Kidney stones, also referred to as renal calculi, are hard deposits that form from precipitated crystals in the kidneys. The most common form of kidney stones are those formed from calcium -- either when it combines with phosphate, another mineral, or oxalate, a naturally occurring substance found in certain foods and beverages, such as tea.
Physiology
Your urine is composed of fluid, waste products and various chemicals that keep these substances in a solution. When your urine contains too much calcium and too much oxalate, the inhibitory chemicals do not work properly. Calcium and oxalate combine and precipitate from your urine, forming hardened crystals that eventually could become kidney stones.
If you eat and drink foods and beverages that contain a lot of oxalate, it can increase the concentration of oxalate in your urine, making it more likely that kidney stones develop.
Oxalate and Tea
If you are prone to developing calcium oxalate kidney stones, a low-oxalate diet, which allows 40 to 50 mg of oxalate per day, is usually recommended.
Black tea contains more than 10 mg of oxalate per 3.5 oz. serving. Because of this, the University of Pittsburgh Medical Center classifies it as a high-oxalate beverage and recommends avoiding it on a low-oxalate diet. Other types of tea, including rosehip tea, black currant tea and Matetea tea, contain 2.5 to 10 mg of oxalate per serving, classifying them as medium-oxalate beverages. On a low-oxalate diet, intake of medium-oxalate foods and beverages is restricted to two to three servings per day.
Considerations
A study published in the "European Journal of Clinical Nutrition" in 2003 reported that adding milk to black tea before consumption may reduce the bioavailability of oxalate in the tea, thus reducing the concentration of oxalate in your urine.
Researchers found that drinking four cups of black tea without milk led to a urinary output of between 6.4 and 8.0 mg of oxalate. When the same tea was consumed with 25 ml of low-fat milk, urinary oxalate output dropped to 3.7 to 5.5 mg.
Green Tea and Kidney Stones
Although black teas and some other types of tea can increase the risk of kidney stones, green tea, which contains less than 2 mg of oxalate per serving, may actually decrease the risk of kidney stones. According to the Linus Pauling Institute, green tea contains a high concentration of substances called catechins that act as antioxidants. These catechins may prevent oxalate from coming out of solution, thus decreasing the risk of kidney stone formation.
In addition to the antioxidant effects of green tea, increasing fluid intake in general can help dilute the concentration of your urine and reduce the risk of kidney stones
References
- "European Journal of Clinical Nutrition"; Bioavailability of Soluble Oxalate from Tea and the Effect of Consuming Milk with the Tea; GP Savage, et al.; 2003
- "The Journal of Urology"; Preventative Effects of Green Tea on Renal Stone Formation and the Role of Oxidative Stress in Nephrolithiasis; January 2005
- Better Health Channel; Kidney Stones; April 2011
- University of Pittsburgh Medical Center; Low Oxalate Diet; October 2006
