Green tea

Tea is an ancient herbal drink known for its stimulant qualities largely attributed to caffeine. In modern times, it also became touted as a health food allegedly helpful in preventing or ameliorating a variety of conditions, from heart disease and cancer, to gum disease, skin aging and weight loss. The major types of tea include black, green and white (oolong), differing by the method of harvesting and processing. The medicinal properties of tea are attributed to flavonoid phytochemicals called polyphenols. The polyphenols found in tea mainly belong to the subtype called catechins. Green tea has more catechins than black tea (about 25% vs 4%). White tea is almost as rich in catechins as green tea but is different in composition and less well studied. The main catechins in green tea include gallocatechin (GC), epigallocatechin (EGC), epicatechin (EC), and epigallocatechin gallate (EGCG).

Tea and general health

Numerous studies have shown tea polyphenols to have protective effects against free radicals, cardiovascular damage, some cancers, infections, toxins and so forth. But there is a catch. Most of the direct studies of the green tea effects have been done in tissue culture (test tubes) and animal models. The majority of human studies have been based on statistical correlations, i.e. the researchers used statistics to link tea consumption and the incidence of various diseases. Such correlational (a.k.a. epidemiological) data is by no means proof or even strong evidence of cause and effect - although it is a useful starting point for designing clinical trials.

As of the time of this writing, the overall impression from the limited human clinical trials is that tea (especially green and white) increases antioxidant capacity of tissues, particularly the blood, and that it improves some aspects of the lipid profile, such as the level and stickiness of LDL (bad cholesterol).

The full heath benefits of tea consumption in humans, if any, may take decades to investigate beyond reasonable doubt. However, considering tea's high safety and a large amount of indirect evidence suggestive of many potential health benefits, switching to tea (especially green or write) from other beverages makes good sense. Due to lack of direct long-tern studies, opinions vary as to how much tea should be consumed for optimal health. Most experts suggest drinking from three to ten cups per day. Those who wish to avoid caffeine or do not wish to bother with tea brewing, can take a supplement of green tea extract. A typical dosage is 100 to 150 mg three times a day of a green tea extract standardized to contain 80% total polyphenols and 50% epigallocatechin gallate. Whether the extract offers the same benefits as freshly brewed tea remains unknown.

Tea and weight loss

Green tea has become a popular ingredient in supplements formulated for weight loss, especially after ephedra (ephedrine) was banned by the FDA. The rationale for that was based on largely indirect evidence that green tea might increase metabolic rate. While the evidence that anything in the green tea besides caffeine increases metabolic rate is modest at best, the evidence that green tea promotes weight loss is even weaker. One small study using oolong tea enriched with green tea catechins found some apparent weight loss benefit. However, several other studies found no weight loss benefits.

See also the index of studies related to green tea.