How much iron are we talking?
We’re talking a pretty decent amount of iron, depending on the condition of your cast iron pan, and what specifically you’re cooking.
To put it into context: men need 8 mg of iron a day, and a serving of tomato sauce cooked in a cast iron skillet can provide 5 mg of iron!
This effect is so reliable that iron cookware has occasionally been used to combat anemia. A variety of studies have shown that iron pots and pans can boost your iron status, substantially increasing the iron content of certain foods (like eggs and applesauce) but not others (like hamburgers).The pan’s iron is in the non-heme form, which isn’t absorbed as well as heme iron from meat. But vitamin C can greatly increase absorption, as can acidity, so recipes containing things like lemon or tomato sauce can boost absorption.
If you don’t want your cast iron pan to leach so much iron, make sure it’s well seasoned. Since acidic foods help transfer iron from the pan into your food, you want to put a barrier between the acid and the iron. And that barrier is seasoning.
References
^Brittin HC, Nossaman CEIron content of food cooked in iron utensilsJ Am Diet Assoc.(1986 Jul)^Geerligs PD, Brabin BJ, Omari AAFood prepared in iron cooking pots as an intervention for reducing iron deficiency anaemia in developing countries: a systematic reviewJ Hum Nutr Diet.(2003 Aug)
^Adish AA, Esrey SA, Gyorkos TW, Jean-Baptiste J, Rojhani AEffect of consumption of food cooked in iron pots on iron status and growth of young children: a randomised trialLancet.(1999 Feb 27)
US 24565 
BR 22786 
GB 12692 
CA 9422 
AU 7896 
IE 5621 
IN 3535 
TR 2939 
