Friday, October 13, 2006

Oxalate & Tea

Appearing in the next issue of The Tea Experience Digest Magazine
in the "Ask the Expert" Section.

Q: Tim asked: I have had kidney stones and have been advised to avoid food and beverages with oxalic acid. Does tea contain oxalic acid?

A: Tim, Oxalic acid or oxalate is found mostly in foods from plants. Some examples are star fruit, black pepper, parsley, rhubarb, spinach, swiss chard, summer squash, sweet potatoes, chocolate, nuts, beans, and tea.

Leaves of the tea plant (Camellia sinensis) are known to contain among the greatest measured concentrations of oxalic acid relative to other plants. However the infusion beverage typically contains only low to moderate amounts of oxalic acid per serving, due to the small mass of leaves used for brewing. Tea brewed in the usual fashion contains about 55 mg of oxalic acid, as tall glass of iced tea contains about 20 mg. It takes a dose of 22,000 mg of oxalate to be potentially fatal in the average human.

Health folks can safely consume oxalate containing foods, but those with kidney disorders, gout, rheumatoid arthritis and a history of kidney stones should be careful. The gritty "mouth feel" one experiences when drinking milk with a rhubarb pie is caused by precipitation of calcium oxalate (CaOx). The calcium in the milk reacting with the oxalic acid in the rhubarb. Interestingly enough, calcium supplements taken along with foods high in oxalic acid can cause oxalic acid to precipitate in the gut and drastically reduce the levels of oxalate absorbed by the body.

About 5% of American women and 12% of men will develop a kidney stone at some time in their life. Calcium oxalate accounts for approximately 80% of stones. High intake of oxalate can induce calcium oxalate stones. Mega doses of Vitamin C can also precipiate these types of stones. Moderate intake of tea should not be a problem in the health individual. However, consuming foods high in oxalate is cumulative. In certain cases of patients who produce calcium oxalate stones, the physician may recommend reducing oxalate intake along with a slight increase in oral calcium intake. It is recommended that these patients have no more than 50 mg of oxalate/day in their diet.

References: J. Clin. Invest.2005 Oct;115(10):2598-608


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