Green tea comes from the plant Camellia Sinensis. The two main varieties of tea plants are Camellia sinensis, a small-leaf variety native to China, and Camellia sinesis assamica, a large-leaf variety that was first discovered in the Assam district of India. Hundreds of cultivars and hybrid plants have evolved from these two plant varieties over time.
Green tea: Supporting literature
Live longer
Studies have found that some compounds in green tea may help you live longer. For instance, Kuriyama et al.(2006) studied 40,530 Japanese adults over 11 years. Those who drank the most green tea — 5 or more cups per day — were significantly less likely to die during the study period. The study found that:
· Death of all causes: 23% lower in women, 12% lower in men
· Death from heart disease: 31% lower in women, 22% lower in men
· Death from stroke: 42% lower in women, 35% lower in men
A previous study( Suzuki et al.2009) examined over 14,001 older Japanese individuals and found that those who drank the most green tea were 76% less likely to die during the 6-year study period.
Improves brain function
Green tea does more than just keep you alert, it may also help boost brain function. The key active ingredient is caffeine, which is a known stimulant. Though green tea doesn’t contain as much as coffee, however, it has more to produce a response without causing the nervous effects associated with taking in too much caffeine.
Two studies(Nehlig et al. 1999; Fredholm, B.B, 1995) found that caffeine affects the brain by blocking an inhibitory neurotransmitter called adenosine. When this happens, it increases the shooting of neurons and the concentration of neurotransmitters like dopamine and norepinephrine.
Another, Ruxton, C(2008) found that caffeine could support many areas of brain function, including mood, vigilance, reaction time, and memory. Apart from caffeine, Nobre et al.(2008) found that green tea also contains the amino acid L-theanine, which can cross the blood-brain barrier.
More studies (Nobre et al. 2008; Nathan et al. 2006; Egashira et al. 2007) explained that L-theanine increases the activity of the inhibitory neurotransmitter GABA, which has anti-anxiety effects. It also increases dopamine and the production of alpha waves in the brain.
This means that two important energy-boosting compounds in green tea combinations could have synergistic effects. This means that the combination of the two can have particularly powerful effects in improving brain function(Kelly et al. 2008; Dodd et al. 2015). This also means that due to the L-theanine and the small dose of caffeine, green tea may give you a much milder and different kind of resistance than coffee. It has been reported that people experienced having more stable energy and being much more productive when they drink green tea, compared with coffee.
Cancers
Boehm et. al. (2009) conducted a systematic review of fifty-one studies with more than 1.6 million participants in studies that looked for associations between green tea consumption and cancer of the digestive tract, gynecological cancer including breast cancer, urological cancer including prostate cancer, lung cancer and cancer of the oral cavity. The result of the review was conflicting with the evidence that the consumption of green tea might reduce the risk of cancer.
Another observation studies review (Ogunleye and Michels, 2009) found that women who drank the most green tea had an approximately 20–30% lower risk of developing breast cancer, one of the most common cancers in women
However, Reygaert (2017) found that case-control studies gave the most consistent results and were positive for reduced cancer risk in breast, cardiac, colorectal, esophageal, gastric, lung, ovarian, pancreatic, and prostate cancers. He conducted a review of research that focused primarily on using human subjects to investigate the health benefits of green tea. In his review, he also found a relationship between breast cancer risk and tea consumption, with the risk being highest in the groups that did not consume tea and lowest in the groups that consumed the most cups per day.
On Prostate cancer, Kurahashi et al.(2008) study found that men drinking green tea had a lower risk of advanced prostate cancer.
On colorectal cancer, Chen et al.(2017) examined 29 studies and found that those drinking green tea were around 42% less likely to develop colorectal cancer. An old study(Litt et al. 1997; Yuan. J.M, 2013) found that green tea drinkers are less likely to develop several types of cancer, but more high-quality research is needed to confirm these effects
Finally, studies have found that adding milk to green tea reduces its antioxidant ability(Ryan and Petit, 2010).
Cardiovascular disease
Two studies from Japan that included nearly 50,000 people found a decreased mortality rate due to CVD based on the consumption of various numbers of cups per day(Nakachi et.al., 2000, Kuriyama et. al., 2006).
Other studies in Japan using a green tea extract found that, after 12 weeks, the subjects had reductions in body fat (10%), blood pressure (6.5%), and low-density lipoprotein (LDL) levels (2.6%), suggesting a reduced risk of CVD.
A large meta-analysis of 17 studies from over 30 years, including data from Europe, the UK, and the U.S., found that increasing the consumption of green tea by three cups per day decreased the risk of myocardial infarction (MI) death by 11% (Peters et. al, 2001).
Another study showed a decreased risk of mortality in patients who had an acute MI and a history of regular green tea consumption for at least a year before the MI. Participants who did not drink green tea had a 14% rate of death due to MI; participants who drank up to 14 cups per week had an 11% rate of MI death; and participants who drank more than 14 cups per week had a 10% rate of MI death (Mukamal et. al,2002).
Another study, by Hartley et al.(2013) found that green tea may improve some of the main risk factors for these diseases, which includes improving total cholesterol and LDL (bad) cholesterol levels.
An old study( Yang and Koo, 2000; Yokozawa and Dong, 1997) also found that green tea also increases the antioxidant capacity of the blood, which protects the LDL particles from oxidation, which is one part of the pathway toward heart disease.
On the other hand, a meta-analysis of tea consumption concerning stroke, myocardial infarction, and all coronary heart diseases based on 10 cohort studies and seven case-control studies by Peters et. al.(2022), showed an interesting geographical effect of green tea consumption.
According to their study, the geographic region where the studies were conducted appeared to explain much of the heterogeneity among coronary heart disease, myocardial infarction, and probably stroke results. With increasing tea consumption, the risk increased for coronary heart disease in the United Kingdom and stroke in Australia, whereas the risk decreased in other regions, particularly in continental Europe.
Finally, (Kuriyama, S, 2008; Shimazu et al. 2007; Miller et al. 2017) studies acknowledge that those who drink green tea have up to a 31% lower risk of dying from cardiovascular disease.
Diabetes
Several human- and animal-based studies suggested that green tea and its flavonoids have antidiabetic effects (Wu, 2004; Iso, 2006; Wolfram, 2006). Green tea flavonoids were also shown to have insulin-enhancing activity (Anderson, 2002). After two months, diabetic patients taking green tea had reduced fasting blood glucose levels (from 135 to 128.8 mg/dL), and hemoglobin A1c (HBA1c) levels (from 6.2% to 6.0%) (Nagao et. al., 2007 and Fukino et. al., 2005).
Side effects of green tea
EGCG of green tea extract is cytotoxic, and higher consumption of green tea can exert acute cytotoxicity in liver cells, a major metabolic organ in the body ( Schmidt et. al. 2005) A study found that higher intake of green tea might cause oxidative DNA damage of hamster pancreas and liver (Takabayashi et. al., 2004). At a high dose (5% of the diet for 13 wk), green tea extract induced thyroid enlargement (goiter) in normal rats (Sakamoto Y et. al., 2001)
Green tea can cause side effects due to caffeine. These can include anxiety, tremors, irritability, and sleeping problems. This is more likely if you’re sensitive to caffeine or take large doses, Green tea extracts may cause liver problems. Symptoms can include yellowing of your skin or the whites of your eyes, nausea, and stomach pain. The negative effects of green tea extract on the liver may depend on how much green tea extract you consume each day.
Interactions
Green tea may change the effects of medicines such as nadolol, a beta-blocker used for high blood pressure and heart problems. It may keep nadolol from lowering your blood pressure as much as it should. Green tea contains small amounts of vitamin K. This means it may decrease how well blood thinner medicines work. Since green tea acts as a mild stimulant, you shouldn’t use it with other stimulants. It may change the effects of other medicines.
Conclusion
Drinking three to five cups of green tea per day seems to be optimal to reap the most health benefits. Try to choose a higher quality brand of green tea, because some of the lower quality brands can contain excessive amounts of fluoride (Lu et al. 2004).
That being said, even if you choose a lower-quality brand, the benefits still outweigh any risk.
NB:
Prof. Nyarkotey has strict sourcing guidelines and relies on peer-reviewed studies, academic research institutions, and medical associations to justify his write-ups. My articles are for educational purposes and do not serve as Medical advice for Treatment. I aim to educate the public about evidence-based scientific Naturopathic Therapies.
The writer is a professor of naturopathic healthcare, a medical journalist, and a science writer. President, Nyarkotey College of Holistic Medicine & Technology (NUCHMT)/African Naturopathic Foundation, Ashaiman, Ghana. E. mail: [email protected]. Visit-prof. Nyarkotey.com
