Re: Why does cooked spinach leave a film that you can feel on your teeth?

Aloha, Kate,

     The answer to your question about the filmy stuff on your teeth from 
cooked spinach is probably related more to oxalates than to iron.  Spinach 
is very high in oxalate crystals (mineral salts of oxalic acid).  In fact, 
people who have problems with kidney functioning should not be eating a lot 
of oxalate-containing foods like spinach, because of the increased risk of 
producing oxalate stones in the kidneys.  Other foods that are high in 
oxalates are rhubarb, beet, chocolate, tea, bran, strawberries, and taro 
(which is a mainstay of the traditional Hawaiian diet, as the vegetable, 
as poi, and as laulau/leaf).
     Anyway, back to your questions and comments.  What may be happening is 
that in fresh spinach, because you chew it for only a short time, not much 
oxalate is released...so little or no sticky film in your mouth.  When 
spinach is cooked, especially canned spinach (heat processed), some of the 
spinach cell wall structure is damaged and oxalate crystals leak out.  It 
is the oxalate that gives your teeth and mouth that 'coated' feeling.
     An experiment:  Next time you have some raw spinach in a salad, chew 
it for a very long time to see if you get the same sensation that you 
report with the cooked spinach.  My guess is that you will.
     One last comment about iron in spinach and other leafy-green 
vegetables:  A number of vegetables contain a fair amount of iron (check a 
food composition table in any introductory nutrition textbook).  However, 
the iron is bound up with the oxalates and other similar chemicals in the 
structure of the plant.  We nutritionists say that the iron in plants is 
not highly 'bioavailable' to the human body.  It goes into the digestive 
tract OK, but the body can't absorb much of the iron into the blood stream, 
across the lining of the digestive tract, because the iron is tightly 
complexed to these plant substances.  Iron in animal products, especially 
in flesh products (meat, chicken, fish) is more bioavailable to the human 
body.  Also, the iron found in vitamin/mineral tablets is probably more 
bioavailable than the iron found in plants.
     Thanks for asking such an interesting question.