Bioavailability Of Catechins Research Articles

Toxicity of green tea extracts and their constituents in rat hepatocytes in primary culture

Recent reports on sporadic cases of liver disorders (acute hepatitis, icterus, hepatocellular necrosis) after ingestion of dietary supplements based on hydro-alcoholic extracts from green tea leaves led to restrictions of the marketing of such products in certain countries of the EU. Since green tea is considered to exert a number of beneficial health effects, and, therefore, green tea products are widely used as dietary supplements, we were interested in the possible mechanism of hepatotoxicity of green tea extracts and in the components involved in such effects. Seven hours after seeding on collagen, rat hepatocytes in primary culture were treated with various hydro-alcoholic green tea extracts (two different native 80% ethanolic dry extracts and an 80% ethanolic dry extract cleared from lipophilic compounds). Cells were washed, and reduction of resazurin, used as a viability parameter monitoring intact mitochondrial function, was determined. It was found that all seven green tea extracts examined enhanced resazurin reduction significantly at a concentration range of 100–500μg/ml medium, while a significant decrease was observed at 1–3mg/ml medium. Decreased levels were concomitant with abundant necrosis as observed by microscopic inspection of the cultures and with increased leakage of lactate dehydrogenase activity from the cells. In a separate series of experiments, the green tea constituents (−)-epicatechin, (−)-epigallocatechin-3-gallate, caffeine and theanine were tested at concentrations reflecting their levels in a typical green tea extract. Synthetic (+)-epigallocatechin (200μM) was used for comparison. Cytotoxicity was found with (−)-epigallocatechin-3-gallate only. The concomitant addition of 0.25mM ascorbate/0.05mM α-tocopherol had no influence on cytotoxicity. In conclusion, our results suggest that high concentrations of green tea extract can exert acute toxicity in rat liver cells. (−)-Epigallocatechin-3-gallate seems to be a key constituent responsible for this effect. The relatively low bioavailability of catechins reported after oral exposure to green tea argues, however, against a causal role of these constituents in the reported liver disorders.

Structures of (-)-epicatechin glucuronide identified from plasma and urine after oral ingestion of (-)-epicatechin: differences between human and rat.

(-)-epicatechin is one of the most potent antioxidants present in the human diet. Particularly high levels are found in black tea, apples, and chocolate. High intake of catechins has been associated with reduced risk of cardiovascular diseases. There have been several reports concerning the bioavailability of catechins, however, the chemical structure of (-)-epicatechin metabolites in blood, tissues, and urine remains unclear. In the present study, we purified and elucidated the chemical structure of (-)-epicatechin metabolites in human and rat urine after oral administration. Three metabolites were purified from human urine including (-)-epicatechin-3'-O-glucuronide, 4'-O-methyl-(-)-epicatechin-3'-O-glucuronide, and 4'-O-methyl-(-)-epicatechin-5 or 7-O-glucuronide, according to 1H- and 13C-NMR, HMBC, and LC-MS analyses. The metabolites purified from rat urine were 3'-O-methyl-(-)-epicatechin, (-)-epicatechin-7-O-glucuronide, and 3'-O-methyl-(-)-epicatechin-7-O-glucuronide. These compounds were also detected in the blood of humans and rats by LC-MS. The presence of these metabolites in blood and urine suggests that catechins are metabolized and circulated in the body after administration of catechin-containing foods.

Analysis of catechins from green and black tea in humans: a specific and sensitive colorimetric assay of total catechins in biological fluids

Catechins are a group of flavonoids widely occurring in fruits, wine and tea, which are strong antioxidants with demonstrated anticarcinogenic effects in animal models. There is currently little knowledge on the bioavailability of catechins due to a lack of suitable analytical methods. Therefore a simple colorimetric assay was developed for the determination of the concentration of catechins in plasma. Following precipitation of proteins with methanol, tea flavonoids were accumulated by solid phase adsorption to alumina. Reactions of adsorbed catechins with 4-dimethyl amino cinnamaldehyde under strong acidic conditions results in desorption and development of a highly selective coloured complex (λ = 637 nm). The method is highly specific for catechins with equimolar responses for catechin, epicatechin, epigallocatechin, epicatechin gallate and epigallocatechin gallate. The detection limit in plasma is 0.05 μmol/L and the within and between day variation was 1.1% and 2.6%, respectively. Following consumption of a single dose of green tea, equivalent to six cups of tea, plasma levels of total catechins of 2.36 ± 0.55 μmol/L (n = 5) were reached after 60 min. This novel method is a rapid and sensitive way to gain more insight in the bioavailability of catechins from tea, wine and foods.