Oxalate content of different drinkable dilutions of tea infusions after different brewing times

Tea Infusions as a Source of Phenolic Compounds in the Human Diet

Phenolic compounds are components with proven beneficial effects on the human body, primarily due to their antioxidant activity. In view of the high consumption of tea and the numerous factors that affect the nutritional value of its infusions, the aim of this study was to identify the effects of tea type and duration of leaf extraction with water on the levels of phenolic compounds and other components that determine biological activity (oxalates, Ca, Na, Cu, and Mn). Based on assays, infusions of red tea prepared for 20 min were found to be the best source of phenolics (202.9 mg/100 mL), whereas the lowest level of these compounds was determined in infusions of black tea extracted from leaves for 30 min (46.9 mg/100 mL). The highest degree of increase in polyphenol content (by approx. 50%) was noted in red and green tea infused for between 10 and 20 min, whereas for black tea, polyphenol levels decreased with time. The biological activity of tea infusions appears to be determined to the greatest extent by the interactions between phenolic compounds and oxalates (r = 0.6209), calcium (r = 0.8516), and sodium (0.8045). A daily intake of three to four mugs (1 L) of tea infusions provides the human body the entire amount of phenolics recommended for health reasons (as regards red tea, this is possible at 1/3 of the volume) and covers the daily requirement for manganese, as well as (partially) copper.

Does green tea consumption increase urinary oxalate excretion? Results of a prospective trial in healthy men

OBJECTIVE

To investigate the impact of green tea on urinary oxalate excretion in healthy male volunteers.

MATERIALS AND METHODS

The oxalate concentrations after different brewing times (2-60 min) of different qualities (2-8 g) of green tea were measured in in vitro experiment. In in vivo experiment, the effects on urine composition were assessed in 12 healthy men with an age of 24-29 years. Each subject was requested to collect two 24-h urine samples under normal dietary conditions. Green tea prepared from tea bags containing 2 g of tea leafs was consumed by the subjects for 7 consecutive days, and 24-h urine samples were collected and analyzed on days 6 and 7. After 3-week washout interval, all subjects consumed green tea containing 4 g of leaf tea for another 7 consecutive days. Two 24-h urine samples were collected on the last 2 days. Urine volume, pH, calcium, magnesium, sodium, phosphate, potassium, chloride, citrate, oxalate, urate and creatinine were measured.

RESULTS

In the in vitro experiments, oxalate in solution increased with brewing time (p < 0.05) and tea quality (p < 0.05). In the in vivo experiment, 24-h urinary oxalate increased significantly (0.24 ± 0.09 mmol to 0.32 ± 0.13 mmol, p = 0.045) when tea was prepared from 2-g bags of green leaf tea. Consumption of green tea containing 4 g of leaf tea resulted in 24-h urinary oxalate increase (0.25 ± 0.25 mmol to 0.34 ± 0.22 mmol, p = 0.041).

CONCLUSIONS

In vitro studies showed that there was a gradual increase in solution concentrations of oxalate that was associated with increased brewing time and increased quality of green tea. Studies in normal men showed that green tea consumption was associated with increased urinary exertion of oxalate.