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Hey, Matthew: What is it in raw spinach that makes your teeth feel gritty and funny? — Downtown Abe

Think spinach makes your teeth feel weird — dig into a dish of rhubarb, friend. Both spinach and rhubarb are rich in crystals of calcium oxalate, related to oxalic acid, part of the plant’s natural defense system. Calcium oxalate is undissolvable in water and in most milder forms of acid, so it will cling tenaciously to your teeth. You’ll not be surprised to hear that synthetic calcium oxalate is used in ceramic glazes. We can digest the crystals because our stomach juices are a dilute form of hydrochloric acid, about the only solvent for calcium oxalate. Another side note makes me wonder about the longevity of Popeye. Oxalic acid in quantity is poisonous; 10 to 20 pounds of spinach or rhubarb at one sitting would knock you cold.

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