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Polyphenols, particularly those called Flavonoids are strong antioxidants. Flavonoids include Catechins (green tea flavonoids), theaflavins and thearubingins (black tea flavonoids) and are mainly responsible for the beneficial effects of tea.

It was thought until comparatively recently that green tea was the most effective antioxidant-containing tea and that green-tea catechins (the unoxidized polyphenols present in tea leaf) alone were the antioxidants giving tea its health-giving attributes. It is now well known that the theaflavins and thearubugins produced by the condensation of oxidized catechins, during the fermentation stage of black tea manufacture, are equally effective antioxidants (Leung et al 2001).
The catechins present in tea flush and as such in green tea are:
                                                           
Expressed as a % of dry weight
                      
Epicatechin                        1 – 3%
Epicatechin gallate            3 – 6%
Epigallocatechin                3 – 6%
Epigallocatechin gallate     9 -13%
Catechin                            1 – 2%
Gallocatechin                    3 – 4% 
 
During manufacture of Black Tea these catechins get oxidized & polymerized (condensed), for example :

Epicatechin + Epigallocatechin gallate + Oxygen —> Theaflavin
 
The paired catechins as they appear in Black Tea are now known to be equally effective antioxidants. The body produces free radicals (FRs) under certain conditions. Carcinogens and radiation from the environment facilitates the formation of FRs. These FRs within the body cause oxidative changes to DNA (the genetic material present in all cells). Changes to DNA carry the risk of cancers. The FRs are inhibited and destroyed by the antioxidants in tea, both green and black tea.

Green and black tea comes from Camellia Sinensis. Green tea is unfermented, steamed immediately after plucking, and retains a lighter colour and flavour. Black tea is allowed to ferment and is then dried, resulting in a darker leaf colour and a more flavour and aroma.

Tea composition varies with climate, season, horticultural practices and variety. Polyphenols are the most important component in tea, as they constitute approximately 36 percent of the dry weight of tea. Other components of fresh green leaf include caffeine, protein and amino acids, carbohydrates, lipids, vitamins and minerals.

Green and black tea have similar chemical make-up. The primary difference between the two types lies in the chemical changes that take place during their production. In black tea the plant Polyphenols are oxidized and this is prevented in the manufacture of green tea.
One of the most important groups of Polyphenols in tea is the catechins in green tea, theaflavins and thearubigens in black tea. A variety of physiological effects have been attributed to tea catechins which are currently best known for their antioxidant activities.

Black tea is all-natural (non flavoured) and contains no additives. It is virtually calorie-free (1 calorie per 100 ml) and sodium free and is therefore a suitable beverage for individuals on calorie-reduced or low sodium diet.  Tea includes fluoride, traces of vitamins A, K, C, B carotene and B vitamins.

Average daily consumption of tea in the United Kingdom, 3.43 cups (650 ml), provides very few calories and only a small amount of fat, whilst contributing valuable minerals and vitamins to the diet. It provides:
 
• Over half of the total intake of dietary flavonoids.
• Nearly 16% of the daily requirement of calcium
• Almost 10% of the daily requirement of zinc
• Over 10% of the folic acid need
• Around 9%, 25% and 6% of vitamins B1, B2 and B6 respectively.