Effect of cooking food in iron-containing cookware on increase in blood hemoglobin level and iron content of the food: A systematic review - https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8266402/

> Thirteen researches were found to be suitable for inclusion in this systematic review. Four studies reported significant increase in blood hemoglobin levels while others reported only a minor increase. Significant improvement in amount of iron in food and iron bioavailability was also observed when food was cooked using iron pot or ingots.

https://www.americastestkitchen.com/guides/cook-it-in-cast-i...

> The Myth: When you cook in a cast-iron skillet, your food will absorb a lot of extra iron so you can effectively supplement your diet by using this type of pan.

> THE TESTING: We simmered tomato sauce in a stainless-steel pan and in seasoned and unseasoned cast-iron pans. We then sent samples of each sauce to an independent lab to test for the presence of iron. The unseasoned cast iron released the most molecules of metal. The sauce from this pot contained nearly 10 times as much iron (108 mg⁄kg) as the sauce from the seasoned cast-iron pot, which contained only a few more milligrams than the sauce from the stainless-steel pot.

> THE TAKEAWAY: Since this occurs in pronounced amounts only with unseasoned skillets, which you wouldn’t use for cooking, we don’t consider this an issue. A seasoned cast-iron skillet will not leach any appreciable amount of iron into food cooked in it.

http://www.wsj.com/articles/can-a-pan-add-iron-to-your-diet-...

> A 1986 study published in the Journal of the American Dietetic Association found that a cast-iron pan used daily for a week transferred less iron to certain foods than a nearly new pan. The same study found widely varying levels of iron transferred to 20 test foods.

> There was hardly any transfer of iron from frying potatoes and cooking green beans compared with cooking in a glass-ceramic control. But cooking apple sauce in the iron skillet added seven milligrams of iron to each 3.5-ounce serving. Cooking spaghetti sauce added five milligrams. Adult women should consume 18 milligrams of iron a day, and men and women over 50 need eight milligrams, according to government guidelines.

The referenced study: Iron content of food cooked in iron utensils - https://pubmed.ncbi.nlm.nih.gov/3722654/

> Twenty foods were cooked in iron and non-iron utensils. Also, three foods were cooked in two iron skillets. Three replications were made, and cooking time and pH for each food were determined. Duplicate samples of the raw and the cooked foods were dried, ashed, and analyzed for moisture and iron content. Iron content was determined by atomic absorption spectrophotometry. Most of the foods (90%) contained significantly more iron when cooked in iron utensils than when cooked in non-iron utensils. Acidity, moisture content, and cooking time of food significantly affected the iron content of food cooked in iron utensils. Perhaps because of differing amounts of previous use, cooking in different iron skillets resulted in some variation in the iron content of food.

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Yes, if you've got well seasoned cookware, it's not significant (compared to cooking with stainless steel).

However, if it is a regular cast iron cookware that is poorly or unseasoned, there can be a significant amount of iron leaching into certain foods.