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I've been researching the nutrient content of various foods and I've found that cooked spinach appears to have more iron than raw spinach (3.57 mg / 100g vs 2.71 mg /100g). Intuitively, I would have expected the opposite.

Is the data I'm looking at wrong?

Maybe, it is just that it is easier to aborb the iron from cooked spinach rather than raw.

Can anyone explain the difference?

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3 Answers 3

up vote 19 down vote accepted

Iron is simply an element, so it cannot be destroyed by cooking (or generally temperature changes), as vitamins and other organic structures potentially can.

Cooked spinach inevitably has a much lower water content, thus the relative density of all other components must increase. So gram for gram, it makes sense that cooked spinach should have a higher concentration of iron (and possibly some other things) than raw. However, the actual process of cooking does nothing to change the amount of iron.

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Also, cooking makes more of that iron available to the human digestive tract. So in effect - as our digestion sees it - there is more iron. –  Daniel Bingham Sep 18 '10 at 1:19
    
@Daniel: I suspect it's slightly more complicated that and depends on the specific food - though I'm sure you're right in many (most?) cases. ('Cooking' raw iron wouldn't make it more digestable of course!) I gave the physicist's answer, heh. –  Noldorin Sep 18 '10 at 23:04
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@Daniel: That what I suspected. After some more research, I've found out that spinach contains high amounts of oxalic acid which combines with the iron and inhibits its absorption. Boiling the spinach breaks down the oxalates making the iron more easily absorbed. (note: the iron in spinach and most other vegetable is more difficult to absorb. Vitamin C helps increase the absorption.) –  Mark Sep 19 '10 at 0:50
    
@Mark: Interesting. I would not deny that's a significant factor. –  Noldorin Sep 19 '10 at 14:37
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@Noldorin It's not about making the raw iron itself more digestible, but about releasing it from its biological container. It's probably wrapped in or affixed to one protein or another in the spinach. Cooking causes those proteins to breakdown (as does digestion) releasing the iron. When you cook it, you're basically predigesting it a little. –  Daniel Bingham Sep 21 '10 at 2:37

I would guess that cooked spinach has lost a lot of the water content, so cooking doesn't add more iron, it just increases the percentage or iron by reducing the total mass.

I.e. if you had 100g of raw spinach and you cooked it (and drained it probably) you would end up with less than 100g of cooked spinach.

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It contains more iron cooked because it has less water in it. For that reason the USDA has it that cooked spinach is a good source of iron. But that is complete nonsense. Spinach contains oxalate that blocks iron take-up by humans. The best we can absorb is 20 percent of the iron in spinach. The fallacy that vitamin C helps us absorb more iron from spinach is a spuriofact. See: http://www.bestthinking.com/articles/science/chemistry/biochemistry/spin-ge-usa-beware-of-the-bull-the-united-states-department-of-agriculture-is-spreading-bull-about-spinach-iron-and-vitamin-c-on-the-internet

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