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However, according to Ferruzzi, the catechins are relatively unstable in non-acidic environments, such as the intestines, and less than 20 per cent of the total remains after digestion.
"Off the bat you are eliminating a large majority of the catechins from plain green tea," he said. "We have to address this fact if we want to improve bodily absorption."
The researchers formulated green tea water extracts in drinks to provide doses of epicatechin (EC), epigallocatechin (EGC), epigallocatechin-gallate (EGCG), and epicatechin-gallate (ECG) of 4.5, 18, 23, and 3.5 mg per 100 mL, respectively.
To these beverages, they added citric acid, BHT, EDTA, ascorbic acid (AA), milk (bovine, soy, and rice), or citrus juice (orange, grapefruit, lemon, and lime), and measured the stability of the catechins under the simulated gastric and small-intestinal conditions.
Adding the citrus juice led to increased stability of the catechins, and resulted in the highest recovery of EGC (81-98%), EGCG (56-76%), EC (86-95%), and ECG (30-55%) for any formulation.
Vitamin C at a dose of 30 mg in 250 mL of tea significantly increased catechin stability of EGC, EGCG, EC, and ECG to 74, 54, 82, and 45 per cent, respectively.
Addition of 50 per cent bovine, soy, and rice milk also produced increased total catechin stability under the simulated gastric and small-intestinal conditions of 52, 55, and 69 per cent, respectively, the authors reported.
But notice they also say:
Ferruzzi confirmed that the study was ongoing, with an in vivo study currently underway to quantify the ability of juices and vitamin C to increase levels of catechins in the intestines and bloodstream of animals and, by extension, in humans.
"This next study is designed to get us past the limitations imposed by our digestive model, which is really just a simple screening process that relies on preset physiology parameters," he said. "Human digestion is a lot more complicated."
The question also remains as to whether the juices and vitamin C actually lead to an increase in bioavailability of the catechins. Ferruzzi added that answers are needed to the question of whether the increased levels of absorbed catechins are not levelled off by metabolic factors.