It's not a myth, nor is it a bad thing, especially for women, kids, and vegetarians, who are more prone to being iron-deficient. Research has shown that cast-iron use does indeed impact the iron level in food. However, how much iron gets transferred and how much your body absorbs is up for debate.
"It's not nothing," says Jennings.
An often-cited study published in 1986 in the Journal of the American Dietetic Association compared 20 foods cooked in a seasoned iron skillet versus in a Corningware dish and found that in all but 2 foods (cornbread and liver with onions), there was a bump upward in iron levels—from 8 percent more iron in fried tortillas to more than 2,000 percent (!) in applesauce.
"Foods with more moisture, more acidity, and longer cooking time increased more in iron content during cooking in iron cookware," the study concluded.
But wait. We're not even supposed to cook acidic foods in cast iron because they'll taste metallic and ruin the seasoning on the pan, right? Nah, says Serious Eats' J. Kenji Lopez-Alt, not for dishes or sauces with a relatively quick turnaround. But if you're planning on simmering something for hours, that's when a different vessel, say, enameled cast iron, will serve you better.
In the same study, researchers compared three foods cooked in two different iron pans, one that had been used daily for a week and one used only a few times prior to the study. While there was no big difference in the iron content of spaghetti sauce and scrambled eggs, applesauce cooked in the lightly used pan ended up with more than double the iron, suggesting that you'll get more iron out of a newer pan than the broken-in one bequeathed to you by Grandma.
More recently, America's Test Kitchen did its own experiment, simmering tomato sauce in a stainless-steel pan and in unseasoned and seasoned cast-iron pans. Lab tests showed the sauce cooked in the unseasoned pan—which isn't ideal from a cooking standpoint—had 10 times more iron than the sauce in the seasoned pan, which in turn had only a few grams more iron than the sauce cooked in stainless steel.
So, it's a little contradictory. From a culinary standpoint, you want a more seasoned cast-iron pan—but from a health perspective, you might get more iron from a new and/or unseasoned pan.
The bottom line, says Jennings? You'll end up consuming some extra iron when you cook with cast iron in general. Just don't depend on your pan to fulfill all your nutritional needs.
"You're still going to need to eat leafy greens and beans and meat," says Jennings.
