- What is Green Tea
- Green Tea and Weight Loss
What is Green Tea
Tea is one of the most ancient and popular beverages consumed around the world. Green tea is the most popular tea in Japan and China. Whilst Black tea accounts for about 75 percent of the world’s tea consumption 1). In the United States, United Kingdom (UK), and Europe, black tea is the most common tea beverage consumed. Oolong and white tea are consumed in much lesser amounts around the world.
Green tea is made from the leaf of the plant Camellia sinensis, shortly after harvesting, tea leaves begin to wilt and oxidize. During oxidation, chemicals in the leaves are broken down by enzymes, resulting in darkening of the leaves and the well-recognized aroma of tea. This oxidation process can be stopped by heating, which inactivates the enzymes. The amount of oxidation and other aspects of processing determine a tea’s type.
Green tea is made from unwilted leaves that are not oxidized.
Black tea is produced when tea leaves are wilted, bruised, rolled, and fully oxidized.
Oolong tea is made from wilted, bruised, and partially oxidized leaves, creating an intermediate kind of tea.
White tea is made from young leaves or growth buds that have undergone minimal oxidation. Dry heat or steam can be used to stop the oxidation process, and then the leaves are dried to prepare them for sale.
Tea is brewed from dried leaves and buds (either in tea bags or loose), prepared from dry instant tea mixes, or sold as ready-to-drink iced teas. So-called herbal teas are not really teas but infusions of boiled water with dried fruits, herbs, and/or flowers.
- Green, black and oolong teas all come from the same plant, Camellia sinensis, but are prepared using different methods. To produce green tea, fresh leaves from the plant are lightly steamed and crushed.
- Tea has been used for medicinal purposes in China and Japan for thousands of years.
- Current uses of green tea as a beverage or dietary supplement include improving mental alertness, relieving digestive symptoms and headaches, and promoting weight loss.
- Green tea and its extracts, such as one of its components, EGCG (pigallocatechin gallate), have been studied for their possible protective effects against heart disease and cancer.
- Green tea is consumed as a beverage. It is also sold in liquid extracts, capsules, and tablets and is sometimes used in topical products (intended to be applied to the skin).
- White Tea is the least processed, and it has a light, sweet flavor. One laboratory study showed white tea sped up the breakdown of existing fat cells and blocked the formation of new ones. Whether it has the same effects in the human body remains to be seen.
- Black Tea is the type of tea that’s often served in Chinese restaurants and used to make iced tea. Black tea is produced when tea leaves are wilted, bruised, rolled, and fully oxidized– a process that allows it to change chemically and often increases its caffeine content. The tea has a strong, rich flavor. Whether it helps with weight loss isn’t certain. But research done on rats suggests substances called polyphenols in black tea might help block fat from being absorbed in the intestines.
- Oolong Tea is made by drying tea leaves in the hot sun. Like green tea, it’s a rich source of catechins. In one study, more than two-thirds of overweight people who drank oolong tea every day for six weeks lost more than 2 pounds and trimmed belly fat.
- Except for decaffeinated green tea products, green tea and green tea extracts contain substantial amounts of caffeine.
All teas naturally have high amounts of health-promoting substances called flavonoids. So they’re thought to bring down inflammation and protect against conditions like heart disease and diabetes.
Teas have a type of flavonoid called catechins (epigallocatechin gallate) that may boost metabolism and help your body break down fats more quickly.
What are the ingredients of tea ?
Tea is composed of polyphenols, alkaloids (caffeine, theophylline, and theobromine), amino acids, carbohydrates, proteins, chlorophyll, volatile organic compounds (chemicals that readily produce vapors and contribute to the odor of tea), fluoride, aluminum, minerals, and trace elements. The polyphenols, a large group of plant chemicals that includes the catechins, are thought to be responsible for the health benefits that have traditionally been attributed to tea, especially green tea. The most active and abundant catechin in green tea is epigallocatechin-3-gallate (EGCG). The active catechins and their respective concentrations in green tea infusions are listed in the table below. These chemical compounds act as antioxidants, which fight against free radicals in the body. Free radicals can alter DNA by stealing its electrons, and this mutated DNA can increase LDL “bad” cholesterol or alter cell membrane traffic – both harmful to our health. Though green tea is often considered higher in polyphenols than black or oolong (red) teas, studies show that – with the exception of decaffeinated tea – all teas have about the same levels of these chemicals, albeit in different proportions.
While the antioxidant action of tea is promising, some research suggests that the protein and possibly the fat in milk may reduce the antioxidant capacity of tea 2). Flavonoids, the antioxidant component in tea, are known to bind to proteins and “de-activate,” so this theory makes scientific sense 3). One study that analyzed the effects of adding skimmed, semi-skimmed, and whole milk to tea concluded that skimmed milk significantly reduced the antioxidant capacity of tea. The fattier milks also reduced the antioxidant capacity of tea, but to a lesser degree 4). Overall, it’s important to keep in mind that tea – even tea with milk – is a healthy drink. To reap the full antioxidant benefits of tea, however, it may be best to skip the milk.
Avoid purchasing expensive bottled teas or teas in coffee shops that contain added sugar or sweeteners. To enjoy the maximum benefits of drinking tea, consider brewing your own at home. You can serve it hot, or make a pitcher of home-brewed iced tea during warmer months.
Table 1. Catechin Concentrations of Green Tea Infusions
|Catechin in Green Tea Infusion||Catechin Concentration (mg/L)*||Catechin Concentration (mg/8 fl oz)*|
*mg = milligram; L = liter; fl oz = fluid ounce.[Source 5)]
Black tea contains much lower concentrations of these catechins than green tea 6). The extended oxidation of black tea increases the concentrations of thearubigins and theaflavins, two types of complex polyphenols. Oolong tea contains a mixture of simple polyphenols, such as catechins, and complex polyphenols 7). White and green tea contain similar amounts of EGCG but different amounts of other polyphenols 8).
Although iced and ready-to-drink teas are becoming popular worldwide, they may not have the same polyphenol content as an equal volume of brewed tea 9). The polyphenol concentration of any particular tea beverage depends on the type of tea, the amount used, the brew time, and the temperature 10). The highest polyphenol concentration is found in brewed hot tea, less in instant preparations, and lower amounts in iced and ready-to-drink teas 11). As the percentage of tea solids (i.e., dried tea leaves and buds) decreases, so does the polyphenol content 12). Ready-to-drink teas frequently have lower levels of tea solids and lower polyphenol contents because their base ingredient may not be brewed tea 13). The addition of other liquids, such as juice, will further dilute the tea solids 14). Decaffeination reduces the catechin content of teas 15).
Dietary supplements containing green tea extracts are also available 16). In a U.S. study that evaluated 19 different green tea supplements for tea catechin and caffeine content, the product labels varied in their presentation of catechin and caffeine information, and some values reported on product labels were inconsistent with analyzed values 17).
Green Tea and Weight Loss
Although many studies have been done on green tea and its extracts, definite conclusions cannot yet be reached on whether green tea is helpful for most of the purposes for which it is used including its use for weight loss. There are just not enough reliable data to determine whether green tea can aid in weight loss 18). Be wary of weight-loss claims. Some advertisements claim that tea can speed weight loss, but research on the effects of green tea and fat reduction have shown little promise of weight loss benefits. Moreover, it’s best to skip any so-called “diet” teas that may contain potentially harmful substances such as laxatives.
In a well-conducted 2012 Cochrane Review 19) the study authors looked at 15 weight loss studies and three studies measuring weight maintenance where some form of a green tea preparation was given to one group and results compared to a group receiving a control. Neither group knew whether they were receiving the green tea preparation or the control. Green tea preparations used for losing weight are extracts of green tea that contain a higher concentration of catechins and caffeine than the typical green tea beverage prepared from a tea bag and boiling water. A total of 1945 participants completed the studies, ranging in length from 12 to 13 weeks. In summary, the loss in weight in adults who had taken a green tea preparation was statistically not significant, was very small and is not likely to be clinically important. Similar results were found in studies that used other ways to measure loss in weight (body mass index, waist circumference). Studies examining the effect of green tea preparations on weight maintenance did not show any benefit compared to the use of a control preparation.
Green tea preparations appear to induce a small, statistically non-significant weight loss in overweight or obese adults. Because the amount of weight loss is small, it is not likely to be clinically important. Green tea had no significant effect on the maintenance of weight loss. Of those studies recording information on adverse events, only two identified an adverse event requiring hospitalisation. The remaining adverse events were judged to be mild to moderate.
Most adverse effects, such as nausea, constipation, abdominal discomfort and increased blood pressure, were judged to be mild to moderate and to be unrelated to the green tea or control intervention. No deaths were reported, although adverse events required hospitalisation. One study attempted to look at health-related quality of life by asking participants about their attitudes towards eating. Nine studies tracked participants’ compliance with green tea preparations. Studies did not include any information about the effects of green tea preparations on morbidity, costs or patient satisfaction.
In another study conducted in 2008, randomised blinded controlled trials that compared catechins (in green tea or capsules) – a epigallocatechin gallate + caffeine mixture, showed a small positive effect on weight loss and weight maintenance. However, the authors stated that further research was needed to assess whether (and to what extent) people are genetically predisposed to the effect of epigallocatechin gallate-caffeine mixtures. And due to the limitations in the review methodology mean that the overall effect size estimate was unlikely to be reliable and the conclusions should be treated with caution.
To get the same amount of EGCG used in the research, you’d need to drink about six to seven cups of your typical green tea every day. You could also try a green tea extract, but it might be risky. Though rare, high-dose tea extracts found in some weight-loss supplements have been linked to serious liver damage.
Green Tea and Cancer Prevention
Among their many biological activities, the predominant polyphenols in green tea―Epigallocatechin-3-gallate (EGCG), Epigallocatechin (EGC), Epicatechin-3-gallate (ECG) and Epicatechin (EC) and the theaflavins and thearubigins in black teas have antioxidant activity 20). These chemicals, especially Epigallocatechin-3-gallate (EGCG) and Epicatechin-3-gallate (ECG), have substantial free radical scavenging activity and may protect cells from DNA damage caused by reactive oxygen species 21).
Tea polyphenols have also been shown to inhibit tumor cell proliferation and induce apoptosis in laboratory and animal studies 22), 23). Research shows benefits for a variety of types of cancer, including ovarian cancer 24) and digestive system cancers 25). Green tea might also have a positive effect in reducing risk of breast, prostate, and endometrial cancers, though more evidence is needed 26). In other laboratory and animal studies, tea catechins have been shown to inhibit angiogenesis and tumor cell invasiveness 27). In addition, tea polyphenols may protect against damage caused by ultraviolet (UV) B radiation 28), 29), and they may modulate immune system function 30). Furthermore, green teas have been shown to activate detoxification enzymes, such as glutathione S-transferase and quinone reductase, that may help protect against tumor development 31). Although many of the potential beneficial effects of tea have been attributed to the strong antioxidant activity of tea polyphenols, the precise mechanism by which tea might help prevent cancer has not been established 32).
Although tea and/or tea polyphenols have been found in animal studies to inhibit tumorigenesis at different organ sites, including the skin, lung, oral cavity, esophagus, stomach, small intestine, colon, liver, pancreas, and mammary gland 33), the results of human studies—both epidemiologic and clinical studies—have been inconclusive. There is insufficient and conflicting evidence to give any firm recommendations regarding green tea consumption for cancer prevention.
The results of this 2009 Cochrane review 34), where fifty-one studies with more than 1.6 million participants, mainly of observational nature were included in this systematic review. Studies looked for an association 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 majority of included studies were of medium to high methodological quality. The evidence that the consumption of green tea might reduce the risk of cancer was conflicting. Although many of the potential beneficial effects of tea have been attributed to the strong antioxidant activity of tea polyphenols, the precise mechanism by which tea might help prevent cancer has not been established 35). This means, that drinking green tea remains unproven in cancer prevention, but appears to be safe at moderate, regular and habitual use.
A more recent 2015 study 36) looked at the cancer-fighting effects of a compound found in green tea when combined with a drug called Herceptin, which is used in the treatment of stomach and breast cancer. Initial results in the laboratory were promising and human trials are now being planned. But this shouldn’t be taken as official advice that drinking green tea while taking Herceptin will make it more effective. This research remains at a very early stage of development. The results from the laboratory and mice studies need to be confirmed by other research groups before the team can consider testing potential treatments in humans.
Green and black tea and the prevention of cardiovascular disease
There are very few long-term studies to date examining green or black tea for the primary prevention of cardiovascular disease. Polyphenols, the antioxidants abundant in tea, have been shown to reduce the risk of death due to cardiovascular disease 37), including stroke 38). In one study of 77,000 Japanese men and women, green tea and oolong tea consumption which was assessed by questionnaires was linked with lower risk of death from cardiovascular disease 39). Other large-scale studies show that black tea also contributes to heart health, with research suggesting that drinking at least three cups of either black or green tea per day appears to reduce the risk of stroke by 21 percent. The study also stated that drinking tea may be one of the most significant changes a person can make to reduce his or her risk of stroke 40). A randomized clinical trial would be necessary to confirm the effect. In a study of green and oolong tea consumption, regular consumption for one year reduced the risk of developing hypertension 41). Long-term regular consumption of black tea has also been shown to lower blood pressure 42). The limited evidence suggests that tea has favourable effects on cardiovascular disease risk factors, but due to the small number of trials contributing to each analysis the results should be treated with some caution and further high quality trials with longer-term follow-up are needed to confirm this 43).
Does green tea cut cholesterol?
A good-quality review from 2013 44) of 11 studies involving 821 people found daily consumption of green and black tea (as a drink or a capsule) could help lower cholesterol and blood pressure thanks to tea and its catechins. The authors of the review caution that most of the trials were short term and more good quality long-term trials are needed to back up their findings.
Another good-quality review from 2011 45) found drinking green tea enriched with catechins led to a small reduction in cholesterol, a main cause of heart disease and stroke. However, it’s still not clear from the evidence how much green tea you’d need to drink to see a positive effect on your health, or what the long-term effects of drinking green tea are on your overall health.
Can green tea help prevent or delay Alzheimer’s disease?
Evidence of a positive link between drinking green tea and Alzheimer’s disease is weak. A 2010 laboratory study 46) using animal cells found a green tea preparation rich in antioxidants protected against the nerve cell death associated with dementia and Alzheimer’s disease.
Whether these lab results can be reproduced in human trials remains to be seen. As such, the findings do not conclusively show green tea combats Alzheimer’s disease.
Can green tea prevent tooth decay?
A small study from 2014 47) looked at how effective a green tea mouthwash was in preventing tooth decay compared with the more commonly used antibacterial mouthwash chlorhexidine. The results suggested they were equally effective, though green tea mouthwash has the added practical advantage of being cheaper.
Are there safety considerations regarding tea consumption ?
Tea as a food item is generally recognized as safe by the U.S. Food and Drug Administration. Safety studies have looked at the consumption of up to 1200 mg of Epigallocatechin-3-gallate (EGCG) in supplement form in healthy adults over 1- to 4-week time periods 48), 49). The adverse effects reported in these studies included excess intestinal gas, nausea, heartburn, stomach ache, abdominal pain, dizziness, headache, and muscle pain 50), 51). In a Japanese study, children aged 6 to 16 years consumed a green tea beverage containing 576 mg catechins (experimental group) or 75 mg catechins (control group) for 24 weeks with no adverse effects 52). The safety of higher doses of catechins in children is not known.
As with other caffeinated beverages, such as coffee and colas, the caffeine contained in many tea products could potentially cause adverse effects, including tachycardia, palpitations, insomnia, restlessness, nervousness, tremors, headache, abdominal pain, nausea, vomiting, diarrhea, and diuresis 53). However, there is little evidence of health risks for adults consuming moderate amounts of caffeine (about 300 to 400 mg per day). A review by Health Canada concluded that moderate caffeine intakes of up to 400 mg per day (equivalent to 6 mg per kilogram [kg] body weight) were not associated with adverse effects in healthy adults. The amount of caffeine present in tea varies by the type of tea; the caffeine content is higher in black teas, ranging from 64 to 112 mg per 8 fl oz serving, followed by oolong tea, which contains about 29 to 53 mg per 8 fl oz serving. Green and white teas contain slightly less caffeine, ranging from 24 to 39 mg per 8 fl oz serving and 32 to 37 mg per 8 fl oz serving, respectively. Decaffeinated teas contain less than 12 mg caffeine per 8 fl oz serving. Research on the effects of caffeine in children is limited. In general, caffeine doses of less than 3.0 mg per kg body weight have not resulted in adverse effects in children. Higher doses have resulted in some behavioral effects, such as increased nervousness or anxiety and sleep disturbances 54).
Aluminum, a neurotoxic element, is found in varying quantities in tea plants. Studies have found concentrations of aluminum (which is naturally taken up from soil) in infusions of green and black teas that range from 14 to 27 micrograms per liter (μg/L) to 431 to 2239 μg/L 55). The variations in aluminum content may be due to different soil conditions, different harvesting periods, and water quality 56). Aluminum can accumulate in the body and cause osteomalacia and neurodegenerative disorders, especially in individuals with renal failure 57). However, it is not clear how much of the aluminum in tea is bioavailable, and there is no evidence of any aluminum toxicity associated with drinking tea 58).
Black and green tea may inhibit iron bioavailability from the diet 59). This effect may be important for individuals who suffer from iron-deficiency anemia 60). The authors of a systematic review of 35 studies on the effect of black tea drinking on iron status in the UK concluded that, although tea drinking limited the absorption of non-heme iron from the diet, there was insufficient evidence to conclude that this would have an effect on blood measures (i.e., hemoglobin and ferritin concentrations) of overall iron status in adults 61). However, among preschool children, statistically significant relationships were observed between tea drinking and poor iron status 62). The interaction between tea and iron can be mitigated by consuming, at the same meal, foods that enhance iron absorption, such as those that contain vitamin C (e.g., lemons), and animal foods that are sources of heme iron (e.g., red meat) 63). Consuming tea between meals appears to have a minimal effect on iron absorption 64).
Green tea extracts haven’t been shown to produce a meaningful weight loss in overweight or obese adults. They also haven’t been shown to help people maintain a weight loss.
Desirable green tea intake is 3 to 5 cups per day (up to 1200 ml/day), providing a minimum of 250 mg/day catechins. If not exceeding the daily recommended allowance, those who enjoy a cup of green tea should continue its consumption. Drinking green tea appears to be safe at moderate, regular and habitual use.
If you like a cup of tea with your morning toast or afternoon snack or on its own, enjoy it. It’s safe to drink as long as you don’t add sugar or artificial sweeteners and the caffeine doesn’t make you feel jittery and shaky; interfere with sleep; and cause headaches. And the limited evidence currently available suggests that both green and black tea might have beneficial effects on some heart disease risk factors, including blood pressure and cholesterol.
Just don’t expect miracles to come in a teacup toward your weight-loss goals. Real weight loss requires a whole lifestyle approach that includes diet changes and activity.
Liver problems have been reported in a small number of people who took concentrated green tea extracts. Although the evidence that the green tea products caused the liver problems is not conclusive, experts suggest that concentrated green tea extracts be taken with food and that people discontinue use and consult a health care provider if they have a liver disorder or develop symptoms of liver trouble, such as abdominal pain, dark urine, or jaundice.
Tea has long been regarded as an aid to good health, and many believe it can help reduce the risk of cancer. Most studies of tea and cancer prevention have focused on green tea 65). Although tea and/or tea polyphenols have been found in animal studies to inhibit tumorigenesis at different organ sites, including the skin, lung, oral cavity, esophagus, stomach, small intestine, colon, liver, pancreas, and mammary gland, the results of human studies—both epidemiologic and clinical studies—have been inconclusive. Therefore we have to say that at present, with respect to tea and green tea and cancer prevention, the evidence regarding the potential benefits of tea consumption in relation to cancer is inconclusive.
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