Green tea is a type of tea obtained from the leaves of the Camellia sinensis plant, but it doesn't appear black because it hasn't undergone the drying and oxidation processes like oolong tea or the more familiar black tea. Green tea was first produced in China and has since spread throughout the world (in Turkey, it is particularly produced in the Eastern Black Sea region). Although the term "green tea" refers to a single type of tea, it actually has several sub-varieties. Globally, black tea accounts for 78% of tea consumption, while green tea makes up about 20%.
Although green tea is portrayed as a "miracle" beverage on social media, mainstream media outlets, and websites, claiming benefits in every area from cancer and cardiovascular disease to controlling blood sugar and fat levels, reducing inflammation, and aiding weight loss, objective findings regarding the benefits of green tea are highly debatable in the academic community. This situation potentially makes green tea another "overhyped diet object."
This article will examine the general problems in green tea research, the claims made to date regarding the benefits of green tea, and the results of individual articles and field studies on this topic.
Problems with Research on the Benefits of Green Tea
As with many studies on nutrition , research on green tea has yielded both positive and negative results: some studies have found a positive association between green tea and a particular health factor, while other studies have failed to demonstrate this positive association, and some have even shown negative associations.
It's normal for scientists to arrive at very different and varied conclusions as they strive to enrich our knowledge of natural phenomena. However, the biased presentation of these findings through the media (for example, only positive articles being featured) leads to a distorted and misleading perception of public opinion regarding green tea or other foods. Many sources cherry-pick research that finds benefits of green tea, highlighting it in headlines while ignoring other studies that have found no positive or even negative effects.
The key point is this: we cannot form an opinion about the benefits and harms of green tea by only looking at studies that yield positive results. What we need to do is consider the entire literature, regardless of the results, rank these studies in terms of statistical power, and arrive at a holistic conclusion accordingly. When this is done, it becomes clear that the claims about green tea (especially the most popular ones, such as those regarding cancer, cardiovascular disease, and weight loss) are greatly exaggerated .
Meta-analysis articles reviewing published results to date have found that green tea research has not provided a sufficient amount of scientifically reliable evidence for any health benefits . [1] , [2] For example, in 2011, at the request of the European Commission, a panel of scientists reviewed green tea research and concluded that the findings did not support the claimed health benefits. Although consuming green tea (as with other types of tea and coffee) can increase mental alertness due to its caffeine-related effects, green tea has not been found to have a significant effect on cancer, cardiovascular risk, or weight loss.
A 2020 review by the Cochrane Collaboration found that, far from being beneficial, green tea has negative effects on digestive tract disorders, liver enzymes, and (more rarely) insomnia, high blood pressure, and skin reactions . [2] When all these are considered together, it can be seen that increasing and prioritizing green tea consumption in order to obtain a benefit in this regard may lead to erroneous treatment attempts, and even erroneous beliefs about green tea may result in the disruption of medical treatment.
The "Miraculous" Benefits and Facts About Green Tea
Having this background information, we can begin to examine the studies on the supposed “miraculous” effects of green tea. In this section, we will first present the academic studies that underpin some of the claims you can find in mainstream and social media, and then (if any) present the findings of meta-analyses conducted in this field. Unfortunately, comprehensive meta-analyses covering all the claims do not yet exist; therefore, we cannot present definitive conclusions regarding them.
We would like to emphasize that the individual studies/findings we will mention here regarding the benefits of green tea are genuinely academically conducted, published in reputable journals, and therefore their results can be taken seriously; consequently, we will not include information circulating in popular culture that is mostly unfounded, unsourced, and/or fabricated. However, as you will see , these individual findings will not be sufficient to claim that green tea is truly so beneficial.
Exaggerated and False Claims About the Benefits of Green Tea
Claim: Green Tea Helps with Weight Loss!
Perhaps the best known benefit of green tea is its contribution to weight loss. When you look at the ingredients of any fat-burning product, you are very likely to come across green tea. This is because green tea accelerates fat burning and boosts metabolism. [3] Caffeine in green tea also enhances physical performance by mobilizing fatty acids and enabling them to produce energy. Studies have shown that caffeine increases physical performance by approximately 11–12%. [4] , [5]
Considering that green tea can increase metabolic rate in the short term, we can assume that it will also help with weight loss. Many studies have shown that green tea helps reduce body fat, especially in the abdominal area. [6] , [7]
In another study, 240 randomized obese individuals were studied for 12 weeks, and the group consuming green tea showed significant reductions in body fat, waist circumference, and abdominal fat. However, there was no clear association between these and weight loss. [8] Nevertheless, more detailed studies are needed on this subject.
What do meta-analyses say?
However, reviews that have considered these studies (and studies with negative results which we have not included here) together have found that green tea has no effect on weight loss or control . [9] , [10]
Claim: Green Tea Protects Against Cancer!
Research has shown that oxidative damage can lead to chronic inflammation and subsequently to chronic diseases such as cancer. Antioxidants can protect the body against oxidative damage. For this reason, green tea can be a very good defense mechanism containing powerful antioxidants. Some studies have shown that green tea components reduce the risks of the following types of cancer. [11] Thus, cancer protection can be added to the benefits of green tea. [12]
• Breast cancer: A comprehensive study showed that women who drink a lot of green tea have about a 20–30% lower risk of developing breast cancer. [13]
• Prostate cancer: The incidence of advanced prostate cancer was found to be quite low in men who drank green tea. [14]
• Colorectal cancer: An analysis of 29 studies showed that those who drank green tea were 42% less likely to develop colorectal cancer. [15]
It is possible to find results indicating that green tea reduces the risk of developing many other types of cancer; however, more qualified research is needed on the subject. To get the best results, it is recommended to avoid adding milk to your tea. Some studies have concluded that added milk can reduce the antioxidant properties of tea. [16] , [17] , [18]
What do meta-analyses say?
Such selective studies are insufficient to support the cancer-preventive effect of green tea. In addition, green tea disrupts bortezomib and other boronic acid-based proteasome inhibitors used during chemotherapy, which are vital for cancer patients, and therefore it is important to consult a physician before drinking green tea during cancer treatment. [19]
Claim: Green tea reduces inflammation in the body!
A meta-analysis conducted in 2015 found no significant association between green tea consumption and a decrease in plasma C-reactive protein levels, an indicator of inflammation. [20]
Claim: Green Tea Keeps the Glycemic Index Under Control!
Although it has been claimed that green tea can lower fasting blood sugar, studies on hemoglobin A1c and fasting insulin levels have not yielded significant and consistent results. [21] , [22] , [23]
Partially True Claims Regarding the Benefits of Green Tea
Green tea contains healthy bioactive compounds!
Green tea not only satisfies our water needs, but unlike black tea, it also contains plant-derived chemicals called phytochemicals , which can offer more than just the benefits of vitamins and minerals, and are often used to treat infections. [24] Derived from the Greek word “φυτόν (phutón)” meaning “plant”, phytochemicals are chemical components that make plants resistant to fungi, bacteria, or viruses. Some of the benefits of green tea come from these phytochemicals.
polyphenol and flavonoid compounds , which are known to reduce oxidative stress, thereby lowering the risk of cardiovascular disease and cancer . They are also thought to delay the onset of Alzheimer's disease. Phytochemicals are also known for their anti-inflammatory, blood sugar-balancing, cholesterol-lowering, and blood pressure-regulating properties.
The most powerful polyphenol it contains is an antioxidant and natural phenol called catechin . Catechins, also known as epigallocatechin gallate (EGCG), prevent cell damage. They protect cells by reducing the proliferation of free radicals that cause aging and various diseases in the body. [25]
What do meta-analyses say?
A meta-analysis published in 2021, claiming to be the first of its kind, found that green tea improves people's total antioxidant capacity and may improve malondialdehyde levels in a dose-dependent manner. [26] A 2013 Cochrane analysis showed that green tea consumption could reduce total cholesterol by only 3–7 mg/dL and LDL levels by only 2 mg/dL, with no effect on HDL or triglycerides. [26] , [27] , [28]
However, there is one thing to keep in mind when choosing your green tea for antioxidant purposes: Some brands of green tea contain high levels of fluoride. Therefore, you need to pay attention to the ingredients when buying your green tea. [29] In such cases, green tea may have both benefits and drawbacks.
Unfortunately, some malicious individuals claim to treat illnesses that modern medicine cannot cure through phytochemicals. However, this is not scientifically accurate. The effect of any chemicals you might ingest through foods like green tea is far less than what modern medicine can provide. They are merely minor contributions/supports. Many of them have not been shown to have significant effects in numerous scientific studies. If you have a health problem, consult a medical professional without delay.
Claim: Green Tea Protects Heart Health!
Cardiovascular diseases, such as heart disease and stroke, have high mortality rates worldwide. Studies have shown that green tea has a positive effect on some key risk factors for these diseases, such as cholesterol. [30]
Green tea also increases the antioxidant capacity in the blood. This protects low-density oxidized lipoproteins from oxidation and prevents the development of heart disease. For all these reasons, there are studies showing that a person who drinks green tea is 31% less likely to die from cardiovascular disease than a person who does not drink it. [31] , [32] , [33]
However, the effects of green tea on heart health are extremely limited: A meta-analysis comparing randomized controlled trials examining the relationship between green tea and blood pressure found that green tea consumption for 3–6 months could only reduce systolic and diastolic blood pressure by 2–3 mmHg. [34] , [35] , [36] , [37] Another systemic review and meta-analysis found that even in those who drank 5–6 cups of green tea a day, systolic blood pressure only decreased by 2 mmHg, while diastolic blood pressure was unaffected. [27]
True Claims About the Benefits of Green Tea
Claim: Green Tea Improves Brain Function!
Perhaps the most prominent benefit of green tea is its effect on brain health. Thanks to its caffeine content , green tea keeps you alert and can also help improve brain function. Of course, it doesn't contain as much caffeine as coffee. Therefore, it's an ideal choice for those who get too much caffeine from coffee.
Caffeine: Benefits and Harms
Caffeine is structurally very similar to adenosine, also known as an inhibitory neurotransmitter. Adenosine is a chemical in our body that promotes sleep. Caffeine blocks adenosine receptors, which are structurally similar to it, and prevents adenosine from doing its job. Thus, with the increase in neurons that cannot bind to the receptors, the mind tires less quickly and can stay awake longer. People who regularly drink coffee, tea, and soda develop a tolerance to caffeine over time. More adenosine receptors are needed to bind, thus reducing the effect of caffeine. [38]
However, caffeine also has negative effects : due to the loss of the alertness effect we mentioned and the development of caffeine tolerance, there are consequences such as fatigue, irritability and headaches that can occur when we stop taking regular caffeine. Unfortunately, withdrawal effects experienced when stopping the regular use of most types of medication are also present in caffeine. [39]
However, it has been found that these effects resulting from caffeine withdrawal occur within the first 1–2 days and may last for 2–9 days (i.e., they end relatively quickly). If you gradually reduce your caffeine consumption, it is thought that you can continue your life as a caffeine-intolerant version within 2–3 weeks. [40] There are studies showing that caffeine can improve mood, attention and memory when used carefully without developing tolerance. [41]
Green tea and L-theanine
The benefits of green tea don't come solely from caffeine, the component that helps us feel more energetic. L-theanine , an amino acid frequently used in nutritional supplements, can cross the blood-brain barrier. The blood-brain barrier is a barrier between the blood and the brain that substances such as alcohol, nicotine, glucose, and caffeine can pass through. This barrier protects brain tissue from foreign substances that may be present in the blood, while also facilitating the transport of some substances from the brain to the blood and from the blood to the brain.
L-theanine, on the other hand, reduces neuronal activity in the central nervous system and increases the production of amino acids that act as neurotransmitters, thus providing an anti-anxiety effect. It also increases dopamine levels and contributes to the production of alpha waves in the brain. [42] Thus, one of the benefits of green tea is that it relieves anxiety, although caffeine triggers anxiety in some people.
Studies show that caffeine and L-theanine have synergistic effects, meaning that when combined they can have enhanced effects. A green tea containing L-theanine along with a low dose of caffeine can create a much different and gentler change than a coffee can provide. Many people say that when they drink green tea they have more stable energy and are more efficient than when they drink coffee. [43] , [44]
What do meta-analyses say?
The patterns we generally see in caffeine consumption also appear to apply to green tea: A meta-analysis examining 21 studies, including 4 randomized controlled trials, showed that green tea consumption has psychopathological effects such as reducing anxiety, cognitive effects such as improving memory and attention, and effects on brain function such as improving active memory. [45]
Unverified Claims Regarding the Benefits of Green Tea
Claim: Green Tea Keeps the Brain Young!
The benefits of green tea include not only protecting brain health but also keeping it young. Green tea not only improves your brain functions but can also keep your brain young as you age. Alzheimer's disease is a neurological disease that occurs with advancing age and causes a decline in thinking, memory, and behavioral functions. Parkinson's disease is a nervous system disease that can be diagnosed as a result of the death of some neurons in the midbrain and a decrease in dopamine production. Some studies show that catechin components protect the brain against dementia types such as these two diseases. [46] , [47] , [48]
Claim: Green tea eliminates bad breath!
Perhaps the least known benefit of green tea is that it helps eliminate bad breath. Tests have shown that catechins reduce bacterial growth and lower the risk of oral infections. [49] , [50]
Streptococcus mutans is a bacterium that causes tooth decay and is found in the human oral cavity. Studies have shown that catechins in green tea reduce bacterial growth in the mouth; however, there is no evidence that green tea consumption has the same effect. The results suggest that green tea may reduce bad breath. [51] , [52]
Claim: Green Tea Helps Prevent Type 2 Diabetes!
Diabetes is becoming an increasingly common disease today. According to estimates by the International Diabetes Federation, by 2040, 1 in 10 adults will have diabetes. This translates to approximately 642 million people with diabetes. Currently, it is estimated that millions of people live with Type 2 diabetes without knowing they have the disease or having access to adequate medical care. Unlike Type 1, Type 2 diabetes occurs because the body can produce the hormone insulin but cannot use it effectively. 90% of people with diabetes have Type 2 diabetes .
Studies have shown that green tea can increase insulin sensitivity and lower blood sugar. A study in Japan showed that individuals who consumed a lot of green tea had approximately 42% less risk of Type 2 diabetes. [21] , [53] Although more studies are needed on this subject, one of the benefits of green tea is that it (possibly) helps prevent Type 2 diabetes.
Claim: Green Tea Extends Life!
We mentioned that green tea can help protect us against cancer and heart disease. Therefore, green tea can also help us live longer by protecting us from these diseases. However, this effect appears to be quite limited: A meta-analysis that examined research on the relationship between green tea and heart health found that drinking 1 cup of green tea a day reduced deaths from cardiovascular disease by a very small amount. [54]
On the other hand, in a study in Japan, officials examined 40,530 people over 11 years. Those who drank the most green tea (5 cups a day) were in the group with the fewest deaths during this period. [55] The risk of death in people who consume green tea was:
• For all causes of death, women had 23% fewer deaths and men had 12% fewer;
• Heart disease is 31% less common in women and 22% less common in men;
• Stroke was 42% less common in women and 35% less common in men.
Another study in Japan involving 14,001 elderly people showed that those who drank the most green tea died 76% less during this 6-year study period. In short, in light of the studies, we can say that people who drink green tea generally live longer and healthier. [56]
What are the side effects of green tea?
Excessive green tea consumption can damage your liver and kidneys due to increased levels of polyphenols in your body. Long-term high-dose green tea consumption can also lead to side effects from caffeine. These side effects include headaches, sleep problems, vomiting, diarrhea, irritability, irregular heartbeat, palpitations, and heartburn.
The antioxidants in green tea can reduce iron absorption in the body. Therefore, those with anemia should pay attention to the dose. Also, the caffeine it contains can increase the severity of symptoms of diseases such as glaucoma, irritable bowel syndrome, and osteoporosis. In addition, green tea consumption can trigger clotting disorders in some cases. One study showed that high doses of green tea can cause some damage to the liver. [57]
A study showed that consuming 6 cups (1500 ml) of green tea every day caused anemia in a 48-year-old businessman. The dangerous dose that can cause serious harm to your body is stated as 10–14 grams (150–200 mg per kg). [58]
How should green tea be consumed?
Evrim Ağacı is to view green tea as another source of nutrition and to understand that, like all foods, it has both benefits and drawbacks. Regarding limits, the information we provided in our article about caffeine also applies here: Green tea consumption should not exceed 2-3 cups per day. It's important to note that everyone may react differently and that adjusting your intake accordingly is crucial.
Since low-quality green teas can contain high levels of fluoride, it is very important to consume them from well-known and reliable brands. Similarly, products sold in open markets can be affected by moisture, contaminants, and aflatoxins, leading to various problems.
Since boiling water (100°C) can reduce the effectiveness of the health-beneficial molecules in green tea, it's important to steep it in water that's close to boiling for about 5 minutes.
You can add flavorful ingredients like lemon, mint, and basil to suit your taste.
Conclusion
Unfortunately, in these types of nutritional discussions, malicious individuals exploit the "recourse to nature" fallacy, presenting relatively rare and ineffective studies on green tea and similar foods as "absolute cures" and "final conclusions." For example, it is unfortunately not possible to cure cancer solely with foods like green tea. A significant portion of the studies mentioned here contain shortcomings that require further detailing. Currently, the best solutions for cancer, cardiovascular diseases, obesity, and other problems are the methods offered by modern medicine. A good diet is always a friend to your health, not a guarantor.
Author: A. Ege Palaz
Editor: Ögetay Kayalı
2. Editor: Çağrı Mert Bakırcı
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Originally published on Medium on October 31, 2022. English translation of “Yeşil Çay Nedir? Gerçekten İddia Edildiği Kadar Faydalı mı Yoksa Zararlı Tarafları da Var mı?”.