Oxalate is another common plant defense. Found in especially high concentrations in rhubarb leaves, it binds metals, especially cal- cium. The majority of kidney stones are composed of calcium oxa- late, and doctors have for years recommended that such patients keep their diets low in calcium. However, a study of 45,619 men, published in 1992, showed a higher risk of kidney stones for those who had low calcium intakes. How is this possible? Dietary calcium binds oxalate in the gut so that it cannot be absorbed. If dietary levels of calcium are too low, some oxalate is left free to enter the body. If, as researchers S. B. Eaton and D. A. Nelson have argued, the amount of calcium in the average diet is now less than half of what it was in the Stone Age, our current susceptibility to kidney stones may result from this abnormal aspect of our modern environment, which makes us especially vulnerable to oxalate.
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