Green tea is considered one of the world's healthiest drinks and contains one of the highest amount of antioxidants of any tea. Researchers commonly speak highly of green tea - Christopher Ochner, PhD, a research scientist in nutrition at the Icahn School of Medicine at Mount Sinai Hospital, is quoted as saying, "...it's the healthiest thing I can think of to drink."
There are 4 main varieties of tea: green, white, black, and oolong. The difference is in how the teas are processed. Green tea and white tea are made from unfermented leaves and reportedly contain the highest concentration of powerful antioxidants called polyphenols. Antioxidants are substances that fight free radicals, damaging compounds in the body that change cells, damage DNA, and even cause cell death.
Researchers think the health properties of green tea are mostly due to polyphenols. In fact, the antioxidant effects of polyphenols seem to be greater than vitamin C. The polyphenols in green tea also give it a somewhat bitter flavor.
Polyphenols contained in teas are classified as catechins. Green tea contains six primary catechin compounds: catechin, gallaogatechin, epicatechin, epigallocatechin, epicatechin gallate, and apigallocatechin gallate (also known as EGCG). EGCG is the most studied polyphenol component in green tea and the most active.
Green tea also contains alkaloids including caffeine, theobromine, and theophylline. They provide green tea's stimulant effects. L-theanine, an amino acid compound found in green tea, has been studied for its calming effects on the nervous system.
Green tea has been extensively studied in people, animals, and laboratory experiments. Results from these studies suggest that green tea may help treat the following health conditions:
Green tea can increase metabolism (oxidation) and aid in weight loss. The polyphenols found in green tea work to intensify levels of fat oxidation and the rate at which your body turns food into calories. Clinical studies suggest that green tea extract may boost metabolism and help burn fat. One study found that the combination of green tea and caffeine improved weight loss and maintenance in people who were overweight and moderately obese.
“Green tea has thermogenic properties and promotes fat oxidation beyond that explained by its caffeine content per se”. That’s what Dr. Abdul G. Dulloo, author of a study published in the American Journal of Clinical Nutrition said to WebMD. Green tea is a smart swap for sugary drinks. "If you sub 1-2 cups of green tea for one soda, [in a] year you'd save over 50,000 calories." -- Christopher Ochner, PhD.
Green tea may reduce bad cholesterol in the blood and improve the ratio of good cholesterol to bad cholesterol. Research shows that green tea lowers total cholesterol and raises HDL (good) cholesterol in both animals and humans. One population-based study found that men who drink green tea are more likely to have lower total cholesterol than those who do not drink green tea.
Results from one animal study suggest that polyphenols in green tea may block cholesterol from being absorbed in the intestines and also help the body get rid of cholesterol. In another small study of male smokers, researchers found that green tea significantly reduced blood levels of harmful LDL (bad) cholesterol.
Population-based studies indicate that the antioxidant properties of green tea may help prevent coronary artery disease, particularly atherosclerosis. Population-based studies follow large groups of people over time or compare groups of people living in different cultures, or with different diets.
Scientists think green tea works on the lining of blood vessels, helping keep them stay relaxed and better able to withstand changes in blood pressure. It may also protect against the formation of clots, which are the primary cause of heart attacks.
Researchers believe green tea reduces the risk of heart disease by lowering cholesterol and triglyceride levels. Studies show that black tea has similar effects. In fact, researchers estimate that the rate of heart attack decreases by 11% with consumption of 3 cups of tea per day.
According to the Harvard Heart Letter, Japanese adults who drank more than five cups daily were at 26% lower risk of heart attack or stroke. While research around mechanisms of action is limited, there were no red flags, so it can’t hurt.
In another study, Athens Medical School took 14 volunteers and had them either drink green tea or caffeinated hot water. Using blood pressure cuffs and ultrasound to check whether arteries were widened or not, the doctors found that those that drank green tea had significantly wider arteries, meaning potentially lower risks for heart diseases like atherosclerosis.
Drinking green tea or coffee on a regular basis is associated with a reduced risk of stroke, according to a study published in the journal Stroke: Journal of the American Heart Association.
The lead author of the study, Yoshihiro Kokubo, M.D., Ph.D., said, "This is the first large-scale study to examine the combined effects of both green tea and coffee on stroke risks. You may make a small but positive lifestyle change to help lower the risk of stroke by adding daily green tea to your diet."
The natural chemicals called polyphenols in tea are what are thought to provide its anti-inflammatory and anti-carcinogenic effects. Green tea is known to aid healthy cells in all stages of growth. There are some clues that green tea may also help destroy cancer cells, but that research is still in its early stages so you shouldn’t rely on green tea to prevent cancer.
Several population-based studies suggest that both green and black teas help protect against cancer. For example, cancer rates tend to be low in countries such as Japan where people regularly consume green tea. However, it is not possible to know for sure from these studies whether it is the green tea that actually prevents cancer in people.
Early clinical studies suggest that the polyphenols in tea, especially green tea, may play an important role in the prevention of cancer. Researchers also believe that polyphenols help kill cancerous cells and stop them from growing.
In one study that compared people with and without bladder cancer, researchers found that women who drank black tea and powdered green tea were less likely to develop bladder cancer. A follow-up clinical study by the same group of researchers revealed that people with bladder cancer, particularly men, who drank green tea had a better 5-year survival rate than those who did not drink green tea. People with cancer should consult with their doctor before adding tea to their regimen.
Studies in animals and test tubes suggest that polyphenols in green tea inhibit the growth of breast cancer cells. In one study of 472 women with various stages of breast cancer, researchers found that women who drank the most green tea had the least spread of cancer. It was especially true in premenopausal women in the early stages of breast cancer. They also found that women with early stages of the disease who drank at least 5 cups of tea daily before being diagnosed with cancer were less likely to experience a recurrence after they finished treatment. However, women with late stages of breast cancer had little or no improvement from drinking green tea.
There is no clear evidence one way or the other about green tea and breast cancer prevention. In one very large study, researchers found that drinking tea, green or any other type, was not associated with a reduced risk of breast cancer. However, when the researchers broke down the sample by age, they found that women under the age of 50 who consumed 3 or more cups of tea per day were 37% less likely to develop breast cancer compared to women who did not drink tea.
In a study done with ovarian cancer patients in China, researchers found that women who drank at least one cup of green tea per day lived longer with the disease than those who did not drink green tea. In fact, those who drank the most tea, lived the longest. Other studies however, found no beneficial effects.
Studies on the effects of green tea on colon or rectal cancer have showed conflicting results. Some studies show decreased risk in those who drink the tea, while others show increased risk. In one study, women who drank 5 or more cups of green tea per day had a lower risk of colorectal cancer compared to non-tea-drinkers. However, there was no protective effect for men. Other studies show that drinking tea regularly may reduce the risk of colorectal cancer in women. More research is needed before researchers can recommend green tea for the prevention of colorectal cancer.
Green tea may reduce the risk of esophageal cancer, but it is also widely thought to kill cancer cells in general without damaging the healthy tissue around them. Studies in laboratory animals have found that green tea polyphenols inhibit the growth of esophageal cancer cells. However, studies in people have produced conflicting findings. For example, one large-scale population-based study found that green tea offered protection against the development of esophageal cancer, particularly among women. Another population-based study found just the opposite...green tea consumption was associated with an increased risk of esophageal cancer. In fact, the stronger and hotter the tea, the greater the risk. Given these conflicting results, more research is needed before scientists can recommend green tea for the prevention of esophageal cancer.
While green tea polyphenols have been shown to inhibit the growth of human lung cancer cells in test tubes, few clinical studies have looked at the link between drinking green tea and lung cancer in people, and the studies that have been done show conflicting results. One population-based study found that Okinawan tea, similar to green tea but partially fermented, was associated with lower lung cancer risk, particularly among women. But a second study found that green tea and black tea increased the risk of lung cancer. More studies are needed before researchers can draw any conclusions about green tea and lung cancer.
Green tea should not be used by patients on bortezomib therapy.
In one large-scale clinical study researchers compared green tea drinkers with nondrinkers and found that those who drank the most tea were less likely to develop pancreatic cancer. This was particularly true for women - those who drank the most green tea were half as likely to develop pancreatic cancer as those who drank less tea. Men who drank the most tea were 37% less likely to develop pancreatic cancer.
However, it is not clear from this population-based study whether green tea is solely responsible for lowering pancreatic cancer risk. More studies are needed before researchers can recommend green tea for the prevention of pancreatic cancer.
Laboratory studies have found that green tea extracts prevent the growth of prostate cancer cells in test tubes. A large clinical study in Southeast China found that the risk of prostate cancer went down with increasing frequency, duration, and quantity of green tea consumption. However, both green and black tea extracts also stimulated genes that cause cells to be less sensitive to chemotherapy drugs. People who are undergoing chemotherapy should ask their doctors before drinking green or black tea, or taking tea supplements. Dr. Katherine Crew, assistant professor of medicine and epidemiology at Columbia University, also conducted a study in men with prostate cancer and found that green tea drinkers had markedly lower prostate-specific antigen, which is linked to prostate tumor growth.
The main polyphenol in green tea is epigallocatechin gallate (EGCG). Scientific studies suggest that EGCG and green tea polyphenols have anti-inflammatory and anticancer properties that may help prevent the development and growth of skin tumors.
Laboratory studies have found that green tea polyphenols inhibit the growth of stomach cancer cells in test tubes, however, studies in people have been less conclusive. In two studies that compared green tea drinkers with non-drinkers, researchers found that people who drank tea were about half as likely to develop stomach cancer and stomach inflammation as those who did not drink green tea. However, a clinical study with more than 26,000 men and women in Japan found no association between green tea and stomach cancer risk. Some studies even suggest that green tea may increase the risk of stomach cancer.
Green tea seems to help keep blood sugar stable in people with diabetes. Because catechins lower cholesterol and blood pressure, they can help protect against the damage a high-fat diet can cause, Ochner says. Green tea apparently helps regulate glucose levels slowing the rise of blood sugar after eating. This can prevent high insulin spikes and resulting fat storage. Green tea has been used traditionally to control blood sugar levels. Animal studies suggest that green tea may help prevent the development of type 1 diabetes and slow the progression once it has developed. In people with type 1 diabetes, their bodies make little or no insulin, which helps convert glucose or sugar into energy. Green tea may help regulate glucose in the body. Research also suggests that regular consumption of green tea may help manage type 2 diabetes.
Population-based studies have shown that men who drink more than 10 cups of green tea per day are less likely to develop liver problems. Green tea also seems to protect the liver from the damaging effects of toxic substances such as alcohol. Animal studies have shown that green tea helps protect against liver tumors in mice.
Results from several animal and human studies suggest that plant chemicals in green tea called catechins, may help treat viral hepatitis, an inflammation of the liver. In these studies, catechin was used by itself in very high amounts, therefore it is not clear whether green tea, which has a lower concentration of catechins, would have the same benefits.
Studies suggests that the chemical antioxidant “catechin” in tea can destroy bacteria and viruses that cause throat infections, dental caries and other dental conditions.
Green tea can help with wrinkles and the signs of aging, this is because of their antioxidant and anti-inflammatory activities. Both animal and human studies have demonstrated that green tea applied topically can reduce sun damage. A 2007 study concluded that green tea could hold promise as a new treatment for skin disorders such as psoriasis and dandruff. Researchers studied an animal model for inflammatory skin diseases, often characterized by patches of dry, red, flaky skin caused by the inflammation and overproduction of skin cells. Those treated with green tea showed slower growth of skin cells and the presence of a gene that regulates the cells' life cycles.
Green tea may help reduce inflammation associated with Crohn's Disease and Ulcerative Colitis, the two types of IBD. If green tea proves to help prevent colon cancer, it would also help those with IBD because they are at higher risk for colon cancer. The anti-inflammatory effects of green tea mentioned above may also help with stomach issues like colitis. A study at the University of Cincinnati College of Medicine found that EGCG found in green tea may help both colitis and Crohn’s disease. Not to mention that green tea’s anti-cancer properties also apply to colon cancer.
According to a study published in the Food Science Technology Bulletin, the catechins found in green tea, specifically epigallocatechin gallate (EGCG) and epicatechin gallate (ECG), can hinder development of many bacterial species. These catechins can stick themselves into the films that cover the bacteria. By lodging into the outside layers of bacteria, EGCG and ECG may upset the bacteria from doing what it is supposed to, which is make you ill.
A study published in the Journal of Trends in Food Science & Technology by Dr. Juneja and team states that theanine acts as a neurotransmitter that increases alpha waves in the brain. Alpha waves are considered to be an indicator of relaxation. Human volunteers were given 50-200 mg of l-theanine and were shown to have increased alpha waves without drowsiness. Sipping tea helps you slow down and relax, Reardon says. A natural chemical called theanine found in green tea can provide a calming effect.
It is believed that green tea may delay the deterioration caused by Alzheimer’s and Parkinson’s. Studies carried out on mice showed that green tea protected brain cells from dying and restored damaged brain cells. What’s good for the heart is usually good for the brain; your brain needs healthy blood vessels, too. In one Swiss study, MRIs revealed that people who drank green tea had greater activity in the working-memory area of their brains. Green tea has also been shown to help block the formation of plaques that are linked to Alzheimer's disease. In a study published in 2011, researchers tested the effect of a component of green tea, CAGTE, after it had been digested, to see how it affected a key protein in Alzheimer's disease. The study showed that at high concentrations, CAGTE protected the cells from the toxic effects of the protein amyloid-beta.
The Alzheimer's Society commented "...this study adds to previous research that suggests green tea might help to reduce the risk of Alzheimer's disease. However, the researchers used a far higher dose of the active green tea chemical than would ever be found in the human body. More research is needed to see whether green tea is protective at a much lower dose, and to understand the mechanism involved."
Researchers at the University of Michigan published results of a study in 2013 that showed a molecule in green tea may help prevent the misfolding of specific proteins in the brain.
Additionally, the results of a study published by the University of Missouri in 2015 concluded that green tea extract and exercise hindered the progress of Alzheimer's disease in mice. They hope that their comprehensive study of EGCG, a compound found in green tea, could eventually lead to treatments for Alzheimer's disease in humans.
According to a study from the Department of Psychiatry, University of Basel Switzerland, sipping on green tea enhances memory. Until last year, the effects of green tea on cognitive function were speculative, but with the improvements in functional magnetic resonance imaging (fMRI), Dr. Stefan Borgwardt could view the increased connectivity in the cortices of the brain after consumption of green tea. Specifically, green tea drinkers showed increased connectivity in key parts of the brain associated with memory.
The study used healthy male volunteers and found that when given a soft drink containing green tea extract, those consuming green tea extract not only showed increased connectivity in parts of the brain, but improved performance on working memory tasks as well.
Green tea’s biggest benefit? "It's all about the catechin content" says Beth Reardon, RD, a Boston nutritionist. Catechins are antioxidants that fight and may even prevent cell damage. Green tea is not processed much before it's poured in your cup, so it's rich in catechins.
When we eat, our bodies have to turn food into energy. As a result, this generates some byproducts within our bodies called free radicals. These chemicals are pretty awful, and can severely damage our bodies through a process called oxidation. Free radicals are also, theoretically, the main driver in aging.
Epigallocatechin-3-gallate (EGCG) is the most studied and bioactive polyphenol in tea and has been shown to be the most effective at eliminating free radicals.
Green tea is approximately 20% to 45% polyphenols by weight, of which 60% to 80% are catechins such as EGCG. These catechins are antioxidants that are said to possibly help with fighting and preventing cell damage.
In clinical trials published by the American Society for Nutritional Sciences, scientists found that numerous studies suggested that the consumption of tea may bring positive health effects. The hypothesis behind these benefits stems from the high levels of flavonoids, antioxidants that scavenge free radicals.
"It's the healthiest thing I can think of to drink," says Christopher Ochner, PhD. He's a research scientist in nutrition at the Icahn School of Medicine at Mount Sinai Hospital.
Of course, no one food will protect you from disease. Your health is wrapped up in your lifestyle and your genes, so even if you drink green tea all day long, you also need to take care of yourself in other ways, like not smoking, being active, and following a healthy diet of mostly fruits, vegetables, beans, peas, legumes and whole grains.
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