Green Tea is one of those products that gets touted a lot as a cure all. If you walk through any Whole Foods or Fresh Market, you will see tons of products containing green tea, and literature praising the health benefits of consuming it. I wanted to do some reading and see if I could find more information than what I've been given by the people trying to sell it, and here's what I found.
Where did it come from?
There is a legend that thousands of years ago a Chinese emperor was making camp in the forest. As a pot of water boiled over the fire, a wind blew up and sent several leaves from a nearby tea plant tumbling in. The emperor was familiar with herbal medicine, so he decided to let it steep, and drank the freshly brewed tea. The emperor was happy with his discovery and eager to share it. Soon the kingdom, followed centuries later by much of the world, was drinking tea. Throughout history tea, and in particular green tea, has been associated with a multitude of health benefits. People over time have noted its ability to prevent and cure diarrhea, which can be a serious problem in areas that lack modern medicine. It has also been thought to prevent dementia, cardiovascular disease and stroke, diabetes, cancer, and a slew of other chronic processes. These associations have been made and reinforced over the course of centuries of consumption, which leads one to believe that there must be something to it.
There are three main varieties of tea. These are green, black, and oolong. White tea exists as well but is a subclass of green tea. All come from the leaves of the same plant, with the difference being how they are processed. To produce green tea, the leaves of the plant are steamed while they are fresh so that fermentation does not occur. Oolong tea is partially fermented and black tea is fermented completely (fermentation also increases caffeine, leaving black tea with more than twice the caffeine of green tea). After steaming, the leaves are rolled and dried, rendering a product that is stable and can be packaged and shipped. Because green tea doesn't undergo the fermentation process it retains it's color and more of the chemicals that are thought to be beneficial to our health than oolong or black tea. These chemicals are called polyphenols. Much as it sounds, a polyphenol is a chemical compound comprised of many phenol groups bound together. The amount of phenol groups and their structural placement give these chemicals their varying attributes. They can come from natural sources or be made in the lab, and are water soluble for the most part, which is why we find them in teas. You may also see the polyphenols in green tea referred to as flavanols or catechins. In any case the polyphenol compound that is believed to be the main source of green tea's healthy effects is called epigallocatechin gallate, or EGCG for short. EGCG is a powerful antioxidant, and has also been shown to have antidiabetic and anticarcinogenic effects in the labratory setting.
There have been thousands of studies done on green tea over the past decade, and so there is a lot of literature to filter through. Green tea poses some difficulty to researchers because it contains such a large variety of chemical compounds, and not every cup of tea is the same. There are differences in the chemical makeup of tea that vary with geographic growing location, processing methods, and brewing methods. This makes it difficult to deliver consistent amounts of green tea polyphenols to test subjects during an experiment. Some researchers have chosen to go with extracts in order to avoid this problem. A factor to consider when using extracts is that the compounds in tea act synergistically, so that studying the effects of only a select group of its constituent compounds in extract form may not be able to tell us all we need to know about how these chemicals work in concert. Also a large part of the research that has shown green tea to be beneficial has been done using animal models or retrospective reviews. Studies done retrospectively, while useful, can be hard to control for confounding factors. And though the results that have been found using animals and in vitro methods have yielded very promising results, it has yet to be seen how those results will translate to human subjects. What we can say is that green tea in normal doses, such as those you would get from drinking a few cups of tea a day, probably won't hurt and probably is beneficial. In very high doses green tea compounds can become cytotoxic, especially to the liver. High doses have also been shown to do some oxidative damage to the pancreas, which is counterintuitive since we usually think of tea as having antioxidant properties. Other negative side effects of green tea usually come from caffeine, disruption of iron absorption, or aluminum. Caffeine can cause anxiety, increased heart rate, insomnia, and increased urine output. Decreased iron absorption is mainly problematic in people with blood disorders or heart problems. Aluminum has been thought to contribute to problems with the nervous system such as Parkinson's and Alzheimer's. Though there are some potential negative side effects of drinking tea, they are normally seen at doses well above what you would find with normal use. Thousands of years of ingestion by large populations of people who are long lived have provided enough proof for me that tea is a good thing.
All in all I think the scientific proof is still somewhat lacking on how exactly green tea is beneficial, but I'm sure in time it will catch up. In the mean time do we really need complete scientific justification to drink something that tastes great and has been associated with long life for centuries? Nah... I think the secret, as in so many areas, is in balance. That, and enjoying the process. Taking a few minutes out of the day to boil up some water and steep a cup of tea, along with the calming smells and warmth it provides can be an instantly soothing experience. No wonder so many cultures have developed social rituals that center around taking time out and drinking tea every day.
A couple quick tips:
Adding lemon to tea can increase the amount of available aluminum, so skip it if possible.
Bottled green tea typically contains almost none of the beneficial polyphenols found in the freshly brewed product. Also it often has a lot of sugar. Brew your own.
The compounds in green tea that make it healthy are the same ones that provide a somewhat bitter taste, so don't be scared to take it a little strong.
To get the most out of a cup of green tea it has to be consumed fairly quickly after being brewed. A Keurig machine is great because you can brew quickly and easily several times a day.
Try to stick with fresh brewed tea instead of extracts, which aren't strictly regulated and can increase the potential of negative side effects.
Sources: Anticancer, a book by Dr. David Servan-Schreiber
The Primal Blueprint, a book by Mark Sisson
Beneficial Effects of Green Tea: A Literature Review, Journal of Chinese Medicine
Cancer and Metastasis: prevention and treatment by green tea
Cancer Prevention by Tea: animal studies, molecular mechanisms and human relevance
