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Green Tea & Kidney Stones
A kidney stone is something like a small rock in
the urinary tract that forms when crystals separate from the urine and
create a mass. While chemicals in the urine normally prevent this from
happening, sometimes they don’t work properly. It's possible for smaller
crystals to travel through the urinary tract unnoticed and leave the
body as part of the urine. But the larger ones or those that have formed
into stones can get stuck in the tract, causing intense pain and
blocking the flow of urine. Symptoms include a sharp, cramping pain in
the side, lower back, abdomen or groin, blood in the urine, nausea,
vomiting and fever.
The most common kind of kidney stone is a combination of
calcium and oxalate, an organic acid found mostly in plant foods. Oxalate
appears to be toxic to cells and increase the formation of free radicals. When
calcium combines with oxalate it forms a salt crystal that is very difficult to
dissolve, and when that crystal is deposited in the kidney (a process called
calcification), it takes the form of a kidney stone.
For people with a calcium/oxalate kidney stone, doctors
often recommend cutting back on foods that are high in oxalate, like black tea.
But green tea, which is low in oxalate, may actually help prevent the
formation of kidney stones.
How does it work? The calcification process is inhibited by
antioxidants, and green tea, of course, has well-documented antioxidant effects.
An animal study done in 2005 found that green tea decreased the amount of
oxalate in the urine, inhibited calcium oxalate deposition, and increased the
activity of the body’s own super-antioxidant, SOD.1 In addition,
giving green tea’s EGCg to animals inhibited the free radical formation induced
by oxalate and lowered the number of crystals formed in the kidneys.2
While green tea appears to have an inhibitory effect on
kidney stone formation in animals, it remains to be seen if this is also true in
humans. However, since calcification is linked to both oxidation and free
radical action, increasing one’s intake of antioxidants just makes good sense.
Interestingly, although those with kidney stones are
usually advised to avoid black tea, those who don’t have stones may be
able to prevent them by drinking it. A study following 81,093 women for eight
years found that for each 8 oz. cup of tea consumed daily, the risk of
developing kidney stones decreased by 8 percent.3 A similar study of
45,289 men showed even better results, with a 14 percent decrease in risk of
kidney stones seen with each daily 8 oz. cup of tea.4
1Itoh
Y, Yasui T, Okada A, et al. Preventive effects of green tea on renal stone
formation and the role of oxidative stress in nephrolithiasis. J Urol
2005;173(1):271-75.
2Jeong
BC, Kim BS, Kim JI, Kim HH. Effects of green tea on urinary stone formation: an
in vivo and in vitro study. J Endourol 2006;20(5):356-61.
3Curhan
GC, Willett WC, Speizer FE, et al. Beverage use and risk of kidney stones in
women. Ann Intern Med 1998;128:534-40.
4Curhan
GC, Willett WC, Rimm EB, et al. Prospective study of beverage use and the risk
of kidney stones. Am J Epidemiol 1996;143:240-47.
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Nadine Taylor, M.S., R.D. presents
GreenTeaLibrary.com, the most
comprehensive collection of scientific information describing the
health benefits of green tea.
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