Green tea differs from black tea in that during processing the leaves of the former are steamed, while the leave of the latter are fermented. The result is that green tea maintains many of the original photochemicals present in the original whole plant leaf, whereas for black tea these substances can be chemically altered and their activity modified. One such group of chemicals that remain in the leaves of green tea but are mostly absent from black tea are the catechins (flavan-3-ols). Catechins have been researched with regard their antioxidant activity, but they appear to also possess antiviral and antibacterial properties. Some evidence has linked tea consumption to improved immunity with regard T-cell function, which may be beneficial in terms of upper respiratory tract infections. Limited studies have investigated the influence of tea consumption on protection from influenza infection, but those that have generally report positive results.
For example, researchers1 have used an observational study to investigate the association between green tea consumption and influenza infections in Japanese schoolchildren. The data was compiled by collecting anonymous questionnaires relating to food intakes to assess green tea consumption in 2663 pupils within a tea plantation area. Infection with influenza was confirmed in the children with an antigen test. The odds ratio for contracting influenza when consuming green tea for 6 days per week compared to 3 days per week was 0.60, indicating a protective effect for green tea. The researchers reported that consumption of 3 to 5 cups per day reduced the risk of influenza infection when compared to 1 to 3 cups per day. However, the data suggested that consumption of over 5 cups per day was not associated with increased protection when compared to between 3 to 5 cups per day.
Moderate green tea consumption may therefore be associated with a reduction in the risk of developing influenza infection in children, although there is no reason why this should not also be the case for adults. These results support previous findings that suggest a role for tea catechins in the enhancement of immunity. Experimental work using cell culture has shown that green tea catechins are able to bind to the haemogglutanin molecule of the influenza virus and may also inhibit the adsorption of viral molecules into cells. Some evidence also suggests that green tea catechins may inhibit the endonuclease activity of influenza RNA polymerase. Of the catechins, those with a golloyl group [(-)-epigallocatechin-3-gallate (EGCG), (-)-epigallocatechin (EGC), (-)-epicatechin-3-gallate (ECG) and] appear to be particularly effective at inhibiting RNA polymerase possibly by binding to the active pocket of the endonuclease domain.
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