Tannins in tea are similar substances to the tannins found in wine, both have very similar properties. Some characteristics of tea, like its colour, its body or its strength, are directly dependent on these polyphenolic derivatives and on the changes they have undergone. It is easy to recognise a tea that is high in tannin by the astringency of the drink, which sometimes translates into bitterness if the tea has been over brewed: tannins are released slowly but in an ever increasing way, so that an overly long infusion considerably raises their concentration and makes the tea bitter. Astringency plays a role in the tightening of cell tissue. Used externally, tea can be used in a bath to close and tighten pores, or in the final rinse when shampooing hair to make it smooth and shiny.

The main polyphenolic derivatives of tea are catechols and flavonoids. Their effect on the human body has been particularly highlighted by research into green teas. The reason for this is that most scientific studies in this area have been carried out in Japan, a country that produces only green tea. However, in recent years this study has been extended to include other families of teas: black teas, Wu Long (Oolong) teas and dark teas. While tannins are present in different types of tea, the fermentation process, on the other hand, changes them and the effects of polyphenols in non-green teas are still not well understood by researchers. Are they the same or are they different from those in green teas? We will have to wait a few more years before we have the answer.

Scientific research has revealed that polyphenols have an effect on bad cholesterol. Thus a daily intake of 5 cups of tea leads to a lowering, after a few months, of LDL-cholesterol - the "bad" cholesterol as opposed to HDL-cholesterol.
Other studies have further explored this matter and have highlighted the effects of green tea in the prevention of cardio-vascular disease, especially arteriosclerosis, an arterial disease associated with hardening of the arteries and fatty deposits.

One digestive effect of polyphenols has been demonstrated: drinking green tea limits the absorption of fats during digestion. So a cup of tea taken at the end of a meal, about 40 minutes afterwards, will aid digestion by activating a process of fat elimination.

Numerous scientific theories pertaining to the antioxidizing effects of polyphenols have also been tested. Polyphenols, which are present in considerable quantities in fruit, vegetables, red wine and green tea, play a vital role in the fight against the free radicals that are responsible for the aging of cells. One of the polyphenols in green tea - epigallocatechol gallate - is the object of very detailed scientific research into the fight against the development of cancerous cells. This polyphenol might inhibit the actions of the enzyme, urokinase, which is responsible for the random multiplication of cancerous cells.

At the present time, this research has been tested only on animals, and the same results still have to be proven for man in order to establish a link between tea consumption and the prevention of certain cancers. It must be added, that this research is not, in any way, taking place within a therapeutic framework but solely in terms of preventative dietary guidelines.