Oxalates

Organic compounds present in high concentration in spinach, beets and rhubarb. Bind to calcium and iron in the intestine, reducing their absorption (up to 90% for calcium in spinach). Cooking reduces oxalates by 30 to 87%. Pairing with a source of vitamin C partially compensates for their inhibitory effect on iron absorption. In people predisposed to calcium oxalate kidney stones, excessive consumption is not recommended.