Evidence Review: Health Benefits of Green Tea

14 min read

Who doesn’t love a good cup of green tea on a rainy day or in the winter time? Those of you who are regular green tea drinkers are probably familiar with some of its health benefits. If you’re not, check out this walkthrough of the beneficial effects of green tea.

As always, I’ve provided a short, condensed summary at the end of the article, if you want to skip the majority of my efforts and information. I suggest reading the article in chunks.

A short introduction

Green tea is made from the leaves of a plant called Camellia sinensis. The origins of its use date a few thousand years ago back to old China. There it was used as medicine to help control bleeding, heal wounds, regulate body temperature, and even digestion.

Tea from this plant is consumed in different parts of the world as green, black, or oolong tea. From these three, green tea has been shown to have the most impressive health benefits on human health. To get green tea, the freshly harvested leaves are immediately steamed so that they can prevent fermentation. This process, as well as others, keep the polyphenols found in green tea. When green tea gets fermented to oolong or black tea, these polyphenols undergo chemical changes because of which their biological properties change. As a result, their health benefits change.

Polyphenols are organic chemicals which have large amounts of carbolic acid in their formula. Hence poly (= a lot), phenol (= carbolic acid)

I would like to focus on green tea in particular as it is the type of tea that’s being consumed most widely.

teacontents

In the picture, you can see the amount in percent (%) of certain macro- and micronutrients for green and black tea, as well as for an infusion.

 

Flavonoids

Flavonoids are plant secondary metabolites, this means that they are not directly involved in the growth, development, or reproduction of a plant. They often have a defensive role against herbivores. In general, they are widely available in plants, are the most important plant pigments for flower colorations, but they have many other roles as well.

Green tea is unique in the sense that it has multiple compounds which have been shown to have some interesting health benefits. These compounds include flavanols, flavandiols, and flavonoids (I am not joking, those are real names). Flavanols are also commonly called catechins. One major catechin which is prevalent in tea and has been extensively studied is epigallocatechin gallate (EGCg). Other catechins found in green tea are epicatechin, epigallocatechin, and epicatechin-3-gallate.

These mentioned compounds, with difficult names to remember, represent around 40% of the dry weight of green tea. Interestingly, the average content of flavanols in a cup of green tea is higher than that of other types of food or drinks. This includes fresh fruits, vegetable juices or wine.

Catechins are natural phenols and antioxidants. They are a part of the flavonoid family.

So all the health claims which you can hear about the mentioned groups due to their flavonoid contents are trumped by the number of flavonoids found in green tea. Moreover, green tea has other important compounds with proven health benefits. These include theanine, caffeine, theobromine, and theophylline. More on this a little further down the line.

I’m really sorry for the hard-to-remember-names. I wanted to mention them so you can remember them for future references in this article. The majority of products derived from green tea are mainly extracts of green tea in liquid or powder form. And these mostly vary in their proportion of polyphenols and the caffeine content.

teaprocess

This picture shows us how the amount of catechins goes up, and the amount of theaflavins and thearubiginis (two polyphenols) go down.

 

Just another thing before I start

I’d like to emphasize something. Green tea is one of those food items which has its real (= proven on humans in multiple randomized clinical trials) health benefits intertwined with wild health claims, speculations, and shoddy science.

I’ve looked far and extensively to pick out health benefits which are supported by randomized clinical trials (RCTs). This means that they took green tea and applied it in different settings to see if it would affect a certain health outcome. After this application period, they observed whether the group, who was administered green tea or its extract, would show a different outcome compared to a group who didn’t receive anything.

Why am I saying all of this?

Because you can find a lot of information lying around on the internet, about the health benefits of green tea, which stems out of observational evidence. This is exactly what it should be – an observation. While researchers fully understand what that means, the public likes to jump to conclusions in some cases, either due to media hype or misinformation.

With that said, observational evidence can only give you a connection between two variables, such as cancer and green tea consumption. However, that connection doesn’t tell you whether it was green tea which influenced cancer outcomes, or whether people who consume green tea also make a variety of other lifestyle choices which influence cancer outcomes in a positive way.

With that out of the way, let’s start now.

 

Cancer

There have been many observational studies that show the preventive effects of green tea consumption. In some, these results are inconclusive [1], others show green tea can be a preventive measure against breast cancer, while others show that drinking a lot of green tea significantly reduces your risk of lung cancer. [2]

You can see that a gathered fraction of observational studies show different results. This is the issue with observational evidence I’ve been mentioning. A lot of such studies are done in different ways, they differ in how they gather data, and how strictly they control variables, as well as a ton of other things which can affect results.

A meta review (a review of a lot of studies done on the topic) from 2012 stated that:

…the health benefits about cancer are so far inconclusive and inadequate.

Colorectal adenoma

Colorectal adenoma is a benign (doesn’t have the ability to invade neighboring tissues) tumor of the colon and the rectum. It is usually a precursor to colon cancer.

Two randomized controlled clinical trials, however, do show promising health benefits. There is a smaller chance of colorectal adenoma relapse, which means there is a smaller chance of it appearing again [3]. It also slows down tumor growth which can lead to prostate cancer. [4]

In the first study, people were supplemented for 12 months with 10 cups of green tea and 1.5 g of green tea extract per day. They had a colorectal adenoma removal prior to the study. They wanted to know whether there would be a smaller chance of it appearing again if they were supplemented.

In the second study, men were receiving 600 mg of green tea extract per day for over a year. Those who were supplemented saw a 3% increase in tumor development, while the other group saw a 30% increase in tumor development.

 

Obesity, weight loss, and thermogenesis

Drinking various amounts of green tea or supplementing with green tea on a daily basis reduces your belly fat and your waist circumference. Most studies use between 500-700mg of green tea extract per day for 12 weeks. Don’t expect miracles, we’re talking about small amounts, around three kilograms was one of the higher recorded amounts. In others, it was much less – the lowest was 0,15 kilograms after 12 weeks. [5, 6, 7, 8]

Due to the cocktail of compounds found in green tea, it has thermogenic effects on your body as well. It increases your bodies energy expenditure when you drink it. Again, don’t expect miracles. One of the mentioned studies observed a 4,6% increase in energy expenditure. This is the foundation of many wild health claims about green tea. I’ll leave the decision whether that number is something to fuss about up to you. The increased expenditure is believed to be due to its caffeine and different polyphenol compound contents.

 

Heart disease, blood pressure, and cerebrovascular disease

Cerebrovascular disease is a group of brain dysfunctions which are related to issues with the brain’s blood supply. Usually, the most important cause is high blood pressure which damages the blood vessel lining which supplies your brain.

High blood pressure is a major risk factor for heart disease and cerebrovascular disease. Drinking more than three cups of green tea per day significantly reduces your risk of stroke as well, possibly by improving the functioning of your inner blood vessel lining. [9]. The latter is the finding of an observational study.

Moreover, it reduces blood pressure in clinical trials as well. [10, 11]

However, two clinical trials actually observed increases in blood pressure after people were given five cups of green tea per day for 7 days [12]. Albeit, these increases were observed only after a short period after consumption. They were also really small and not clinically significant in any way. Furthermore, these studies lasted for 7 days, while the above-mentioned two lasted for 24 and 3 weeks respectively. This explains the differences, it seems that the there are no increases in long-term health, but there are some increases in a short time period.

 

Cholesterol

Green tea reduces overall LDL cholesterol, in some even total cholesterol [1011, 13]. People were given between two 500 and 700 mg of green tea extract for various durations, between 3 and 24 weeks. This depended on the study. All of them converged to the same effects. However, I must mention that a decrease in LDL cholesterol doesn’t necessarily reflect an increase in healthiness. Larger LDL molecules, which are more effective carriers of cholesterol, are not a good predictor of an increased risk for heart disease, as was previously thought.

Are you still with me? Things are about to get interesting.

 

Antioxidant effects

Let’s explain a few concepts first.

Reactive oxygen species

Reactive oxygen species (ROS) are chemically reactive molecules which contain oxygen. Usually, they are a natural byproduct of normal oxygen metabolism, they also get produced during radiation, and at high temperatures. However, at times of increased stress from your environment or your own body, they can increase dramatically. This damages cell structures. They are useful for the destruction of cancerous cells in higher quantities, but not in normal cells.

Oxidative stress reflects the imbalance between a manifestation of an increased number of reactive oxygen species, and your body’s ability to detoxify these molecules or to repair their resulting damage.

An increased accumulation of ROS is the main sign of oxidative stress and can cause damage to proteins, our DNA, and even lipid damage [14]. And these events are strongly connected to different types of cancer, heart disease, and even neurodegenerative diseases and the aging process. [15, 16]

This is where the antioxidant effects of green tea come in.

Antioxidants prevent oxidation of other molecules, and oxidation reactions (among others) are the ones that produce reactive oxygen species.

Drinking green tea increases your plasma antioxidant capabilities, this makes your body more effective in fighting off free radicals [17]. It also causes significant reductions in oxidative damage to your DNA and increases the amount of antioxidants in your body [18]. As noted in people who drank two cups of green tea (equal to 250 mg catechin) for 42 days.

Furthermore, pump workers, who are often exposed to benzene; a chemical which induces oxidative-stress related toxicity, lessened their benzene-induced toxicity and saw significant improvements in their antioxidant status, after they had to drink six cups of green tea per day for over six months. [19]

Given this knowledge, we are safe to assume that drinking green tea on a daily basis will give us important health benefits as far as the antioxidant capabilities of our bodies go.

But I must mention that the common perception of antioxidants is very wrong:

  • They work synergistically – you need consume a number of antioxidants for them to work properly, if you’re missing one, it’s like you did not consume anything.
  • Increased amounts of antioxidants can have harmful effects on your body, this can be reflected in a variety of ways.

And a quote straight from Wikipedia:

Although first studies suggested that antioxidant supplements might promote health, later large clinical trials with a limited number of antioxidants detected no benefit and even suggested that excess supplementation with certain putative antioxidants may be harmful.

Which really makes you think how strong the supplement industry is. If you would like to read more, the article on Wikipedia about antioxidants is really well written.

 

Brain health

And now to the less studied part of green tea.

When you have Alzheimer’s disease, there is an increased accumulation of so called amyloid beta plaques. In animal studies that have been done so far, the before-mentioned EGCg and catechin decreased this accumulation. This suggests it could be used to cut cognitive deterioration and improve your brain health.

Observational studies support a smaller chance for Alzheimer’s and different types of dementia if you consume green tea as well [20]. Some evidence even suggests that catechins have other positive brain health benefits which are not related to Alzheimer’s. They improve your brain’s metabolism, how well the neurons in your brain fire, and how much energy it produces. [21]

One particularly interesting study observed increased brain activity in the part of your brain where your working memory is [22]. However, a greater brain activity does not necessarily mean you will automatically be able to calculate infinitely greater numbers than you could otherwise. It simply means those parts of the brain are more active. Further studies are needed to test whether green tea improves working memory.

 

Immune system and bad breath

It reduces bad breath and can be used as a preventive measure against dental caries. [23]

EGCG from green tea can help boost your immune system as well. One study noted that EGCG prevented the binding of human T-cells,  which is the first step in an HIV infection. However, they strongly emphasized that green tea is in no way a cure, a safe way to avoid infection, nor a treatment, it can simply improve the quality of life for people who are infected with HIV. [25]

And something I’m not sure about.

It’s possible that green tea reduces your stress levels and improves your subjective comfort. L-theanine is an amino acid which is also found in tea and has a tranquilizing effect on the brain. L-theanine has been linked to subjective reports of being more relaxed in one study. However, I’d remain very skeptical of subjective reports in studies. They are even worse than observational evidence, and are often a consequence of a placebo; a dummy treatment with no real effect, or an experimenter expectancy effect; the experimenters receive results that they want because they act in a way to maximize the probability of receiving such results throughout the study.

However, to counter my skepticism, some reports show that it induces a certain pattern of brain waves which basically reflect a relaxed but alert mental state. This pattern was especially observed in women who were highly anxious. This suggests it could be possibly beneficial for such women.

 

The side effects

You might be thinking by now:

Well, gosh, I should drink a ton of green tea per day since its so healthy”

And that would be a smart idea if it weren’t for its toxicity in very high doses, partly due to its antioxidant contents. Its compounds can cause headache, nausea, confusion, and muscle pain as well. Besides the usual symptoms you can experience from caffeine; however, the caffeine contents are around 30% lower in green tea than in regular coffee. Moreover, drinking green tea is not recommended for patients who are taking beta blockers, lithium drugs (for bipolar disorder), estrogens, clozapine etc. The reason for this is that catechins bind to the drug molecules and make those drugs less effective, some results even suggest it even reduces the effectiveness of blood pressure lowering drugs. [26, 27]

 

The grand summary

  • Green tea reduces blood pressure, however, the effect is very small.
  • Green tea has preventive effects in colon and prostate cancer, reduces relapse rate and slows tumor growth.
  • Green tea reduces LDL cholesterol and total cholesterol, which is not necessarily a good indicator of a healthier blood profile.
  • Green tea helps you lose waist circumference, belly, fat, and increases your basic energy expenditure. However, these effects are really small. If you’re thinking “Well, then I’ll just increase the dose”, Green tea can be toxic, see below.
  • Green tea increases antioxidant activity in your body, which isn’t something that is necesarilly beneficial.
  • Green tea in high quantities can be toxic with adverse side effects (nausea, muscle cramps, confusion …).
  • Green tea in animals helps with brain health through various ways, it helps with Alzheimer’s, different types of dementia, and can improve brain performance overall.
  • Green tea reduces bad breath and dental caries.
  • Green tea may help boost your immune system.
  • Green tea may help you feel calmer and more tranquil.
  • Green tea isn’t recommended for people who take beta blockers, lithium drugs, clozapine, or estrogens, because it can make these drugs less effective.

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