I just ate a tasty and fresh spinach salad and my teeth feel completely funky, kinda chalky. Now this is not the first time this has happened, but this is the first time I researched the issue.
First off, the term "spinach teeth" cannot yet be found in one of my favorite resources - the Urban Dictionary, but the phenomenon none the less exists. It is caused by the high quantities of oxalic acid found in spinach, not iron like many spinach lovers might think. The oxalate crystals leak out as you chew, and even more, when you cook spinach and these crystals coat the teeth. Drinking milk with spinach can exacerbate the chalky sensation. Rhubarb, beets, kale and chocolate are high in oxalic acid as well. People with kidney problems should not eat large quantities of spinach or any food high in this acid since it can lead to because of the increased risk of producing oxalate stones in the kidneys.
Fit's tip: The oxalic acid contained in spinach can prevent your body from absorbing iron and calcium. To improve iron absorption, spinach should be eaten with foods that contain vitamin C. This is a good reason to squeeze fresh lemon on cooked spinach!