Simultaneous ingestion of dietary proteins reduces the bioavailability of galloylated catechins from green tea in humans

To investigate the influence of dietary proteins (casein, soy protein) and skimmed milk on the plasma kinetics of green tea (GT) catechins. In a randomized cross-over design with one-week intervals, 24 healthy normal-weight women consumed a test drink containing 1.75 g GT extract with or without the addition of different proteins. Treatments were GT (control), GT with skimmed milk (GT + M), GT with caseinate (GT + CS), or GT with soy protein (GT + S). Venous blood samples were taken before and several times during a period of 4.5 h after consumption of the test drink. Plasma concentrations of catechins were analyzed by HPLC with electrochemical detection. Compared to control, consumption of GT with milk, caseinate, or soy protein significantly reduced the bioavailability (mean area under the plasma concentration-time curve) of total catechins (means +/- A SEM; GT + M, 87 +/- A 5%; GT + CS, 79 +/- A 5%; GT + S, 88 +/- A 4%), epigallocatechin gallate (GT + M, 68 +/- A 4%; GT + CS, 63 +/- A 5%; GT + S, 76 +/- A 5%), and epicatechin gallate (GT + M, 68 +/- A 5%; GT + CS, 66 +/- A 6%; GT + S, 77 +/- A 6%), while the bioavailability of non-galloylated catechins such as epigallocatechin (GT + M, 134 +/- A 9%; GT + CS, 118 +/- A 9 %; GT + S, 123 +/- A 8%) and epicatechin (GT + M, 125 +/- A 10%; GT + CS, 114 +/- A 11%; GT + S, 110 +/- A 8%) significantly increased. No significant differences in bioavailability of GT catechins were observed between the treatments GT + M, GT + CS, or GT + S. Simultaneous ingestion of dietary proteins reduces the bioavailability of galloylated catechins from GT in humans.