Oxalate blocks the absorption of calcium in plants that contain it.
Oxalate is a naturally occurring substance in some vegetables that binds to calcium in the digestive tract, reducing its absorption.
Some calcium-rich foods, such as spinach, beets, and rhubarb, contain significant amounts of oxalates. These molecules form insoluble complexes with calcium, preventing its absorption in the intestine. This is why spinach calcium (5-6% absorption) is much less bioavailable than dairy calcium (30-35%). Cooking can partially reduce oxalate content, but not completely eliminate it. People with a history of kidney stones should particularly limit their oxalate intake.
Oxalate antagonizes intestinal calcium absorption by forming unassimilable chemical bonds.
Limit consumption of very oxalate-rich foods if you have kidney stones. Prefer calcium sources with low oxalates like dairy products, broccoli, and curly kale. If you eat spinach, combine it with another well-absorbed calcium source.
Oxalate makes spinach calcium nearly inaccessible: seek your calcium elsewhere.
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