Polyphenols, including flavonoids, are a naturally occurring plant chemicals (called phytochemicals) that are found in tea and have strong antioxidant properties.
Tea contains a particular variety of polyphenols known as catechins and epicatechins. These are considered to have properties that protect or act against cancer (anticarcinogenic), tumours (antitumorigenic) and unwanted genetic changes (anti-mutagenic).
Polyphenols also help regulate blood sugar (glucose). As glucose rises in the blood insulin shoots in from the pancreas to signal the cells to start metabolizing the glucose. Polyphenols assist this process and make cells more sensitive to insulin's effects. This can also assist in lowering blood pressure and in some cases as with consumption of pu-erh tea, which has increased levels of polyphenols, can reduce bad cholesterol.