Almost everyone has heard of the benefits of green tea, but it's not widely known that it comes from the same plant as the black tea we're used to. Globally , 78% of tea consumed is black tea, while only 20% is green tea. Although this plant, which is cultivated especially in the Eastern Black Sea region of our country, originates from China and India, it has now spread to many parts of the world.
How should green tea be consumed?
To get the maximum benefit from green tea and avoid its potential harms, it's important to consume it correctly. First and foremost, it's necessary to adopt the general principle that "too much of anything is harmful." Consuming 2-3 cups of green tea a day is harmless, but exceeding 5-6 cups a day can lead to the effects mentioned below. It's important to note that everyone reacts 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.
Benefits of Green Tea
Green tea has become increasingly popular in recent years due to its well-known significant health benefits. However, this has also led to a great deal of misinformation surrounding it. In this article, we will address the benefits and potential harms of green tea using proven scientific studies to clarify these misconceptions.
IMPORTANT: The effects mentioned here should be considered as a supplement to your diet, not a treatment. Some of these studies may even be controversial or flawed. If you have a health condition, you should consult a medical doctor.
1) It helps with weight loss by increasing fat burning.
Perhaps the best-known benefit of green tea is its contribution to weight loss. If 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 [ 14 ]. The caffeine in green tea also increases physical performance by mobilizing fatty acids and enabling them to produce energy. Studies have shown that caffeine increases physical performance by approximately 11–12% [ 15 , 16 , 17 ].
Given that green tea can increase metabolic rate in the short term, we can assume it will also help with weight loss. Many studies have shown that green tea helps reduce body fat, especially in the abdominal area [ 1 , 2 ].
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 correlation between these and weight loss [ 3 ]. Nevertheless, more detailed studies are needed on this subject.
2) Contains Healthy Bioactive Compounds
Green tea not only satisfies our water needs, but it also contains plant-derived chemicals called phytochemicals, which, unlike black tea, can offer more than just the benefits of vitamins and minerals, and are mostly used to treat infections [ 4 ]. Phytochemicals are chemical components that make plants resistant to fungi, bacteria, or viruses. Some of the benefits of green tea come from these phytochemicals.
Phytochemicals derive their name from the Greek word φυτόν (phutón), meaning plant.
Some of these are 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.
Unfortunately, some unscrupulous individuals claim to treat illnesses that modern medicine cannot cure through phytochemicals. However, this is not scientifically accurate. Any chemicals you consume through foods like green tea have a far less significant effect 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.
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 [ 5 ].
However, there is something you should pay attention to when choosing your green tea. Some brands of green tea contain high levels of fluoride. Therefore, it is beneficial to pay attention to the ingredients when buying your green tea [ 6 ]. In such cases, green tea may have both benefits and harms.
3) Improves Brain Functions
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 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, preventing adenosine from achieving its purpose. This increases the number of neurons that cannot bind to the receptors, resulting in slower fatigue and longer alertness. People who regularly drink coffee , tea, and soda develop a tolerance to caffeine over time. The continued formation of adenosine receptors creates a greater need for caffeine to bind, thus reducing its effect [ 7 ].
Negative Effects of Caffeine
Because of the loss of the alertness effect we mentioned and the development of tolerance to caffeine, 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 [ 8 ].
However, it has been observed that these effects, which are predicted to occur within the first 1-2 days of caffeine withdrawal and last for 2-9 days, will quickly subside. 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 [ 9 ]. Studies have shown that caffeine, when used carefully without developing tolerance, can improve mood, attention and memory [ 10 ].
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 [ 11 ]. Thus, one of the benefits of green tea can be given as relieving anxiety, however, 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 [ 12 , 13 ].
4) Contains antioxidants that fight 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 [ 18 ]. Thus, cancer protection can be added to the benefits of green tea.
• 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 [ 19 ].
• Prostate cancer — The incidence of advanced prostate cancer was found to be quite low in men who drank green tea [ 20 ].
• Colorectal cancer — An analysis of 29 studies showed that those who drank green tea were 42% less likely to develop colorectal cancer [ 21 ].
Green tea also reduces the risk of developing many other types of cancer, but more qualified research is needed on the subject. For 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 [ 22 , 23 , 24 ].
Again, some malicious individuals, resorting to the "natural fallacy," present these rare and low-impact studies as absolute cures. Unfortunately, it is not possible to cure cancer with foods like green tea. A significant portion of the studies mentioned here may contain shortcomings that need to be detailed in many aspects. Currently, the best solution against cancer is the methods offered by modern medicine. A good diet is always a friend to your health, not a guarantee.
5) 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 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 [ 25 , 26 , 27 ].
6) 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 [ 28 , 29 ].
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 obtained suggest that green tea can reduce bad breath [ 30 , 31 ].
7) It can help prevent Type 2 Diabetes.
Diabetes is becoming an increasingly common disease today. According to estimates by the International Diabetes Federation, 1 in 10 adults will have diabetes by 2040. This translates to approximately 642 million people with diabetes. It is estimated that millions of people currently 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 when the body can produce the hormone insulin but cannot use it effectively. 90% of people with diabetes have Type 2 diabetes [ 32 , 33 ].
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 [ 34 , 35 ]. Although more studies are needed on this subject, one of the benefits of green tea is that it (possibly) helps prevent Type 2 diabetes.
8) 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 [ 36 ] .
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 [ 37 , 38 , 39 ]. In short, one of the benefits of green tea is its protection against cardiovascular disease.
9) It can prolong life.
We mentioned that green tea can help protect us against cancer and heart disease. Therefore, green tea can also help us live a longer life by protecting us from these diseases. In a study in Japan, officials examined 40,530 people over 11 years. Those who drank the most green tea were in the group that had the least cups during this period, with 5 cups a day [ 40 ].
In people who consume green tea, the risk of death is lower.
• 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 had a 76% lower mortality rate during the 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 [ 41 ].
Possible Side Effects of Green Tea
It's worth adding that, as with any substance, consuming too much green tea can be harmful in some cases. This is because excessive intake of polyphenols can potentially damage your liver and kidneys. If you consume high doses of green tea over a long period, you may experience some side effects due to caffeine. These side effects can manifest as headaches, sleep problems, vomiting, diarrhea, irritability, irregular heartbeat, palpitations, and heartburn. However, you are unlikely to experience so many side effects if you consume green tea in normal doses.
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 [ 42 ].
However, there's nothing to worry about if you don't consume more than 4–5 cups of green tea a day. Green tea was found to cause anemia in a 48-year-old businessman who consumed 6 cups (1500 ml) a day after several years. The dangerous dose that can cause serious harm to your body is stated as 10–14 grams (150–200 mg per kg) [ 43 , 44 ]. As long as you don't exceed these dangerous doses, you will experience the benefits of consuming green tea, as described above, rather than its harmful effects.
IMPORTANT: The information contained herein is for cultural purposes only and is not medical advice. If you have a health problem, you should consult a medical doctor as soon as possible.
Translated and Adapted by: Ege Palaz Contribution: Ögetay Kayalı Editor: Ögetay Kayalı
References
1. “10 Evidence-Based Benefits of Green Tea” <https://www.healthline.com/nutrition/top-10-evidence-based-health-benefits-of-green-tea>
2. “Disruption of astrocyte–vascular coupling and the blood-brain barrier by invading glioma cells” < https://www.nature.com/articles/ncomms5196 >
3. “What is Cancer?” https://hsgm.saglik.gov.tr/tr/kanser-nedir-belirtileri
4. “Turkish Diabetes Foundation” < https://www.turkdiab.org/ >
5. “International Diabetes Federation (IDF) 7th Diabetes Atlas” < http://www.diabetcemiyeti.org/c/diyabet-istatistikleri >
6. “Disruption of astrocyte–vascular coupling and the blood-brain barrier by invading glioma cells” < https://www.nature.com/articles/ncomms5196 >
7. “Green Tea Supplements Could Be Hurting Your Liver, Researchers Say” < https://fortune.com/2018/04/18/are-green-tea-supplements-safe-liver/ >
8. “11 Harmful Green Tea Side Effects” < https://senchateabar.com/blogs/blog/green-tea-side-effects >
Originally published on Medium on March 06, 2025. English translation of “Yeşil Çayın Faydaları ve Olası Zararları”.