Instant Genius - The potential health benefits of green tea

Episode Date: April 11, 2024

Be it drank first thing in the morning, as an afternoon pick-me-up or to soothe our souls after hearing some bad news, we Brits love a cup of tea. But over recent years, the traditional cup of black builders’ tea has found a new rival vying for our attention in cafés and on supermarket shelves – its more traditional cousin green tea. Many people are making the change not for reasons of flavour but for its supposed health benefits. But is there any truth in this? In this episode I speak to Justin Roberts, a professor of nutritional physiology based at Anglia Ruskin University in Cambridge. He tells us all about the nutritional content and potential health benefits we can gain from drinking this ancient beverage. Learn more about your ad choices. Visit podcastchoices.com/adchoices

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just as the artist intended. Visit name audio.com to learn more. Hello and welcome to Instant Genius, a bite-sized masterclass in podcast form. Each week, you'll hear Will leading scientists and experts talking about the most fascinating ideas in science and technology today. I'm Jason Goodyear, commissioning editor at BBC Science Focus. Be it drunk first thing in the morning, as an afternoon pick me up or to soothe our souls after hearing some bad news. We, Brit. love a cup of tea. But over recent years, the traditional cup of Black Builders tea has found a new rival, firing for our attention in cafes and on supermarket shelves. It's more traditional

cousin, green tea. Many people are making the change, not for reasons of flavour, but for its supposed health benefits. But is there any truth in this? In this episode, I speak to Justin Roberts, a professor of nutritional physiology based at Angela Ruskin University in Cambridge. He tells me about the nutritional content and potential health benefits we can gain from drinking this ancient beverage. First off, Justin, welcome to the show. Yeah, thank you. Thanks for inviting me to be here. So we're talking about green tea today and it seems to be everywhere at the moment. But what exactly is it, you know, sort of bottomly speaking? So this is a really interesting question because when I first got into research around green tea, it wasn't because of tea, it was for another

reason, which I'm sure we'll talk about. But I didn't actually realize just how many different types of tea there were. We've got, you know, green tea, black tea, white tea, uh, white tea, uh, uh, uh, uh, uh, so there's lots of different types of tea. You know, what exactly is green tea? Well, all tea comes from generally the same plants. Uh, it's called chamelea synensis, which is a type of plant. So the green tea that leads from that plant is how we get green tea. But we also get black tea from the same plant, um, as well, um, as well, we're, um, well. So it all comes from the same general tea plant. So what's the difference between green tea and black tea then? That's a great question. So it's often to do with how the tea leaves are

processed. So as I said, green tea, black tea are all made from the leaves of this particular plant called camellia scents. The key difference is that black tea is effectively oxidised, whereas the green tea is not oxidised. And what that basically means is that to make the tea black or to make black tea, the leaves are basically rolled, then exposed to the air. And because they're exposed to the air, it triggers this oxidation process. So therefore, this might affect the type of nutrients or compounds in the tea, specifically being the difference between green tea and black tea. So as an example, we know that teas contain some very potent nutrients called antioxidants,

and a subclass or a category of that is called polyphenols, which are plant-based antioxidants. and there may be differences between the types of antioxidants in the teas. So as an example, black tea contains lots of different antioxidants, but one in particular called a thiaflavin, whereas green tea contains higher amounts of what are called catechins. And these compounds have potent antioxidant effects, but they may differ between the two.

So that's the kind of key difference, how they're actually processed or treated after the leaves are taken from the plant. So another type of tea that people might have heard of is called matcha, and you see it in all sorts of things like flavouring ice creams or whatever. But what's the difference there between the sort of regular green tea and this matcha? Great question. So green tea is often, in traditional speak, is called sentia tea. Matcha tea is called tentia tea with a tea, tentity. And what it is is that mature tea involves shading the plant from the sun.

so there's less intense sun get into the plant. And then when the tea leaves are ready to be harvested, the leaves are steamed, then they're dried, and then they're ground into a powder. So effectively with matcha tea powder, you're getting the whole tea leaf, if you like. So the theory is that it may have more active compounds within the matcher tea,

so therefore it may actually give you more nutritional benefits than a regular cup of green tea. But before I say anything else, it's important to remember that, you know, the research in this area is, whilst there's a lot of research already, the research into Machete specifically is still kind of young and we still need to learn a lot about matcha tea. So let's have a look at there. How much caffeine does it have been compared to other drinks, you know, black tea or, say, a cup of coffee? Again, a great question. But so I think the first thing to say here is it really depends on a lot of different factors. You know, if I start with kind of the generic caffeine content, if you look at, say, a cup of green tea, you might have, you know, anywhere between around 10 and 30 milligrams of caffeine for, say, a 200-mill cup of tea. If you look at match a tea, it might be, you know, over double that.

So maybe sort of around 50 to 80 milligrams. And in contrast, if you compare that to, say, coffee, a cup of strong coffee might give you around 120 to 100. So, you know, you tend to see that the matcha tea and black tea is quite similar in caffeine content, and it's more than green tea, but it's typically less than a coffee, for example. But as I said at the beginning, you know, this comes back to a lot of different factors like the time when the leaves were harvested, the age of those leaves. How you brew them makes a big difference as well. So the temperature of which they've brewed, how long the tea is sitting in the water can also have to an extent an impact. So all of this may affect the caffeine content. And this is why you see in the literature, you see a very high variance in types of teas. So talking about brewing then, is there any difference between making it using loose leaf tea compared to a tea bag? To an extent, yes, that's a good question. So basically, loose leaf tea may have more of the bioactive nutrients than if you like a higher quality leaves or tea bags.

But the leaves in tea bags are just cut smaller, which means that you might actually get other nutrients. it's in there. So for example, you know, you might have more particular antioxidants. I would say it all comes down to how long you leave the tea bag in. So if you put the tea bag in some hot water and just, you know, two seconds and it's out and you've got your cup of tea, that's completely different if you leave it in there for, say, two to three minutes or even if you leave it in there for like 20 minutes, which I'm sure most people don't. So I think it comes down to how long the tea is brewed for and also the quality of that tea. So you've mentioned this a little bit earlier, but chemically what inside the green tea is supposed to bring these health benefits that people talk about.

Again, that's a kind of tricky question to answer because, you know, you're probably, if you're reading around tea and particularly green tea, you'll hear the word catechins or polyphenols a lot. But we need to remember that all the teas that are out there have what are called bioactive nutrients in them. So if we look at green tea as an example, it will contain things like catechins. There will be things like saponins, chlorophyll, minerals, vitamins, there's a lot of different nutrients in our teas. One of the big differences between, say, green tea and black tea is maybe the content of catechins, which are these antioxidant components of green tea. They're more potent in green tea. But then things like black tea might have more of a

nutrient called fiendy, which is another nutrient that has slightly different effects. But if we just focus on the categents, for an example, we know that the green teas contain, you know, a cup of green tea would contain around 60 to 80 milligrams of catechins, which is kind of, you know, more like double that of black tea. But if we look at matcha tea, the theory and some of the research I've seen suggests that there might be nearly three times the amount of catechins per serving. And in fact, there was one study I saw that suggested that matcha tea had a staggering 137 times more catechins than regular green tea. And that's only one kind of report, but the point being is that match a tea might be quite potent in these antioxidants and then

green tea, then black teas, if we're just focusing on the catachines. So let's have a look at some of these potential benefits then. So I think your work's focused on the link between green tea drinking and an increase in metabolic rate and potentially fat burning. So what do we know about that? Yeah, this is where my original interest started from. I've actually been quite interested in sports nutrition for a number of years and kind of more into health nutrition. And one of the things that caught my attention was this idea that there are numerous compounds and numerous foods or supplements that might help us to burn more fat when we exercise. And the reason I'm interested in this is because for many people, that's a challenge in terms of their ability to not only

sustain exercise, but also to get the most out of their exercise. So we did a study firstly with some healthy individuals to look at whether a supplement of green tea. So not taking green tea as a drink, but supplementing it, which I can come back to. Whether that had any benefits in the short term on how they utilise fat as an energy source during exercise. And we found that it had a positive impact, which supported some of the other literature in this area. We then went on and looked at a much bigger study looking at older individuals or less active individuals. And we found that the green tea had an effect to an extent. but it took a lot longer, it took almost double the amount of time for it to have an effect

if we look at supplementing these particular extracts. And so the reason we did that is because with an extract, number one, you can standardise it and control it, but also we can give you quite a high amount without necessarily having to drink, say, five or six cups of tea a day. So that's the reason we did the supplement approach. Ambition comes in all shapes and sizes. At First Citizens Bank, we roll with your goals.

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Try it for yourself at a focal powered by name boutique. visit vocal powered by name.com for more information. So another thing that I've read is that it's been suggested that drinking green tea can help to lower your blood pressure. I mean, is there any truth to that? Yeah, there is. Whenever you're looking for research, one of the first things that a lot of people do is they will go to search engines like PubMed or other engines, and they will look for what is called a meta-analysis or a systematic review. And so if you're listening to this and you're thinking, I want to learn more about a particular area, You could, for example, go online and look at something like public. And you could type in, you know, what is the effect of green tea on blood pressure,

meta-analysis or something like that. And what you'll get is some information on what are called collection of studies. So this is where researchers have tried to pull together different randomized controlled trials and to look at the effects between those trials or overall. So in terms of blood pressure, there is a number of meta-analysis that have been recently published. And what they've done is they screen the literature for lots of different studies, but then they try to, using a standardized method, pick out the kind of top papers that then they can compare. And so when it comes to green tea consumption, there is a meta-analysis that has suggested that

things like blood pressure. So with blood pressure, you've got what's called your systolic blood pressure, and you've got your diastolic blood pressure. So think about when your heart is beating, it kind of squeezes and then relaxes. And so when it's squeezing, you've got more pressure going through the heart. That's kind of referring really to the systolic blood pressure when it's going through the relaxation phase of the heartbeat. That's really relating to your diastolic blood pressure. Now, in some people who have high potential or high blood pressure, it's kind of an advantage to see a reduction in some of these measures. And there is a meta-analysis suggesting that green tea consumption, I believe that's by tea drinking, not necessarily supplementation, has an impact

on reducing both systolic and diastolic blood pressure by a couple of units of blood pressure. So it doesn't sound like much, but what it suggests is that green tea is having an antioxidant effect at some level, and then that might be impacting on our blood vessels, if you like, and how we control blood pressure. So one of the key mechanisms that comes out is that green tea may support what's called nitric oxide production. And what nitric oxide does is it helps, if you like, increase the vascularisation, so it allows the blood pressure to be more normalised as opposed to constricting our blood vessels. And within that, there have been studies showing both in the short term and the long term that Brinty can have a favourable effect on blood pressure.

So another thing sort of related to that is I've read some studies saying that perhaps can lower cholesterol. Yeah, again, so whenever I get a question like this from a researcher perspective, I will go and look at what's being done in the literature to start with to see if there's any information on this and also have to get a sense of what's been done prior to that. And again, I would probably start with either a systematic review or a meta-analysis. And if we're looking at, for example, cholesterol, there are, again, meta-analysis on this that look at cholesterol in the context of cardiovascular risk factors. Interesting enough, there's a couple of meta-analysis that have slightly conflicting information. So there was one meta-analysis I found where they looked at over 50

randomized controlled trials and they showed that supplementation of green tea. So now we're looking at the supplementation of green tea, not necessarily drinking green tea, significantly had an impact on things like total cholesterol and potentially lowering total cholesterol, but also lowering what's called LDL or low density lipoproteins, which some people refer to as bad cholesterol, if you like, while simultaneously increasing what's called high density lipoproteins, which is often referred to as good cholesterol. So what we're seeing is potentially an impact of the green tea supplementation on lowering cholesterol whilst changing the sub-components of cholesterol in favour of what are called high-density lipoproteins.

Now, what's interesting, if you ever go to those studies, they also look at it in terms of how long people supplement for and what dose they supplement for. And often what you read in the literature is that actually the effects can be seen in the shorter-term, particularly with lower dose green tea consumption. There are other studies that have not challenged this, but sort of suggested we do see a change in things like LDL cholesterol and total cholesterol, but not necessarily high density, libo-protecholesterol.

And this one is more to do with the consumption of green tea as a drink. So what that tells me is that maybe it's the type of categens and those of those categings that are having an effect. So another one that I was interested in was the link between drinking green tea and staving off cognitive decline. So, I mean, is there any truth to that? Yeah, this is a tough one to answer because, yes, there is evidence out there, and it kind of depends on how you read that evidence. So one of the things that's quite interesting behind this is that green tea contains compounds like caffeine, if it's caffeinated green tea, you can get decaffeinated extracts as well. And it also contains things like elthianin. And the idea is that they may have impacts on cognitive function

particularly attention, and they may have an impact on things like alertness, decision-making, memory, and focus. And in fact, I've seen a really good paper looking at Matcha Tea, where they've given participants a fairly reasonable dosage of matcha powder. I think it was about four grams in that study as a drink. And they scored better on cognitive tests, which looked at things like how you process information and how you remember things. But there's also been a suggestion that taking several cups of green tea a day may impact

or even prevent, I think it was the language used in the paper, on age-related cognitive decline, which properly comes back to a combination of these stress-reducing or calming effect of el-thenein that you find in tea, but also the antioxidants, particularly the catechins, which may be particularly high in matcha-tree. So maybe it's a balance or a combined effect of these nutrients in tea that is having a protective effect. Having said that, just going back to the concept of, randomized controlled studies, when you start to look across studies, you do see evidence that in the longer term, so if I'm drinking green tea for a year, should we say, or even longer, that you may not necessarily see significant changes, particularly in elderly populations.

So all of this sounds really promising, and I bet many people listening are going to rush to the shops after listening and go and buy themselves some green tea. But is it possible to drink too much? that's again you're asking really good questions here so well how much is too much let's start with that and i think if you're going to look at what is too much we know that there are positive benefits of green tea if you kind of step back from all of the research that's being done there's an overwhelming evidence to support the health benefits of green tea and tea drinking generally because of these polyphenols in them we know that the tea contains these antioxidants called flavonoids which is a type polyphenol, which may have a protective effect on your health. However, we also know that tea contains

caffeine. And so if we're focusing on, for example, the amount of caffeine we're drinking, then maybe we should be mindful of not overdoing it. So some of the recommendations I've seen suggest that if you want to get positive effects of drinking green tea, somewhere between three and six cups a day seems to come out from the literature. And maybe if you want to not overdo the caffeine, the upper end of the consumption might be somewhere between 10 and 15 cups a day. And I'm sure not many people are actually doing that repetitive, which you would say. However, if you like stronger teas, maybe you want to bring the number of maximum cups a day to under 10. So we've talked about a lot of different things there that green tea's been linked to.

What do you think is next for the future of green tea research, you know? What do we want to find out? It's a million dollar question. So, you know, ultimately, we need to learn more. Let's start with that. I mean, there's a lot of research out there, but there's also a lot of questions that haven't been answered yet. So, for example, in my area, we still need to understand more about why certain green teas might impact metabolism and why others don't. We still need to look at dose responses in terms of why do some studies show an effect with a lower dose, but not a higher dose and vice versa. What's interesting with match a tea is because it's It's in a powdered form. You could actually add it to foods so you can have mature foods. And that hasn't really been explored in a food context as far as I'm aware to a high level. So maybe we could look at the use of mature tea within certain foods and whether that has an impact.

We probably need to learn a lot more about how green tea and particularly match of green tea affect the gut, specifically the gut flora and how that might impact our health. Although there are studies sort of suggesting that green tea, has a beneficial impact on our gut. We probably also need to learn more about what are called signaling pathways. So as we age, we know that these different signaling pathways in ourselves could be linked to how the cells protect themselves, how they defend damage, if you like. And polyphenols in plants, particularly the cations in green tea,

might have what's called a calorie restriction memetic effect, which basically means that they stimulate the body via these pathways, similar to if you were on a calorie-controlled or calorie-reduced diet. And that might also link to how we burn fat, but also how we age as well. So I think some of the research could look more towards that direction. And within that, you could ask questions around, are there differences between male-females? Are there differences between younger, older people?

are there differences between normal weight and overweight people, for example? So there's a whole wealth of research we probably still need to learn more about. And then also the other thing that, you know, when you're doing research, you're trying to control, if you like, or standardise the way you set up the study. So the only thing you're really looking at is the variable you're tested, in this case, green tea. But often when you do that, you try to control the diet. So you often recommend that people go on a lower polyphenol diet whilst you're supplementing green tea. My question is, if you are eating a normal mixed diet with fairly high amounts of these polyphenols in your foods, would the green tea be as effective?

So maybe looking at the dietary control with or without a low polyphenol content could be useful as well. So it sounds like there's still plenty to be learned. Absolutely. And it's an area of research interest that I'm very passionate about. So if any of your listeners want to reach out and connect on this, I'll be very happy. Thank you for listening to this episode of Instant Genius, brought to you from the team behind BBC Science Focus. That was Professor Justin Roberts. If you liked what you just heard, please consider subscribing to Instant Genius on your preferred podcast platform. The current issue of BBC Science Focus magazine is out now.

Pick up a copy wherever you buy your favourite magazines or download a copy on your app store of choice. You can also find us online at sciencefocus.com. This podcast is sponsored by Name, Audio and Focal. The texture and emotional depth of music can be lost through digital sources or poor signal. Name Audio believes you can have digital precision with analogue warmth. Alongside French acoustic specialist focal, name creates high-end audio systems, combining innovation with craftsmanship, so you can listen to music, just as the artist intended.

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