Tea is the second most consumed beverage in the world, behind water, and is a product of the leaves and leaf buds of Camellia sinensis, the tea plant, which originated in China. Tea may have been used initially as a medicine, and then progressed to a beverage.
Herbal teas often don't contain tea at all but, instead, are infusions of fruits and/or herbs such as chamomile tea, hibiscus or rosehip teas.
Green tea is tea that has undergone a minimal amount of oxidation during processing. It is known to contain antioxidants which may have a protective effect against cancer. And it's been discovered that chemicals in the tea shut down a key molecule which can play a significant role in the development of cancer. Research suggests that green tea may help against rheumatoid arthritis and also lower cholesterol levels.
White tea has similar healthful properties, however, having undergone the least amount of processing the heathful properties of white tea diminish more rapidly with the passage of time.
Although derived from the same plant there are different types of tea, classed by the degree of oxidation undergone by the leaves.
Health Benefits of green tea are a result of it being "rich in catechin polyphenols, particularly epigallocatechin gallate (EGCG). EGCG is a powerful anti-oxidant: besides inhibiting the growth of cancer cells, it kills cancer cells without harming healthy tissue".
Black and oolong tea are made from oxidized leaves, which results in the EGCG being converted into other compounds that are not nearly as effective in preventing and fighting various diseases.
Harmful effects: To date, the only negative side effect reported from drinking green tea is insomnia due to the fact that it contains caffeine¹ . However, green tea contains less caffeine than coffee: there are approximately thirty to sixty mg. of caffeine in six - eight ounces of tea, compared to over one-hundred mg. in eight ounces of coffee. To lessen caffeine intake from drinking green teas avoid the green tea teabags as they contain more of the caffeine and are of lesser quality tea; loose green tea leaves are of a higher quality and can be infused about three times (a better choice all around). Other methods of lessening the degree of caffeine when consuming green tea is to brew the green tea at a lesser strength (use fewer tea leaves), or by discarding the first infusion as it, especially if brewed for a longer period, will contain more caffeine.
Tea is now grown in India, Sri Lanka, Indonesia, Japan and other places as well as China. There are many different varieties and blends.