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Does Raw Spinach Cause Kidney Stones?
Oxalates are naturally occurring chemicals that are found in the human body, as well as in animals. But they are most common in fruits and vegetables. As a general rule, the leaves of a fruit and vegetable will contain more oxalates than the stems and shoots of the plant. Some health experts encourage people to lightly steam high-oxalate vegetables such as spinach and chard. Other pop nutrition folks say avoid eating them raw altogether to prevent kidney stones. So what’s the real truth of the matter?
A study was published April 2007 in the Journal of the American Society of Nephrology1 regarding oxalates and kidney stones. It examined the relationship between oxalate intake and nephrolithiasis (kidney stones) in health professionals. Food frequency questionnaires were used to assess oxalate intake every four years. A total of 4,605 incidents of kidney stones were documented over a combined 44-year period of follow-up. There was no statistically significant difference in oxalate intake between participants with and without kidney stones.
Studies show a large amount of urinary oxalate is derived from the metabolism of glycine, glycolate, hydroxyproline, and dietary vitamin C. One study showed that a diet high in hydroxyproline (protein from gelatine) increased urinary oxalate excretion by 42%.2 Studies of dietary oxalate and stone risk also must account for the intake of other dietary factors such as magnesium, which may decrease urinary oxalates. The researchers said that their data did not support the contention that dietary oxalate is a major risk factor for kidney stones. The risk that was associated with oxalate intake was modest even in individuals who consumed diets that were relatively low in calcium.
Foods High in Oxalates
Vegetables: These vegetables all contain some forms of oxalates: celery, collards, dandelion greens, eggplant, green peppers, leeks, okra, parsnips, potatoes, sweet potatoes, pumpkin, rhubarb, rutabagas, spinach, squash (yellow and summer), turnip greens, watercress and yams.
Fruits: Fruits containing some forms of oxalates include blackberries, blueberries, dewberries, dried figs, gooseberries, raspberries (red and black), grapes (Concord), strawberries and tangerines.
Herbs and spices: These herbs and spices all contain some forms of oxalates: chocolate, ground cinnamon, cocoa, cocoa powder, ginger, lemon peel, lime peel, orange peel, parsley, pepper, pokeweed, sesame seeds, and sorrel.
In conclusion, magnesium helps prevent calcium from binding with oxalates and forming kidney stones. The American diet is quite deficient in magnesium. But interestingly some of the dark green vegetables like chard and spinach that are particularly high in oxalates are also quite high in magnesium. It looks like these vegetables came packaged just right. I don’t recommend you avoid eating vegetables raw that are high in oxalates, but rather making sure you have enough magnesium in your diet. Include dark leafy greens often in your juices and green smoothies.
1 http://jasn.asnjournals.org/content/18/7/2198.full (accessed May 25, 2012)