The hidden link: Depression, processed food, and your gut microbiome

Could depression, dementia, and poor oral health all have one uniting link? The gut microbiome contains 200 more genes than human cells, and research shows its strong impact on mental illness, brain health, and chronic inflammation.

Professor of epidemiology at King's College London Tim Spector breaks down the science of how gut microbes produce the chemicals that shape your mood, your immune system, and your cognitive health, and why the standard Western diet has been systematically destroying them for 50 years.

My name's Tim Spector, I'm an MD, I'm a professor of epidemiology at King's College in London and I'm also co-founder of the nutrition and gut health company Zoe. Today on Big Think, I'm going to talk about the connection between what's going on in your gut, your gut microbes and how that affects your brain and your mental health. I'll also be giving you tips about how you can improve your gut health to improve your overall health.

How the gut microbiome works

The gut microbiome, I like to think of as, it's like a virtual organ in our bodies that we've only recently discovered, something like discovering we had a liver. And essentially it's made up of trillions of little tiny microbes that you need a microscope to see, bacteria, fungi, parasites, viruses and other things we still don't exactly know about. This collection forms a community mainly inside the lower part of our colon, the large intestine. They essentially need to be thought of as mini-pharmacies. They convert the food that we eat into chemicals, into hundreds and thousands of different chemicals and those chemicals have this amazing impact on our body that we're only just beginning to realize.

This collection of microbes has something like 200 times more genes than we have in our own human cells. That means they're much more versatile, they're more flexible in what they can produce in terms of chemicals and they're actually more numerous than our human cells. This is important because we evolved from these bacteria and the very first mammals and then humans were built because they contained these colonies of microbes that could interact with them and without these microbes we really wouldn't function and particularly our brains don't develop normally.

One way to think about our gut microbiome is either as the environment of a jungle or an amazing natural zoo where you've got these thousands of different species of microbes that have taken over little niche inside our gut that is very specialized to them. They're not all fighting for the same food, they've evolved just like animals have to have very specific tastes. The example I like comes from our own research where we found there is one bug that only really eats or drinks coffee. It is so fussy, it hangs around maybe for years just waiting for that first cup of coffee before it will replicate and really start to be noticeable.

If we start thinking of coffee as a great example then you think about all the other bugs that we might have lying dormant in our guts that are waiting for us to eat something unusual, say a bit of baobab or some beetroot or some particular type of nut or herb or spice. That's a really important concept because we now know that the more diverse your gut microbes are, the healthier you are. So that's why we need to be thinking how can I best feed these very different gut microbes, some of which might like coffee, others will only like the leaf of a bit of cabbage, others might like seeds or nuts, and others will like the leftovers from what the other microbes have eaten.

This is also a really interesting concept because they working together, they make sure there's no waste. They are these perfect ecological beings so that the concept of waste doesn't really exist. And what they've shown in some mouse studies is that if you have a really strong colony of gut microbes that are very diverse, then you eat your standard diet, you have no waste left over. But if you only have a few microbes rather poor colony, they're not very well diversified, you get leftovers and when you get the leftovers then you get what we call the bad bugs, we'll eat those because they've got something to eat. So a really good healthy colony of microbes like a zoo is one where all the animals have a role, they've got something to eat, there's no real waste and everything is used up. It's really a marvellous model that we all love, a sustainable gut society.

Although we all have thousands of species of these trillions of bugs inside our guts, we're actually really very different one to another. And this is because as kids, when we're born, we're born sterile, so it's slightly random which microbes end up colonizing our own colon. And it turns out that the average person only shares about 20% of their gut microbes with another. And even in identical twins, they don't share many more than that. So it's a process that's part random, part designed that way and not very genetic. This means that we all have a different set of microbes producing slightly different functions.

Until recently, we didn't really know how good bugs and bad bugs were interacting. And it turns out they're fighting for resources. The good bugs, these are ones that eat your, for example, plants and change those plants into healthy chemicals that your body can then use, and that has a health effect. If they do well, they leave no food left over for the bad bugs — by which we mean bugs that increase when you have a bad, an average American diet with fast foods and burgers, etc., lots of fat. Those ones will increase and cause bad outcomes.

So it turns out that if you're only having a good, diverse diet, there's very little waste to supply the bad bugs. This concept has changed quite recently because we used to be thinking, okay, how do we get rid of the bad bugs that are causing inflammation, irritation in there? But now we know it's focusing on the good, it's increasing those guys as much as possible, essentially giving them a greater diversity of food so that everybody gets something to eat. And we can really use the full armory of all those good microbes to produce the biggest possible array of chemicals that the body can then use in the best way.

An unhealthy gut leads to an unhealthy brain

When I started this research, we thought that the gut microbiome was really just involved in things like obesity and metabolism. And it's only recently we've realized that it does so much more. I like to think of it really interacting in sort of three key parts of the body. Yes, it does influence metabolism, energy levels. It sends signals to the brain about appetite and body weight and distribution, etc. And it's probably important in things like type two diabetes.

But one of its key roles is acting on the immune system. Our immune system is pretty crucial for virtually every common disease that we're encountering at the moment. From autoimmune diseases, food allergies, to things like preventing cancer by these immune cells killing off early cancers and the aging process itself. So the immune system is a huge part of what the gut microbiome does and without a really good microbiome, you can't have a good immune system.

The third key thing that the microbiome does is interact with our nervous system. And it does this through a huge network of nerves that are sometimes called our second brain. They're as big as a cat's brain and they probably evolved around our intestine before even our brain was formed as when we were in the womb. And this network of nerves connects to the brain primarily through the vagus nerve. This is like a fast internet cable that picks up what's going on in our gut microbes through the gut lining, picking up signals in every single layer of the gut. So right through the tiny connections right on the inside of the mucosa through every layer. So they are actively sensing everything that's going on there.

All these neurons interconnect. They're picking up signals from the gut microbes that are sending chemicals such as short-chain fatty acids, what they make when they break down fiber. They might be getting directly from the microbes or from the cell wall of the microbes. They might be getting it from some hormones that the microbes are making. These endocrine hormone sensing cells in the gut lining like GLP-1 — that's what people now know is how Ozempic works. They're picking up hostile chemicals, good chemicals. Everything is going through to these little tiny neuron endings and they're being fed into this vagus nerve through this incredible cable to our brain where it tells the brain what's going on.

This discovery has really changed our whole idea of the gut-brain axis because we used to think the brain was telling the gut what to do. Turns out most of the traffic is from the gut to the brain. People conceptually thought the brain was separate to the body and this is the Cartesian view really that has penetrated and been very persistent in medicine for centuries and still means the reason that psychiatrists and medical doctors don't really mix and that we have separate hospitals and separate training. And yet now the latest science is showing that there really is very little difference. The brain is just another organ. It is not the brain controlling the rest of the body. It is just another organ and it relies on the gut for most of its information.

"The brain is just another organ. It is not the brain controlling the rest of the body. It relies on the gut for most of its information."

Many people have vaguely heard about inflammation. I was unusual because as a kid I heard about it a lot because my father was a professor who focused on inflammation. Before it was trendy — and it was trendy briefly then it went out of fashion. Now it's definitely back and everyone needs to know what it means. It's a normal response we all have to a threat in our bodies whether it's stress or it's an infection or something alien to us. Our body responds. It sends all these white cells from our body to that site and triggers a whole careful reaction that gets rid of that threat as quickly as it can and goes back to normal.

Now we currently have a problem of what's called chronic inflammation which means it lasts too long. Instead of getting a brief spike of inflammation in response to a threat — say some bad food — that returns to normal, our bodies have become in some way changed so that that threat level stays up. So we are suddenly going through life at a threat level that isn't our baseline. That may be due to our environment of terrible food. It could be due to pollution, pesticides, all kinds of things but it could be due to our stressful lives and social media. Whatever the cause, it's true that in the West we have stress levels based on the levels of inflammation or our immune system that are higher than you would find in Africa or India or where our ancestors came from.

I also learned a lot because I was a rheumatologist originally in one of my early careers before I changed several times, and saw patients with rheumatoid arthritis, autoimmune disease who suffered a lot and they were constantly tired because of the inflammation in their joints which spread to their blood. I didn't think much about it because we thought they were tired and depressed because their joints hurt and that's really what we were told. Oh, it's normal they are tired and depressed. They've got quite bad crippling arthritis. But it turns out it was just a reaction in the brain to what was going on in the rest of the body.

We now know that many mental health problems — virtually all of them that have been studied so far — have above average levels of inflammation and they also have abnormal gut microbes. So there's a link between mental health disorders, which nearly all of them have reduced energy and reduced mood to some extent. Even in forms like psychosis and schizophrenia, or even in ones you don't regard as mental health — brain problems like epilepsy or migraine — this is also true. So there's a general increase in inflammation in the blood, there's also a general abnormality of the gut microbes compared to people who don't have any mental health problems.

So we're seeing this picture developing whereby we are getting much more common ground to explain this explosion in mental health problems — that's based not on the old theory of the deficiency in certain brain chemicals or in excess of brain chemicals, but due to this underlying inflammation which probably starts in the gut and then sends those signals to the brain.

So an obvious question is: where does this inflammation come from? What's triggering the body to in a way overreact and keep overreacting to stimuli rather than dealing with a problem and going back to the base state? We know that's where our healthy ancestors were. Inflammation is both a cause and a consequence of many things we're talking about.

The most obvious cause in the West of this pandemic of mental health problems is the fact that since the 1970s we've been eating an increasingly bad diet. The standard American diet — the SAD diet — which is denuded of fiber, plenty of saturated fats and full of ultra-processed food. All of which have been shown in particularly mouse models to cause inflammation in the gut, which leads to many more bad bugs compared to good bugs. There's not much for the good bugs to eat, the bad bugs take over. They like an inflammatory environment and they actually produce chemicals that will stir up the gut lining and that environment to in a way be more toxic and raise the levels of the immune cells in the body that think they're infected or diseased in some way.

So I think that's the unifying idea of how bad food on a long-term basis causes this increase in the immune cells. You've got to remember that the gut microbes, probably by evolution, are there to pick up signals from the environment, tell the brain — that can't really see outside the little skull — what's going on, gives an idea of what's going on. It's this consciousness and it's picking up these signals. And if those signals are bad food, inflammation, heightened fear, it's like: gosh, there's something bad going on here. Let's signal that back to the brain.

What does the brain do? It goes into some sort of lockdown mode — a sickness behavior, some people call it — which is quite natural. It's getting a signal from its main communicator, the gut. Along the vagus nerve it tells it all kinds of bad things are happening. This is not good. We're getting really bad signals. How do we react in evolution? We'd say it's like an infection. We lower our mood, we lower our sociability, we lower activity. We tell the body we're tired, so we rest. And that's what you get — mood change and all the other associated factors — and you end up with what we're calling depression.

That's my way of thinking about this problem for perhaps the commonest factor that's changed in our lives in the last 50 years and particularly why in kids we're seeing this pandemic. Kids that have had mothers who had terrible diets, who passed that on to them, and then basically they've had ultra-processed foods for the whole of their lives without ever having any real food to nourish the few good microbes there are. Only the bad ones have been surviving in that jungle, just on the fats and the scraps and the artificial sweeteners and the chemicals, etc. I think that for me is the key model.

You can also think of other ways that you might get inflammation. My research in this area is not that I've done it myself, but looking at the literature: many of the risk factors for mental health disorders are all the same. So it doesn't matter which of these so-called 300 different disorders you have, the factors are the same. Childhood stress, trauma, poor diets and living in urban environments — all of these things increase stress, and we now know that that stress, you can see it as an increase in inflammation in the immune system. So just psychological stress can also cause this as well as, in a way, dietary or gut stress — and they end up with the same common pathway.

After a bad night's sleep or some stress, your body seems to crave fatty and sugary things that are unhealthy. And we've already experienced that after a poor night's sleep or a hangover or whatever it is, you reach for the one thing that you think you need, but it turns out you're wrong. Your brain's given you the wrong directions and this is going to make you probably feel worse. So this sort of explains this loop people get in, particularly with depression or anxiety. The brain is telling them to do one thing that actually is making it worse. And so it really explains why people with low mood and depression end up eating badly and can't get out of it and it's making them worse.

The dementia connection

I hadn't really thought about the gut-brain connection quite as much, but about nine years ago my mother suffered a stroke. She ended up in a care home with dementia. Each time I visit her, I am reminded of the fact that so many of us now are potentially facing this fate in old age. And it's not something that anyone really wants. Dementia is probably the worst way to go. So it's made me think about this more and start to look at how my own research could potentially have an impact or help not only myself, but other people prevent dementia or at least delay it considerably.

It turned out my mother, with her stroke and subsequent acceleration of dementia, probably had what's called vascular dementia, which is a fairly general term that affects about one in three people who get dementia. It's the second commonest form and it tends to occur in phases so that blood supply to the brain is getting progressively diminished and knocked off probably by atherosclerosis in the arteries supplying the brain.

The other type of dementia that many people hear about is Alzheimer's, which is probably the commonest form, perhaps affecting 60% of all cases. And it can start in a similar way with loss of memory or cognitive function and then progresses at a usually more sustained pace that doesn't have these separate phases. And that's caused slightly differently, distinguished by having these protein folds in the brain — these amyloid and tau proteins that have led to some of the damage in the brain as the brain has been shrinking. Now, we don't know they are causal. It could be that these proteins are just part of the damage after it's occurred. But we believe that there are some common causes of both these types of dementia, that the risk factors seem to be quite similar.

In both cases we can link this back to poor gut health. The picture of gut health where you have a low ratio of good to bad bugs, poor diversity of species, and an inflammatory component going on — that is a major risk for both types of dementia, as is things like type 2 diabetes, which probably involves the energy supply to the brain and affects both. So for practical purposes, you should be trying to prevent both kinds of dementia because they have a big overlap in the causes, although they can look very different at the end.

Gum disease and the brain

So we have gut microbes all through our intestinal tract — that means from our mouth to our anus. And some bits are harder to access than others. It's very hard to know what's going on in the middle bit, the small intestine, because it's very hard to reach. And so we've relied generally on what's happening in stool samples, but we also can look at what's happening in our mouths, saliva, which is teeming with microbes that are there basically to protect us, but also start the first bit of digestion of food.

And just like in our colon, there's a little colony of microbes there that interact with each other. You have good ones and you have bad ones. And if you provide an environment that doesn't clear away your food, leaves food waste, the bad bugs will accumulate and they can lead to inflammation. And this tends to happen at the interface between your teeth and your gums. And we know gum disease is very prevalent in the West — periodontitis.

So you might see some of the redness and swelling, which is a sign of inflammation. You can generally see it there — your gums might be slightly tender, might be swollen, might be red, and in certain cases they will bleed if you floss them or brush them hard. So you've got inflammation there, the bad bugs living there, eating the bits of food. They form plaques — a complicated structure made by these microbes that builds up like a protective shield so that they can keep digging away and are protected from you brushing them away.

So our teeth, it turns out, are a really good place to look for inflammation. And remarkably, we learned about 10 or even 15 years ago that dental decay was related to heart disease. Now we're able to look much more broadly at all the microbes. We know that poor mouth hygiene means high rates of bad, pro-inflammatory microbes. And if you're not brushing and flossing your teeth, you have roughly a 25% increase in heart disease, and some studies are showing anything between 20 and 50% increase in dementia and cognitive decline.

So suddenly we've got a really good example of how uncontrolled inflammation in our bodies can have a direct effect on areas we hadn't really thought linked at all, like our brain. And this gives people the idea of how important it is and how preventive action can work, because if you then brush your teeth regularly, you'll reduce that risk, perhaps by half — so rather than 50%, you're down to 25%. If you floss, which gives you an extra 30 or 40% of cleaning, you do that really well, you can get rid of the plaque and get rid of the bleeding and the inflammation. You can get right back to zero. So I think that's a marvellous example of how we need to battle inflammation in our bodies really to save our brain health.

"If you're not brushing and flossing your teeth, you have roughly a 25% increase in heart disease, and some studies are showing anything between 20 and 50% increase in dementia and cognitive decline."

So it's very easy for the average person to know what their gut health is like. Basically it means that your trips to the toilet are regular. Look at your stool — is it fairly soft? If you're a cow pat you don't want to be a sheep dropping. You want something in between, and you are going once or twice a day. If that's the case and you're not getting any bloating and you're not having constipation or diarrhea or lack of regularity, then you're probably pretty healthy.

If the opposite is true and you're having any of these problems, your stools look rather strange, they're varying, you're only going once or twice a week, then it's highly likely you have gut problems and you need some dietary help. We talked earlier about thinking about your mood and energy: are you fatigued, is your mood lower than it should be, are you getting some bloating? These are signs you do have gut health issues and you should address them.

If you want to test your gut microbiome — which I think will be standard of care in five years time — it's very difficult now to know who to go to. Your general family physician may or may not be able to help. A functional medicine doctor probably will, and we're seeing more of those. But it's not something that is very easy to do. I think a good gut microbiome test is much more useful to you than getting your genes tested or your DNA. It's really your metabolic blood pressure. If your gut health is good, most of the rest of you is going to be pretty good. And if it's bad, you're going to have problems.

8 tips for maintaining a healthy gut ecosystem

The old way of thinking about nutrition — the way I was taught and doctors are still being taught — is that essentially there's only four things you need to know about nutrition: its calories, how much fat there is, how much protein there is and how much sugar there is. If you get those four things right, everything else will follow. We've been following that advice for the last 40 years and it's been a complete disaster. It doesn't make sense scientifically anymore. It's only really helped the food companies sell us more crappy products and it's led to a disaster of unbelievable scale for public health.

Real food, unlike fake food that is made just from a few ingredients, contains thousands of chemicals. And all of these play a role in our body because they then nourish the trillions of microbes inside our gut to produce thousands of other chemicals that our body can't produce. It's only by focusing nutritional health guidelines on this new organ in our body — our gut microbes — that it all makes sense. We have to have a seismic shift in our idea about nutrition. It'll make healthy progress and focus on gut health, and what are the key principles to improve all of our gut microbes which will help us.

As I've been studying the gut microbiome, I've been obviously thinking of how I can translate that science into progress and practical tips for people in various ages and states of how they can improve their gut health.

1. Mindfulness

The first one is what we call mindfulness and this seems a bit strange. What's he talking about mindfulness for? Well, I remember when I started to change my diet, I decided to be vegan. It was an easy way to change your diet as a temporary fix if you like, because it meant that when I was going to medical conferences, I would stop and think before I just took something from that tray, from that buffet. I would say, "What's in it? Was there meat in it? Is there dairy in it?" And we don't need to be that extreme. All you need to do is to think about what you're eating just for a second before you grab it or you put it in your mouth.

So, mindfulness in this context means just stop and think: Am I hungry? Do I need this? What's in it? And then what effect is that going to have on me if I do eat them? These sort of three steps are important. Mindfulness is an important realization that food is much more than just calories. Is it going to affect my mood? How am I going to feel in three hours' time? What are the long-term consequences? And also, is it going to be really tasty? Am I going to really enjoy it? Or am I just being forced by marketing, by big food, to eat this rubbish that I don't really want? So there are three steps in this concept of mindfulness. We just want people to stop and think.

2. Eat a diversity of plants

So, the second principle — probably the most important that we've found scientifically — is to eat a diversity of plants. It's not about eating the same plant many times a week. We think that the science is pointing us to something around 30 different plants a week gives you optimum gut health. This is a study that I took part in with the British Gut and the American Gut about 10 years ago where we saw the sweet spot for gut health was people eating around 30 different plants a week. It looks like it might be slightly more than that, but certainly 30 is a really good target.

It turns out it doesn't matter if you're vegan or vegetarian or omnivore. It's the people that eat the most plants that have the healthiest gut, which I think is really interesting.

30 plants — many people say, that sounds way too much. The average American probably has between 10 and 12 plants a week. How do I get over double that? Well, you've got to remember what a plant is. It's not only a fruit and a vegetable, but it's also a nut. It's every different type of seed. It's an herb. It's a spice. It's even things like coffee, which is a fermented bean. So when you just change your mind a bit about what plants are, then it becomes easier and fun to add them to your plate. Think about more rather than less. Most diets are all about excluding things. What we're trying to do for our gut health is to include more things. We've got little microbes just waiting around for that bit of baobab or that particular nut or seed or berry, the chemical that comes from it. So the more you can give your gut microbes, the better.

3. Eat three fermented foods daily

My third guideline is to eat three fermented foods daily. So I've spent the last three years researching fermented foods and have a book on that subject. And what was really interesting is I didn't think it was a topic worth discussing three years ago because the science wasn't really up to it. But I've been convinced by a couple of studies. The first was from my colleague Christopher Gardner at Stanford — a very detailed but small study of adults who were told to eat five ferments a day, comparing that group to a high-fiber group. And what they showed was that not only did the gut health improve in the fermented group, but particularly the immune system was impacted and they had around 25% less inflammation.

So suddenly, moving from anecdote and lots of small not-very-good studies in the literature — they were all pointing the same way, that fermented foods were good for you — to suddenly a mechanism that this can really impact your immune health in different ways, rather than just eating lots of fiber and plants.

People think, what is a ferment? Fermented food is something that's been transformed by microbes into something better. You transform milk into cheese — much more interesting than milk. You transform grapes into wine. You're transforming cabbage into sauerkraut. So think the four Ks: kefir, kimchi, kraut, kombucha. Kombucha is fermented tea, which is all really good for you. Most cheeses, if they're not made artificially in factories. Good yogurt is great. All the soy products that you see in Japan — miso paste, all these krauts and pickles that are fermented in salt — are really good for you. Variety is important. They've all got plenty of different microbial species, and they're better for you than just taking probiotics from the pharmacy or the drugstore. So my recommendation is to build up to three ferments a day as an extra boost for your immune system and your gut microbes.

4. Diversify your protein

The fourth guideline is to diversify your protein. We're going through a protein craze at the moment where every product has added protein in it. Every American thinks they're deficient in protein, and there's nothing wrong with protein. It's not particularly harmful, but if you don't need it, you lose it. You can't store it, so you pee it out or it gets converted to fat.

If you are trying to get more protein — and perhaps 10% of people are lacking in protein because they're unwell, they might be elderly, they're trying to build body mass or trying not to lose it too much as they're on these weight-loss drugs — then you should be trying to think about other protein sources rather than just red meat. The ones that come to mind are beans and lentils, things that have really high protein content, but also contain lots of fiber. This means your microbes can actually use that at the same time as your body's getting that protein. That's a real reason why we should be switching, eating many more beans and pulses than we are currently. They're very cheap, they're accessible, they're good for the planet, and our microbes love them.

5. Focus on quality, not calories

Guideline number five is focus on quality, not calories. For decades, we've been obsessed with the calorie as the main source of describing food and nutrition and whether it's healthy or not. There are hardly any nutrition scientists who now believe in the dominance of the calorie. It's only a tool for the food companies to make rubbish food seem more healthy because it is lower in this magic term.

The science tells us that in the vast majority of people these diets fail. It's really impossible for most people to follow a calorie-restricted diet for more than a few weeks. Even if they do lose weight, as most people do, everybody regains it because your body's appetite signal is turned up. The brain has finely tuned to know if you're not getting enough energy and it switches up its thermostat to ramp up your hunger signals. Your body is designed by evolution to overcome that pure calorie signal.

The other problem with calories — not only the fact that it's virtually impossible to calorie count and lose weight — is that it disguises the food in ways that gets us to stop thinking about the quality of what we're eating. That's what the food companies have been doing for the last 40 or 50 years, trying to switch our thought away from the fact that they are just making this crap food out of a few ingredients that have government surpluses, away from the food we should be eating, which is whole foods — which is what our gut microbes want to eat. They want to eat a variety of foods that are in their original state, that have the casing that contains most of the fiber. They want the skin on it. This is where most of the nutrients and ingredients are that we need for our bodies to work really well.

This is a really important step as people try to disregard the calorie, which effectively doesn't have a major role in any of our plans. We ignore the calorie completely — tell people it doesn't matter. It's all about the quality of the food, how it's going to impact your gut microbes. And once you do that, it's very liberating for people to think, I don't have to worry about this. And generally, if something's got "low calorie" on it, it's a poor quality food. You should be avoiding it.

6. Avoid high-risk ultra-processed foods

Number six: avoid high-risk processed foods or high-risk ultra-processed foods. These are the foods that make up 50 to 70% of the American diet, higher in kids, and most of those foods are really harmful for us. There isn't a universal definition of an ultra-processed food. We think that ultra-processed food itself is too broad a term because most people use anything that's got an additive. Some of those additives can be like vitamin C — therefore not particularly bad for you — whereas others are dreadful.

The worst ones, the ones we categorize that make up 25% of the American diet, have a number of components that make them particularly harmful. Remember, these are foods mainly made from cheap ingredients that are government-subsidized — like maize, like soy and wheat and sugar. These are things that are plentiful and taxpayers are paying for them to be cheap, and they have a surplus of it, so they've got to create foods with them. And they do that really effectively, and they try and upsell them by adding additives that change their color, and they change their texture, and they change their taste.

And some of these additives are not too bad, others are quite unhealthy, and others are really unhealthy for us. Some of them are associated with cancer, some of them associated with mental health problems in kids. You have emulsifiers which glue them together, which are bad for our gut microbes. You've got artificial sweeteners, which many of them developed from the petrochemical industry, which our bodies are not used to, that cause our microbes to produce strange chemicals in response, and are generally harmful. You've got others that we still don't know how bad they are.

Then you've got the hyperpalitability — the companies use a mixture of salt, sugar and fat in those combinations to make that food override your normal fullness signal. So they do this very deliberately. We all know we've eaten a big bag of chips or whatever it is in front of the TV, and it's got potato, which is starch in it. It's got salt, it's got usually some fats in there, and it's that sugar salt fat in the perfect combination that these brilliant food scientists have created that will tickle our brains into thinking we can keep eating them and we just don't get full.

So hyperpalitability is a component of 70% of these foods. The other component that you need to be looking out for is: has its structure been taken away? They strip off all those bits of plants that make it necessary to chew it, and you've just got the inside bits, so we end up eating baby food. They melt in your mouth, dissolve away, you can keep eating them without even noticing it without exercising your jaw. And then finally, the best from the food companies' point of view are foods that you can get lots of energy in really quickly without noticing it. This is called calorie density.

So these are the four categories, the ones to look out for, the ones that have all of those components — and that makes up 25% of the American diet. Breakfast cereals for kids, most children's yogurts, most savory snacks, most cookies, most ready meals, most sodas and drinks, even if they are low calorie. I don't think it's realistic to cut them all out completely, and my view is that you can still have the odd treat, but don't have any of these as a regular part of your diet.

7. Eat the rainbow

Number seven: try and eat the rainbow. You may have heard that before, certainly been around a while, but we now know why it's good to eat the rainbow. Rainbow plants, especially fruits and vegetables, have rich colors, and that's what we're looking for when we're trying to select something to put on your plate — to try and get as bright and as varied as possible. Because plants that have these bright colors tend to also have high levels of a defense chemical called polyphenols.

Plants, like humans, have a defense mechanism, and this is their defense mechanism. They are fighting off insects and sunlight and drought, etc., with these chemicals that turn out to be actually very good for us. They used to be called antioxidants, which is a rather vague scientific term. We now know that our microbes can use these polyphenols as a form of fuel. If they're getting this, it's like they're getting extra gasoline. This engages all our microbes to be healthier. As well as breaking down the fiber in these plants, they're also getting these extra chemicals, which are acting as a fuel, which makes them even more productive.

Nature has given us these clues. Bright colors, for example — you've got a store, you've got iceberg lettuce there, and you've got a radicchio, an Italian one with lots of purple leaves on it. Always go for the brightly colored one. That will have a thousand times more polyphenols than the ubiquitous iceberg lettuce, whose only advantage is that it stays forever in your fridge. It has no nutritional value. It will still be there after your vacation.

The other clue in nature is they can be bitter as well. Slightly bitter-tasting plants like your broccoli, the cruciferous vegetables, nuts and seeds. You've got extra virgin olive oil, which is my oil of choice — it's really high in polyphenols. The best ones will make you slightly cough, and you want them to be really nice and young. You've got dark chocolate — over 70% is generally healthy because that cocoa bean has really been fermented and is really strong in polyphenols. As is coffee. If you like a drink, the only one I'd recommend for your gut health is probably red wine, which has high levels of polyphenols, again in moderation. So they're the tips that nature has given us to identify why eating the rainbow and bitter foods are actually good for us.

8. Give your gut a rest

The final tip is try and give your gut a rest. And by that I mean time-restricted eating. Twenty years ago, and certainly when I was at medical school, the idea was that we should be grazing, not feasting. Things have now changed and we now realize that constant grazing is what your gut microbiome doesn't like at all.

We know how important sleep is for our health. Getting eight hours a night can really extend your lifespan and improve your immune system. Well, the same is true for our gut microbes. They have a circadian rhythm. They actually go to sleep. And they need this time to repair the gut. So if it's eating all the time and you're having late night snacks, early breakfast, they're not getting the natural time they need — that our ancestors had for millions of years — to clean up our gut, particularly the lining of the gut where the mucosa is. This is really important as an immune barrier and the healthier that is, the healthier generally our immune responses are.

So give your gut 12 to 14 hours to recover overnight. And this is the basis of time-restricted eating. The way time-restricted eating seems to work is through a number of mechanisms. The first is by leaving that gap in time — your microbes are not focusing on eating and you have a defense team that comes out at night. A bit like in American football, you have different teams for offense and defense. Defense comes out. It cleans up the lining of your mucosa. That tidies up. The gut lining makes it more efficient, helps the immune system. You're also getting metabolic advantages so that your gut health is generally better the next day because it's all been tidied up for you. There isn't that much debris lying around and you get less unhealthy bugs over time.

It also seems to be better for your mental health. A lot of this is anecdotal, but we did do a large study in Zoe of 140,000 people, asked them to do time-restricted eating. A third of people hated it. They were snackers and they just found it really difficult. A third loved it and they're still doing it years later. And what we learned was the people that were adhering to it had improved mood, energy and less hunger from doing it. So there do seem to be other benefits that we still are looking at. It also helps you avoid snacking and we know that mindless snacking accounts for about 25% of calories in the United States and UK.

"The people adhering to time-restricted eating had improved mood, energy and less hunger. A third loved it and they're still doing it years later."

Time-restricted eating is a form of intermittent fasting, but you're not restricting calories. You're just eating the same food you would normally eat, but in a more limited time frame. So for people wanting to give it a go, I would say the easiest thing is to follow the little rule: leave at least two hours before going to bed where you're not eating anything else except maybe some black tea or water. That way your gut will have a chance to recover and hopefully you'll notice the difference the next morning. And if that works, then you can extend it and maybe say, okay, I can try this for a 12-hour overnight fast as well.

We are moving to the idea that many people can do very well just having two main meals a day, maybe combining breakfast and lunch — the concept of brunch. Many people are enjoying this around the world and seeing, actually, I feel quite good at a weekend when I do that. I don't necessarily need to eat first thing in the morning. And I think we're realizing that this idea of three big meals and maybe three snack events has been forced on us by the food industry. Mr. Kellogg was probably the first one to say that everyone had to have a breakfast before doing anything, otherwise you'd be unable to work.

I personally find — and I'm doing this interview now — I decide not to have breakfast because I'm actually sharper when I haven't eaten just before it. So for me, two or three meals, no major snack events, eating relatively late. I finish eating maybe at eight-thirty at night, and then I'll try and not have anything to eat before about ten-thirty the next morning. That suits me. When we asked the 140,000 Zoe members what they preferred, they found two thirds preferred that same rhythm. One third couldn't miss breakfast. They said, oh, I really like my breakfast. I'm hungry when I get up. I want to have my breakfast, but then I'll finish eating about six, six-thirty. So people have different rhythms as we discovered.

I think there is a bit of personalization to this and you have to find the one that suits you. But my advice would be: don't follow what the big food companies want you to do. Do experiment, find out what works for you, and follow that.

So there you go. There are my eight guidelines. The key message is you've got to eat for your gut microbes. If they're happy, you're going to be happy.

Why the internet gets dieting all wrong

I think it's very hard at the moment for the average public who's not a scientist, not a doctor, to know what to do for their health. Reports one day say that coffee is bad for you, another day coffee is good for you. Eggs are good for you one day and eggs are bad for you. Public has to realize that the media thrive on these stories, on these scare stories, and they love food stories. So newspapers and social media are full of all this stuff.

So the key is to not overreact and to realize that your core diet is the most essential, and that if you are looking after your gut microbes and you're doing most of those eight things we discussed, it doesn't really matter what else you do or don't do because the majority of your plate every day is going to be full of good stuff. Whether you have eggs, whether you have coffee, whether it's good or bad, is not going to be crucial to your health. What is crucial is that the majority of the time you're doing the right thing. The 80% rule is what I love — 80% of the time you're doing things right and you don't worry about the odd time. You may have the odd trip to McDonald's once a month. If your gut health is good, it will cope with the rest.

All good scientists should change their minds when the data changes. And I've made lots of mistakes in my career. There was some data on margarine being healthier than butter. So I changed our fridge back — this is 20 years ago — to having these low-fat spreads and kicked out the butter. My wife objected. She's French speaking, so she wouldn't have any of it. She kept the butter, so we had separate compounds. And I realized that I got it wrong and now there's no low-fat, artificial ultra-processed spreads in my fridge. So I'm constantly changing my view on many of these things. I'm much more open-minded than I think I was before. So it's an exciting time to be a scientist.

What is important to realize is that science is moving on at a pace. It takes on average 20 years for doctors to change their practice from the time that a new scientific discovery has been proven. So there's a big lag between what the latest science is delivering and what the latest practice is — or what your insurance is going to pay for, etc. The public is stuck in the middle here. And I think what's important is to see when there's a consensus, when there have been a number of studies that are all pointing in the same direction and they've got someone they trust that is telling them, yes, the summary of this data convincingly is saying you need to do this.

I think the lesson here is that the public needs to up their game a bit to be a bit more educated. You need to disregard test tube studies. I can do the same thing in mice that will probably be pretty irrelevant. Getting people to think, has this been tested in humans? Was there a placebo arm? Is this really going to change the practice of most people without just jumping on every bandwagon? At the same time, realize that the guidelines that are out there from government are very behind the times — and realize that a lot of the stuff you might be seeing on social media is probably too far ahead of the time, just so someone can talk about a test tube study or a mouse study that sounds exciting or wacky or accounts for some weird theory.

Why fad diets fail your gut

There are lots of people looking at paleo diets or keto diets. People who only eat meat or people who only have really high levels of fat above 70% that claim miraculous benefits. Most of these diets work short-term because if it's away from ultra-processed foods, then generally it's going to be healthy to that extent. People who go on a carnivore diet — which is an idea of what our ancestors ate, a very selected idea of what our ancestors ate — they will feel benefits for a few weeks, usually lose weight. Those people will then tell everyone about it. Isn't it amazing?

People who go on very strict gluten-free diets, keto diets, carnivore diets, seed-oil-free diets — they like the idea of excluding things so that suddenly you're belonging to this diet. You're belonging to this club of people that are excluded. Perhaps that's the attraction, and that's why most religions have some food-based ideas that keeps that community together so that they become a little group. They feel better for the first six weeks, and that's when the problems start. They may be very hard to sustain, like a keto diet is really hard to sustain. Those high levels of fat — most people feel rather sick.

Long-term, your gut is suffering. Any restriction to the amount of plants that are going into your gut will start to have an effect of killing off the good ones and making the overgrowth of the bad ones that are just eating the scraps of what you're eating. Long-term, you have immune problems and gut health problems and mental problems down the line. I think exclusion diets will work short-term and people might want to do that as an experiment to see how they feel, but then they've got to long-term start looking after their gut health, which is the thing that's going to keep them alive and keep them free of diseases.

What I like about our message is that it's not about exclusion. It's about inclusion. At the very best level, all that matters is you're getting that big variety of those 30-plus plants.

On organic food, pesticides, and fish

I get a lot of questions about organic food, for example. Should we only be eating organic food? How bad are these pesticides and these herbicides? These are great questions that don't have simple answers. It's always better, I think, to eat non-organic vegetables than not to eat them at all. Yes, fruits and vegetables are sprayed with pesticides and herbicides that are probably slightly negative for our health, particularly if you're pregnant or a young child or you have some immune problems or you're sick. For most of us, it's a low-level harm.

We do know that these pesticides harm our gut microbes. They've been shown to be relatively safe for our genes — they don't alter our genes — but they do interfere with the genes of our microbes. In theory, I don't want to eat pesticides at all. The reality is it's tough. At home, what I do is I get organic food delivered on a regular basis, but when I'm out and about, I'm not that worried about it, because it's better to eat vegetables with some chemicals on them, wash them off, than not to eat them, and deprive your gut microbes of a meal.

Some foods I would worry more about than others because they're not all the same. If you look at oats, oats have a terrible record because they're sprayed multiple times before harvesting, so they can have many times the levels of other crops. I'd always go for organic oats. Similarly with berries — if you can get organic strawberries that aren't sprayed, they'll be much healthier. If you're talking about things like oranges or avocados, I'm less worried because you're not eating the skin. A lot of the pesticide is in that skin.

It's about not exposing yourself to things over long periods of time — not the occasional event — that's really important.

Another thing people ask me about is mercury content of large fish. If I'm a fish fanatic and I'm having fish two or three times a week, I wouldn't want to be having those same large fish because I would be building up high mercury levels. I'd be going for smaller fish, which are also more sustainable for the oceans. My own personal choices of food are that I'm thinking about the health consequences of the food, some ethical considerations about it, but also the effect on the planet. Fish is particularly pertinent to that because most of the fish we get is grown in fish farms, some of which are good, some of which are bad. So it's good to be thoughtful, but in general, go for small fish that are more likely sustainable and also have very high levels of the omega-3s that are really good for your brain.

On salt

I'm often in restaurants and people are very worried about the salt content and they're asking for no salt to be added to their food. This is a subject that I was interested in because I had high blood pressure diagnosed — came on suddenly about 15 years ago — and I tried salt restricting for a couple of weeks to see if it would change my blood pressure. I didn't notice the difference, so I started researching this. And it turns out that for the average normal 50-year-old, cutting out salt virtually completely from your diet improves your blood pressure by about 1-2%. If you have a problem with hypertension, it might increase to about 4%. And when I looked in detail, it turns out that there's only about one in five people that are really sensitive to salt. Four out of five of us are not that sensitive.

What I also found out is that eating just something like beetroot can reduce your blood pressure three times more than salt restriction. Similarly, just changing your salt to a potassium salt can have three times the effect of salt restriction. So most people don't have to worry nearly as much as they are, and I think it destroys a lot of the pleasure of eating and stops them maybe enjoying vegetables as much or fermented foods as much because they're worried about the salt content.

There's really good data showing the salt you add at your table has a negligible effect on your health compared to all the rubbish you're getting from consuming too much ultra-processed food. When I was researching fermentation, I looked at kimchi eaters — and anyone who's had kimchi knows it really tastes really salty. And when you make fermented foods, you have to add a lot of salt. They looked at over 10,000 kimchi eaters in South Korea compared to non-kimchi eaters of the same age, and they actually had lower blood pressure than non-kimchi eaters. So the benefits of having fermented foods and plants — and plants contain potassium — outweigh any effect of the salt.

On supplements

A lot of people also think that diet isn't that important if you've got a rich supply of supplements, and there's a lot of marketing directed at people to say you don't have to worry about your diet — you've just got this green powder you take every morning with 80 different chemicals and vitamins, and that's all you ever need. Again, that falls into this idea of reductionism, a nice simple solution to a complex problem that by definition is usually wrong.

All the studies suggest that eating the food is always better than eating the chemical that has been derived from that food, because there are other things in the food that aren't represented in that highly purified chemical — often made artificially by yeast in factories, mostly in China, with all kinds of additives — that make it very different to just having that from a natural plant.

The example I like is that for years I was giving calcium replacement tablets to my patients in rheumatology, thinking I was helping their bones, and they'd be taking maybe a gram a day. It turns out that taking something like that, your body doesn't deal with it, and it's now been associated — over six studies — with increased risk of heart disease and atherosclerosis, with the hardening of the arteries. So the calcium is being deposited in places that you don't necessarily want it, because it's given in a form that the body's not used to. We normally break down calcium in tiny amounts from all the foods we're eating or from mineral waters, and it gradually gets incorporated into our body. So taking one big dose of it can have very different effects.

"If someone's offering you a single solution to a nutrition or a gut problem, it's probably rubbish. They're probably just trying to sell you something."

And things like vitamin D — the body converts sunshine into vitamin D and that's how we get most of it, and that's how we've evolved. But taking vitamin D is not a replacement for sunshine, because there are many other things in sunshine that we still don't know about that provide our bodies with a lot of benefits. There's a lot of epidemiology comparing people who go in the sun and out of the sun showing that even after having a melanoma, for example, people who go in the sun have a better immune system and a better outcome than people who are maybe just taking vitamin D tablets.

So often we mistake a subset of a food for being better than the whole food or the whole environment like sun. We love to reduce foods to one thing, we can manufacture that in huge quantities, marketing does the rest, and what seems like a good idea is often really a pretty poor one and can often be harmful.

The science of nutrition, as I've got into it really without biases over 15 years, is one of the most complex sciences we have. We were told it was the simplest. All calories and macronutrients. Turns out it's really complicated. That means there isn't one single product or solution to it. There isn't one vitamin that's going to cure you. We need to take a more holistic view of it — and that's a really important message.

We need to be doing things that aren't just for a six-week diet, aren't just to lose weight. It's: how do you change all your habits that you've accumulated, all these bad habits, how do you change them in this direction going forward and do this for years so that your body is much healthier, your brain is much healthier.