside your cell is from the genes that are the most diverse genes in all humanity. So that means that every one of us is slightly better or slightly worse at presenting up at the surface samples being made from different kinds of viruses, because the genes that make those protein molecules that show samples are enormously different between each of us. The practical take-home is that if you and I got infected with the same cold virus, you might get better in two days, I might get better in a week. And one of the reasons for that might very well be that we've got different versions of the genes that are making the protein molecules that show up at the surface of cells samples made by that virus But if you and I got infected with a different cold virus, I might get better in two days, you might get better in a week because in that situation, my genes are a bit better at presenting the samples being made by that kind of virus. So that is why we are enormously diverse in our genes, and in a molecular level, we have an incredibly deep, powerful understanding of why we're all different in every kind of illness.

Tim: And there's some evidence, isn't there, that some people are better at fighting viruses are worse at fighting fungi or better at fighting bacteria. So we've all got some superpowers, but just we may not be using them much because we're not overwhelmed with fungal infections at the moment, but we might have been in our ancestors' time.

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Jonathan: Why is the diversity across? Why wouldn't we all just have the same immune system?

Daniel: It is to make us all strong as a population. It's also true that there are really well-worked-out examples where a genetic inheritance in one situation is extremely good. For example, the type of gene that you might have inherited to help you fight off HIV is the exact same type of gene that you might have inherited that makes you more susceptible to a particular autoimmune disease, ankylosing spondylitis. So in one sense, it's a great inheritance to have, and in another sense, it's a bad inheritance to have. There's a diversity, and it makes us strong as a population against all other kinds of illnesses. But there's even more magic to it, which is quite subtle and a bit hard to get your head around, which is, in fact, it doesn't just work at the level of all the population because every single individual person has actually inherited multiple copies of those kinds of genes. So we all actually in us as individuals have a diversity in those kind of genes that makes each of us as individuals stronger. It's not just about the population, actually.

Jonathan: And Tim, I know that you've studied genes, particularly in twins, that you've been studying for thirty years and the immune system. What would you add to what Daniel's just been discussing?

Tim: We looked at rates of colds, for example, in identical twins, and there is a, there is a small genetic component. It's not a hundred percent, interestingly. So genes aren't the only thing that are important, and we've got things like epigenetic factors whereby genes can be changed by the environment, et cetera. And that also makes you think that things that we're going to talk about, like gut health, could also be important,, as extra variables in that altering the immune system that isn't dependent on our genes. We also just published a paper in Science, a study we started fifteen years ago, longitudinally looking at twins, and this is three hundred and fifty twins, where for nearly a decade we were taking regular blood samples, looking at the changes in the genes And it turns out, of our 20,000 genes, about 5,000 are regularly changing,, with age. It's not everyone's changing the same way. We're changing in lots of different ways. And so some people's genes are,, improving with age, if you like, to fight disease, and other people's are getting worse. And this probably also explains why,, some people might find that they suddenly are no longer having colds as they're getting older, and others are actually getting them when they didn't used to, because a certain lot of these immune genes are actually also changing with time. And I think this is really interesting. But it's not just the genes. There are these other factors. There's the environment, there's the epigenetic changes, where you can switch genes on and off, and there's also, of course, the gut microbiome.

Jonathan: Amazing. So Daniel, as we age, like what is going on inside our immune system, and would we expect that to make us more susceptible or, like, less susceptible to germs?

Daniel: Yeah. So I suppose there's a broad brush analysis of what happens as you age, is that there is a background level of inflammation that exists in the body. So just to get it down on a kind of molecular level, immune cells send signals to each other to coordinate their activities and their response to germs through particular proteins called cytokines. And what happens as we age is that these levels of cytokines tend to be a bit elevated in blood. So there's a background level of inflammation, which really, actually, what that really means is your immune system is kind of slightly alert, slightly aroused, because these molecules that immune cells use to communicate with each other are slightly elevated in blood. That means that your immune system, roughly speaking, is slightly less good at switching on sharply when it should, and slightly less good at coming back down to its normal resting state when a virus has been cleared from the body. So as we age, there tends to be this general situation of a background level of inflammation, which makes it slightly harder for the immune system to switch on and slightly harder for the immune system to come back down to its resting level. Also, that might affect the chance of developing autoimmunity, for example. So some autoimmune diseases do become more frequent as we age. And again, that might be linked to this background level of inflammation building up.

Tim: There's virtually no disease that doesn't involve inflammation at some level. So what you're saying is also true for cancer-

Daniel: Mm...

Tim: mental health problems, dementia, everything. It's mind-blowing really when you start thinking about how important the immune system now is and how we know it's integrated with everything else

Jonathan: So we're gonna talk about sort of lifestyle factors and how they influence our immune system and what we can do about it, I think for much of the second half of the podcast. But other than aging, what other things might make some people more susceptible to germs than others?

Daniel: Well, the whole history of previous infections you've been exposed to will affect the state of your immune system as well as the genes you've inherited. You know, your exposure to germs is infected by who you're with, where you've been brought up, how your immune system's been trained when you're younger affects your immune system very deeply. , the microbiome affects your immune system profoundly.

Jonathan: What about just your overall state of health? Is that gonna have a big impact on your immune system, or actually is that sort of one of the last things that gets affected and it's not that relevant really to-

Daniel: Well, it's absolutely true that the immune system is interconnected with all other aspects of your health. I mean, one of the things that Tim briefly mentioned was how your immune health and your mental health are connected. We know that when you have a fever you do feel different as well. You might be mentally more foggy and you just don't feel yourself. Even just from that, you already know that these molecules that immune cells produce directly act on the brain, and that does affect our mental health. I think my takeaway from that would be that it might affect how you think about your mental health situation, but it's also true that we don't really know enough about it yet to give direct medical advice.

Jonathan: So it sounds really clear that our genes and aging are both really important for our immune health. But there's nothing I can do about it, right? Like, my genes are what they are. My current sta-- you know, how old I am is how old I am. So I'd love to move on to things that, you know, we have some control over, and I'd like to start with sort of immune wives' tales. So the stories that we were all told growing up about what we can do to try and make sure that we don't get sick, and also, like, what the risks are. I mean, can I start with the first obvious one, Daniel, which is it really true that we're more susceptible to colds and flu during the winter?

Daniel: Yes, that is, that is definitely true.

Jonathan: Why is that?

Daniel: So there are many things going on,, during, you know, different seasons. And essentially during winter there are several things happening. We are less exposed to sunlight, which affects our levels of vitamin D,, that we may have in the body. We may stay indoors more. We may mix with groups of people in an indoor environment where the windows aren't open, the-- so there's less ventilation, so we might be more exposed to,, other people's infections. Plus the viruses themselves,, that cause colds are able to live longer or survive longer outside, the body or in droplets when it's cold. So there's kind of lots of reasons why there are these seasonal effects of different kinds of illnesses

Tim: I did read somewhere that the cold and dry conditions might actually affect the nose- Yeah so that it's more likely that viruses can land there and multiply rather when it's hot and humid. Is that right?

Daniel: Absolutely true. So your mucus is also a key part of your defense against,, many of these kinds of viruses. Mucus is actually another miraculous, magical thing in the body that contains an enormous concoction of different kinds of molecules, including things that directly kill viruses, and that also, the levels of mucus you have also vary.

Jonathan: So my snot is magical.

Daniel: It is. I don't know about yours.

Jonathan: Yeah,

Daniel: yeah. Some people's.

Tim: A key part of the immune system. Yeah, that's right. It's,, really vital, and people who don't produce saliva or mucus have real problems with infections. When the COVID started, one of the biggest outbreaks were in,, European ski resorts, where, you know, they had infection rates that were absolutely huge, sort of 70% of people going there were getting infection. And you think there, the conditions, you've got lots of people together, you've got cold weather, it's super dry, there's no humidity there, and this is just the perfect conditions for these viruses to,,, you know, to get into our respiratory system.

Jonathan: That's fascinating. So I want to come back again to my nan telling me that getting cold and wet would cause a cold. Is that true?

Daniel: Not really because your body temperature is basically sustaining itself when you're-- if you have wet hair, you're not affecting your actual body temperature.

Tim: And they did do a great study, didn't they, on conscientious objectors,, in I think it was the Second World War, where they exposed them to freezing conditions, cold showers, not allowing to dry off,, to see if they all got,, colds. And they did. They were very miserable, but they didn't get actually more colds.

Jonathan: I'm sitting here. You guys are both like, "This is really obvious." I'm stunned. It seems to me everybody knows you're not allowed to leave the house on a cold day with wet hair. It's like a rule. I'm sure that we have said this to our children. But it's just nonsense.

Daniel: Pretty much nonsense.

Jonathan: What about if you're on the edge? So if you're not feeling very well, 'cause this often happens, right? Some-- you're not feeling very well, and you feel like you have to go to work, or you have to go to school, and it feels as though the act of just commuting to work is just so tiring that you get worn out. Does that increase your chances? And so does also going out in the cold weather when you're on the edge, like, does that really make a difference, or is this again just nonsense?

Daniel: I would say there's some balance to that. I mean, your body healing, your body's immune system fighting off any kind of germ does take up quite a lot of energy in your body. You know, you wouldn't want to, with a fever, run a marathon, right? You wouldn't want to push yourself. You do need to give your body time to heal, I think.

Jonathan: Let me move on to a few other potential immune wives' tales. Are there any vitamin supplements that are worth taking to stop you getting sick?

Tim: This is something I keep looking at, and the data changes. At the moment, there is some weak evidence that if you have high vitamin D levels, you slightly reduce your risk of getting respiratory infections by about ten percent or so, which isn't a large amount, and so some of that could be experimental error. Everything else It slightly depends on whether you're really deficient at the beginning. There are lots of conditions that if you completely stop some nutrients, if you have no zinc or you have no copper, or these sort of really super unusual, ultra-rare things, you can get infections. There's very little data to show if you've got normal levels, any supplements really help you prevent an illness.

Jonathan: And what about vitamin C? I feel not only was I told that as a child, but I feel that people continually say to me when I'm not very well, "Oh, take this vitamin C right away."

Daniel: So I think, I, I completely agree with Tim that most people are gonna be okay with the number of vitamins they have. Vitamin D, as Tim said, has a really small effect. The actual data is that 33 people taking a supplement of vitamin D continuously for a year will get one less infection overall. So it's a really small effect, but it's a bit stronger the evidence for vitamin D protecting against autoimmune diseases. And the NHS guideline is actually that during the winter months, people could supplement themselves with vitamin D. So I think at some level, those guidelines from the UK government through the NHS is essentially a consensus of where scientists and medical professionals are at. Vitamin C, I mean, vitamin C is important for health in lots of ways. Absolutely true. But vitamin C in protecting against colds is absolutely a myth. . So having a high dose of vitamin C will not stop you catching a cold. If you are a person who supplements yourself with a high dose of vitamin C, then you get over a cold on average 8% quicker. So if you're ill for three days, that's a few hours quicker, but people who supplement themselves with high dose vitamin C are doing lots of other things for their health. So it's quite hard to know from that correlation whether it's really about the vitamin C. And we have that myth because someone who won two Nobel Prizes published a book in 1970 called Vitamin C and the Common Cold, and that made everyone around the world think we need to have a high dose of vitamin C to stop getting a cold. So be careful of any one person's view.

Jonathan: So vitamin C is out. What about zinc? I feel I hear that all the time as well. Do I need to take more zinc?

Daniel: I don't think there's any clear evidence,, around zinc. When you hear something that this particular thing is going to boost your immune system, protect you from infections, I mean, it's not necessarily easy to do, but step back and try and think, "Well, how do they know that? What actually is the evidence?" And actually, some of these very simple sounding questions, does zinc help you get over infections? How are you gonna study that? What is-- what experiment actually could you do? It's actually really hard to answer some of those questions.

Jonathan: And Tim, I feel like I've seen a lot of supplements that claim to boost the immune system. I mean, what's your view on that?

Tim: My view is that this is down to labeling rules in Europe and the US that are so lax that they allow,, you to say things about the immune system based on no data whatsoever. So in Europe, we can say that vitamin C, vitamin D, the B vitamins, manganese, zinc, copper,, selenium all have some magical immune power, and you can stick that on the label if you've just got a certain amount of it. In the US, you don't need any amount of it, just a sprinkle. And it's nonsense, because this is all based from studies 30 years ago from some African tribes that were low in selenium or zinc or manganese that had abnormal immune systems. So it's been twisted round to say that, yes, we know they're important for an immune system. We couldn't exist without these nutrients, but doesn't mean that if by taking it in a cereal packet or something or a yogurt, we're gonna suddenly boost our immune system. So all those claims are actually false. The wording's clever, and so all these things when people say, ", good for your immune system." So people, when they have a cold or they, you know, it's cold/flu season, they'll buy all these products thinking they're doing themselves some good, and it's just complete rubbish, and we should be changing the legislation to let the science actually catch up.

Daniel: I couldn't agree more. If there's some rallying cause that comes out of this podcast, it should be that, to change the legislation, actually.

Tim: But Daniel, what really- Yeah... gets me upset is the fact that you've got things like prebiotics, probiotics, things that help your gut microbes, which you know is integral to the immune system, which we're not allowed to say on any label is, could potentially even have a slightest interest compared to something like zinc or selenium, which makes, you know, makes a mockery of the latest science, just doesn't catch up.

Jonathan: I'd like to move on to another old wives' tale. I was always told feed a cold, starve a fever Is there any truth in this?

Daniel: It's quite an interesting question because the-- it's much, even much broader than that. So the broadest version of that question is, when I feel like I want this or that food, is it because my body is somehow influencing my appetite to know that I want that type of food? I do know it's true in flies. There is really good evidence that if you starve flies of particular types of things that they need, then they go and prefer the type of food that gives them that type of thing that they're lacking. So there is, there is an emerging sense that organisms do change their appetite according to what it is that the body actually needs, but it's an emerging thing rather than very clearly established.

Jonathan: So if I'm sick and I am not hungry, and I think that quite often happens if I feel like really sick and I'm like lying in bed and I just don't want to eat anything, should I follow that feeling, or I-- should I be sort of trying to eat food despite the fact that I'm not hungry?

Daniel: I mean, it is important you don't get dehydrated. I think everyone agrees on that, so that's very clear. 'Cause you do tend to, you know, not want liquids, but you do need to keep hydrated. But yeah, I think just normal sensible advice, try and eat a bit.

Tim: Our ZOE studies have shown that after a poor night's sleep, we make terrible food choices the next day. We have high fats, high sugars. Similarly, we know from vaccine studies, when you have vaccines, you know, during COVID, so we got a lot of vaccine data, a lot of people went into sickness mode for twenty-four hours 'cause the brain was being fooled thinking it was having an infection. And so the brain's prediction was, I'm in sickness mode, lockdown. But it's not always correct, and I think we just have to keep an open mind.

Jonathan: Final question around all of these sort of myths and stories. Daniel, since COVID, I think we've all been more aware of how easily germs pass between people. So if we're trying to get fewer colds this winter, should we be trying to avoid crowds?

Daniel: I mean, it's true that obviously germs spread between people, and avoiding crowds would make you less likely to be exposed to those germs. But equally, I wouldn't recommend you hide in a cave and don't interact with people because obviously life is about interacting with people

Jonathan: It's incredibly hard to understand today sort of what's true and what's nonsense when we're just surrounded by all this stuff on social media and in traditional media now with people claiming things with absolute confidence that isn't based on a lot. So I'd like to talk about habits that really can impact our immune system ability to cope and what we can do about it. And the team has sort of five habits to talk through. So I'd love to go through one after the other and understand whether they're real and then, like, some simple practical advice about what you can do. And Daniel, could we talk first about sleep? So firstly, is sleep important for immune health?

Daniel: Absolutely sleep is important.

Jonathan: So what happens if we don't get enough?

Daniel: There are many studies that show the importance of sleep. For example, if people were told only to sleep from one AM to five AM, four hours sleep a night, and then were given a vaccine and their antibody levels were measured in response to that vaccine, they produce much less antibody. The vaccine worked less well if they were told to sleep for only those four hours per night. So there is really good evidence that the ability of your immune system to respond is affected by your sleep.

Jonathan: You're saying that for these people who've been made to have very short sleep, it's like their immune system isn't working as well? Not...

Daniel: It's not working as well. There are also other studies where people were made to stay up, and there were sort of markers of inflammation in their blood. But it does come back down to its normal resting level, normal state once you have more sleep. The very best evidence for how much sleep you need comes from the cognitive studies, because it's much easier to measure how well you do in some kind of test where you're answering cognitive questions. And so there- People do better in a cognitive test if they sleep for seven hours. So if you sleep less than seven hours, you do worse. If you sleep more than seven hours, a bit more surprising, you also do worse. And the most important frontier about sleep is that we're all very individual in our sleep needs and how we sleep, and we understand very little about that individuality. We know even from everyday life that some of us stay up late at night, some of us wake up early, some of us nap in the daytime. We really understand very little about the individuality of sleep. So it's an important frontier, but broad brush, yes, sleeping is important.

Jonathan: And Tim, do you have any tips for people who are listening to this and struggling to get enough sleep? And I know because we get a lot of comments around that. You know, a lot of people are probably feeling more anxious right now because they worry about their sleep for other reasons, and now they're hearing it hits their immune health as well. What can they do?

Tim: Yeah. Well, likewise, I was worried about my sleep and improving it. And so the first thing to do is to take it as seriously as you would exercise. And what I've done that's,, helped me is change the environment in my room. So you got proper blackout blinds and, you know, if it's summer, I also wear,, an eye shade., I think, you know, making sure your brain knows it's dark is really important. I've experimented with mouth taping and found that it suits me. It's not for everybody, and some people can cause problems, but, you know, give it a try for a week or so., that means I don't wake up all the time with a dry mouth and probably improves my general breathing, possibly might help my brain. Got little ear pods., you can get these really soft ones now that play sounds of the sea and things like this, so if you're living in a busy place, you're not distracted by that. And,, I go to bed half an hour earlier than I used to. So I'm spending longer in, in bed, and I also turn off all my computing equipment, phones, everything, you know, an hour before. So I'm sort of not distracted and read a book. And I found that's really improved my ability to sleep, not only slightly longer but also better quality, so I'm getting up less.

Daniel: One of the,, studies that looked at this compared people having three different smells in the room, and then would they sleep longer? And the truth is that whatever smell was in the room, it didn't... There was no kind of magical smell that allowed people to sleep better. But if you thought that one of those three smells was particularly pleasant, then that is what correlated with you having a better night's sleep. So I would take away from that there's an enormous amount of individuality in this. So whatever it is that works for you to sleep better is what to do.

Jonathan: Brilliant. Daniel, I'd, I'd love to move on to the second habit or situation if you like, which is stress. And again, my first question is, does stress really have an effect on my immune health?

Daniel: Of all the things that we're gonna discuss about lifestyle, stress is by far the one thing that all scientists will agree on affects your immune health. Many of the other things we're talking about, there's a kind of edge to our knowledge which is a bit vague, and a lot of the time it's based on correlations, especially with things to do with exercise or, or nutrition or food. There's a lot of correlations that are really important in guiding what we think. With stress, there is all those correlation studies as well, but there's also a really precise molecular level understanding of exactly what happens during stress which directly affects our immune system. Stress has an enormous impact on the body in lots of different ways. So your body goes into this flight or flight response to deal with any kind of emergency situation, which could be, you know, in an actual fight or just stuck in a traffic jam. So what happens is your body produces these hormones that get your body in this fight or flight response. Cortisol is one of those hormones. I know that affects your immune system because if I take cells out of your blood right now and mix them up with cancer cells in a lab dish, and looked at your immune cells killing those cancer cells, they would probably be quite good at killing off those cancer cells. If I spiked into that dish cortisol, your immune cells would be much worse at fighting off those cancer cells. So that kind of reductionist lab experiment where I can just see the effect of a stress hormone on your immune cells makes me very confident in thinking that stress directly affects your immune health.

Tim: I think there's a lot of studies also, epidemiology one's really strong, where they've taken kids who've gone through very stressful early life experiences and then 10, 15 years later done blood tests and seen that their immune system is markedly different to those kids at the same time who had a, you know, relatively stress-free childhood. And so th- there's long-term... You're seeing these both term effects, but also these long-term effects on the body.

Jonathan: That's amazing. So both that you're saying if I'm really stressed right now, my cortisol has gone up high and literally you're telling me my immune system is suddenly failing to deal with cancer at the same level. But also that if I'm really, you know, have some terrible stress,, earlier in life, it, it can even be seen in my immune system in the, in the long run.

Daniel: Yeah, if there's a chronic level of stress. But also we do need to be a bit careful perhaps about, because stress is part of life, and so you can't avoid stress. If you do a sky jump, which is a stressful event and has some actual risk to it, when you land after a sky jump, your immune cells are changing number in your blood, and that's absolutely clear. In that moment, yes, your immune system is probably less good at fighting off some particular infection. But an hour later, it's come back to its normal resting level. So we didn't-- So, you know, that's why watching a stressful movie, it will change your cortisol levels, actually, but it won'

Jonathan: Give you a cold.

Daniel: No. Chronic stress is the thing to be careful about when you're, you know. And again, that can't always be avoided, right? Raising young children is very often a happy experience for people, but it does have stress.

Jonathan: You're both saying you can't really avoid it. To how might we try and reduce its impact? Maybe I can start. Like, Daniel, is there any evidence for Tai Chi or mindfulness or these sorts of things that people talk about reducing,, stress actually working and therefore having an impact on your immune health?

Daniel: So it's definitely true that mindfulness or Tai Chi or even just taking a walk in nature can help reduce your cortisol levels, which should then improve your immune health. The only caveat, the only issue about all of that is it's quite hard to do the actual experiment where you would say, you know, take a large group of people. You guys do Tai Chi. You guys don't do Tai Chi. Now I'm going to give you some deadly infection to see what happens. So the actual evidence is that it should help, and at l- in terms of levels of cortisol, it has an effect. Whether it literally stops you know, more-- less likely to develop cancer or you're better at fighting infections, in all of these things, it's hard to do that actual experiment.

Tim: Again, there's ob- observational epidemiology show that people who do these sort of activities,, will end up having less cancer, and we now know that cancer is very much linked to the ability of your immune system to fight off early disease. I mean, there are lots of other factors, but that would be an example where you can sort of extrapolate and say that generally these lifestyle approaches, which includes, you know, good sleep, also socializing, all these other things which we know are good for humans and aging, are also true for the immune system, which you can see, but it, it's all a bit indirect. Pretty hard to do, you know, deliberately stress someone, randomize them to, you know, you're doing Qi Gong, you're doing,, you know, swimming, you're doing,, nothing, and then follow them for 10 years. It's never gonna happen.

Jonathan: But what I am hearing is there is a really strong,, sort of proven link between stress and immune health, and so you're therefore both being more cautious about, like, the proven evidence about things that reduce stress. Mm. But as an individual, if I can figure out the things that really can reduce my stress in a sort of sustained,, duration, it sounds like I sh- that's a reasonable expectation to think that would actually be helping my immune health.

Daniel: Yes, I think so. It's actually even true that just your state of mind, like if you go into a stressful situation with a sense of that stress is still a part of my creating something that I would be happy about, you also spike cortisol levels less. So your state of mind in general is really important.

Jonathan: Last question on this. What about laughter? Is that really the best

Daniel: medicine? Well, that's exactly the kind of thing I was just saying. So actually, it's true that you do tend to spike cortisol levels less if you have a-- you're, you're in a state of mind of being happy generally.

Jonathan: I love it. What about alcohol? I've reached an age where, like, even one or two glasses of wine seems to have an effect on my sleep and seems to affect how I feel the next day. So definitely different from when I was younger. Does alcohol have any impact on my immune system as well as those things?

Daniel: It do- it certainly does,, at a h- if you have a high level of alcohol intake. At a mild level of alcohol, like one glass of wine or something, actually, it's a bit mixed, I think, what the evidence really is. But by and large, you're right that alcohol is not-- it's certainly not helping your immune system.

Jonathan: And if I mention alcohol, I guess I have to mention tobacco. Does that have any impact on immune health?

Daniel: Yeah, that is actually really clear that, you know, everyone will already pretty much know that tobacco is pretty bad for you. But it's es- it is also really bad for your immune system, especially in your lungs. It very directly actually stops immune cells working.

Jonathan: No rescue for tobacco there. Tim, the next one that of course we have to talk about is our diet, and we talk about diet all the time on this show with you and many other scientists. But today we're talking specifically about our immune system. So is our diet important for our immune system?

Tim: It's crucial. It's the main reason that I don't get sick. I think we have to now look at the immune system with the microbiome together, because the majority of our immune system and our immune cells are i- in or lining the gut And the microbes are talking to the immune cells and the lining of our gut, and our immune cells are talking to our gut microbes in a, in a two-way process. So essentially, the immune system is using our gut and our microbes as a sort of window to the world. It's saying, "What's going on out there? Tell us what's happening." And a lot of that is through the food that i- is being taken in. And a lot of our training as babies, our immune system is all through our gut. It's working out what's safe to eat, what's a potential pathogen that we should be building up antibodies for. So as you change the state of your gut microbes, you change your immune system. And if you have a healthy gut, then you're much more likely to have a healthy, well-balanced immune system. And we know that very poor gut health means that you're gonna have an immune system that's out of balance, as we've discussed. It's either more likely to be overreactive, cause food allergies and autoimmune disease, or not working well enough and getting a- accelerated aging and cancers.

Daniel: It's absolutely true. I mean, Tim's exactly right that the gut microbes are absolutely critical to how the immune system is set up, how it functions. And we are starting to see very clear molecular-level understanding of exactly what happens. Some of the gut microbes produce particular molecules that very directly act on immune cells. So one of the really clearly established things is that certain types of bacteria in the gut produce what's called short chain fatty acids. One of those happens to be called butyrate. They very directly help a particular type of immune cell, which is called a regulatory T cell,, function properly. And that type of immune cell is fascinating in its own right, because most of the time we're thinking about the immune system switching on to fight, to fight germs. This particular type of immune cell does the complete opposite, and it actually switches off an immune response to stop you attacking your own body's normal healthy cells and tissues, which happens in an autoimmune disease. So we do have really kind of very clear molecular-level understanding of how the gut microbiome produces particular kinds of molecules that very directly set the immune system to be in the correct balance.

Tim: And that means of eating things like fiber, which we're always talking about on this show, means you produce more of these short chain fatty acids, and you get a better control against autoimmune disease and these other things where the immune system goes wrong. So there are now increasingly sort of clear links between the actual foods that are going in, how they're affecting the gut microbes, and then how that affects the cells of the immune system, and then how that plays out with inflammation across the body.

Jonathan: So you're saying that if I have a much healthier diet- I could actually reduce my risk of, you know, getting a cold, having an infectious disease.

Tim: At the height of COVID, we,, did this mass citizen science project with followers in the US and the UK, gave them food,, diaries to record what their diet was like. And then we asked them, "If you had contracted COVID, how bad was it on a scale naught to 10? Did you go to hospital, et cetera? Do you need drugs, et cetera?" And with this mass of data, we clearly worked out that the people who had the best diets Had about 50% less risk of having any serious COVID symptoms. So your diet was directly l- linked in this massive study, even when we'd adjusted for all kinds of factors like obesity level and diabetes and other medical problems, so to your immune system's response. So you had a much better,, journey through the infection. You still got infected just as much, interestingly, but the consequences were very much less if you had a healthy diet. And we believe that healthy diet is working through the gut microbes, which are communicating with the immune system. It was a massive study of over a million people.

Jonathan: Over a million people.

Tim: So it was the largest citizen science project ever done, and only because of COVID we managed to do it so fast., and everyone volunteered like a shot, and,, it was a unique chance to do these kind of mass experiments. But it showed how important,, things that we take for granted, like our basic nutrition and our food, are to something that it was killing millions of people.

Jonathan: You're saying that the quality of their diet had a really big impact on whether they had a very mild experience or a really severe experience with COVID- Yes even after you'd adjusted for their health and all these other things?

Tim: Yes., people who had the very worst diets had a three to fivefold increased risk of severe COVID that leave them hospitalized or even dead.

Jonathan: Wow. Let me move on to the next on the list, which is exercise. Again, we talk about exercise a lot on this show. Everybody agrees that it's good for your long-term health, but does it actually have any impact on my immune system and whether I'm gonna get sick?

Daniel: We spoke earlier about the immune system using particular molecules called cytokines for different cells to communicate with each other, to coordinate the right kind of response. Actually, your muscles also do produce cytokines, so they very directly affect your,, immune system. So again, one of the reasons why we can be really confident is we have the epidemiology, we have the correlations, we have the analysis that shows exercise affects your response to vaccines, your susceptibility to infections. But also, we have a kind of molecular level view of exactly how this would play out in the body, which makes us really confident the actions of muscles directly affects the ability of your immune cells to do this or that inside the body. So it's, it's, it's very clear.

Tim: But there is this thing about too much exercise can overstress your immune system and cause problems, whereas intermittent one says a little bit of stress does you good. I think that's the phrase I've heard.

Daniel: Well, it's definitely true that, - Some level of exercise helps your immune system, and the best benefit tends to come to people that are not doing much exercise, then they start doing some exercise

Jonathan: And so if I'm worrying about my immune health, you know, and I don't want to get sick in the winter, like, how much exercise do I need to do?

Daniel: So that's,, not an easy question to answer. You know, we commonly hear this 10,000 steps thing., but in fact, that's really a kind of psychological tool to give yourself a target that you might be able to reach 10,000 steps. It's any amount of exercise, and generally speaking, the more you do, the better, until you get to very strenuous levels of exercise. And as I said, the best benefit comes to people that don't really do much at the moment, but start doing some exercise

Tim: And probably having a rest day as well. So, you know, rather than having, you know, seven days a week overdoing it, there is some evidence from some mouse models, aren't there, that,, giving a-- having a little break is good for you

Jonathan: What about if you're listening to this and you haven't done any exercise early in your life, and maybe you're in your sixties or seventies or eighties. If you start to do more exercise now, and let's assume that the doctor says that it's safe, can that help your immune system now, or is it all too late if you didn't do it, you know, when you were younger?

Daniel: I think it can help your immune system now. So, you know, we've spoken a lot about these correlative studies and the complications in trying to disentangle parts of your life from every other part of your life because it's very complicated. So that is why some scientists turn to doing experiments with animals, just because you can control a one aspect of their life. So you can have mice in a cage, and they have a running wheel where they can run four to six kilometers per day, and other mice that do not have a running wheel, or they have a running wheel that doesn't turn, so they cannot run. Then I can do an experiment where some mice are getting exercise, some mice aren't. Now I can inject those mice with some cancer cells and see how many tumors develop. That then is a measure of how good was their immune system at fighting those cancer cells before they would develop into proper tumors. Okay? Mice that are allowed to run four weeks before I inject them with cancer cells produce about half the number of tumors than mice that are not allowed-- haven't been running at all. So a term running before they were exposed to a cancer made them much better at fighting off that cancer

Jonathan: Amazing. I'd like to wrap up with one question for each of you, for our listener who's coming through all of this and saying, "Okay, I now want to get to some really simple advice about what I should do right now to hopefully try and not get sick." Tim, if you were just gonna give no more than three simple tips about what I should eat to improve my immune system, what would you say?

Tim: Eat a diversity of plants so that you're getting lots of fiber. Try and reach 30 plants a week. Try and get three fermented foods a day. And I would limit the amount of ultra-processed food that you're ingesting that could have a negative impact on your immune system and,, your gut microbiome. Do those three things and,, yeah, you'll definitely like to be healthier.

Jonathan: And Daniel, if someone's listening to all of this and they're feeling a bit overwhelmed, right? We talked about so many different things, and they don't really know where to start, what would you advise them?

Daniel: Okay, so firstly, I would say the thing that most scientists agree on will be long-term stress can affect your immune system. So try to avoid long-term stress, which isn't always easy. But long-term stress, try to avoid that. Secondly, I would say be a bit skeptical of sound bites and advice that you get online or even with products. Thirdly, I would say if you're feeling overwhelmed about the amount of health information you're being told, if you get better from the cold in two days and I'm taking a week to get better from that same cold, I might well feel guilty that I haven't done enough exercise. I haven't slept well. I'm not eating the right diet. I'm doing everything wrong. But it might actually just be that you had an immune system that was hardwired to fight that cold more effectively than me, and I shouldn't feel guilty about that. If we were infected with a different virus, I might well do better because there's an innate, profound diversity in humanity, and we are all fundamentally different.

Jonathan: I'd like to do a little summary, and please both of you correct me if I've got any of this wrong. The first thing that I think of is my snot is magical It's apparently a key part of the immune system. Then maybe getting a little bit more serious, a lot of what we're told, and I think I took for granted about how to keep yourself healthy, is just wrong. Products that claim to boost the immune system and actually have those labels like written on them in the supermarket are nonsense unless we're literally deficient in, you know, you said selenium, and apparently I'm totally fine on my selenium and my zinc and all these things. So they just don't work. However, there are real things that you can do, and I think there were five things that you both really agreed on actually really can help with your immune health. The first one which to me is a real surprise, was you said long-term stress absolutely plays a big role in immune health. There's no debate about it. The second one is sleep. Sleep really can affect your immune health. The third was alcohol. So lots of alcohol is bad for immune system as well as bad for other things. The fourth is food. Food that you're eating has a direct impact on your immune system. 30 plants a week, try and get to three ferments a day, limit the amount of ultra-processed food. And then last but not least, interestingly, you talked about exercise. And again, I think everyone knows that exercise is good, but really fascinating that there's this evidence that exercise is directly helpful for your immune health. And my big takeaway from that is sort of however old you are and however little exercise you might have been doing, actually if you start now doing some more, it can actually help. So this is something where anyone listening to this can do something right now that can really affect and improve their immune health.

Tim: Pretty good.