Epidemiological evidence suggests that green tea may have beneficial health properties, particularly with regard to cancer and cardiovascular disease. This beneficial effect may be due to the high levels of antioxidant polyphenols present within the leaves of the Camellia sinensis plant, that remain upon processing to green tea. Green tea is a complex mixture of a number of polyphenols, mainly from the flavan-3-ol class of flavonoids. The major flavonoids in green tea are (-)-epigallocatechin-3-gallate (EGCG), (-)-epigallocatechin (EGC), (-)-epicatechin-3-gallate (ECG) and (-)-epicatechin (EC). Research suggests that these flavonoids may contribute to the health benefits of green tea. The flavan-3-ols (catechins) in green tea appear to be bioavailable in humans as they can be detected in plasma following ingestion. However, extensive metabolism of the flavon-3-ols may occur during absorption in the enterocytes of the small intestine and also within the liver during first pass metabolism.
Following the consumption of 5 grams of green tea powder in hot water, EGC was detected in human plasma with the highest level (63 to 142ng/mL) occurring around 2 hours after consumption1. In another study2, plasma levels of EGCG, EGC and EC were 46-268 ng/mL, 82-206 ng/mL and 48-80ng/mL, respectively, 1 hour following consumption of 1.2 grams of decaffeinated green tea. The catechins detected in plasma were predominately in their conjugated form with most of the catechins being excreted in the first 9 hours. Ingestion of 1.5 grams of green tea solids in 500 mL hot water resulted in maximum plasma concentrations of EGCG and EGC of 326ng/mL and 550ng/mL, respectively3. Increasing the dose to 3.0 grams of green tea increased the maximum plasma concentration by 2.7 to 3.4 fold, but increasing to 4.5grams did not increase the plasma concentration again significantly.
These studies suggest that green tea is bioavailable, and that the predominate forms of catechin in plasma are conjugated. This supports research investigating other classes of flavonoid, that shows extensive conjugation in the enterocytes of the gut and in the liver prior to detection in plasma. As with other flavonoids, the main forms of catechins in the blood appear to be glucuonides, sulfates and mixed conjugates of free form and methylated catechins. Studies looking at green tea supplements have also shown that green tea in capsule form is bioavailable. Subjects ingesting 3, 5 or 7 capsules of green tea extract (containing 224, 375 and 525mg ECG and 7.5, 12.5 and 17.5mg EGC) showed a dose dependent increase in plasma catechins 90 min post consumption4. Mean plasma levels were 657, 4300 and 4410 pmol/mL and 35, 144 and 255 pmol/mL following consumption of 2, 5 or 7 capsules of EGCG and EGC, respectively.
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