Does cooking in cast iron increase the iron in food?

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First, a little bit about Iron…

Iron is an important mineral which the body uses to preform important everyday functions. Iron is utilized to make hemoglobin and myoglobin, both of which are important proteins used in the distribution of oxygen throughout the body (National Institutes Of Health, 2023). In addition to this vital function, iron also plays an important role in immune function, the production of certain hormones, and thyroid function. 

Iron can be found in foods in two main forms: heme and non-heme iron. Non-heme iron is found predominantly in plant foods as well as some animal-based foods, while heme iron is only found in animal-based foods (National Institutes Of Health, 2023). While we typically associate high dietary iron levels with red meat, a number of plant-based foods such as spinach actually may contain more per gram iron than red meat (see this link to compare iron content of different foods). This is however a little misleading since the bioavailability (ability to be utilized by the body) of the non-heme iron found in plant-based foods is lower than that of heme iron found in animal-based foods. Approximately 25-30% of dietary heme iron is absorbed by the body, whereas only 1-10% of non-heme iron is absorbed (Skolmowska & Głąbska, 2019). Those on a plant-based diet may need to be more aware of ways to maximize their iron absorption throughout the day in order to reach the recommended daily allowance of iron for their demographic (see this link to find out your iron RDA), such as pairing iron containing foods with those high in vitamin C (Péneau et. al., 2008). 

So...how does cast iron cookware fit into this?

Many cooking materials appear to impact the nutritional end product of our meals, with many cookware items releasing minerals based on the material and age of the cookware used (Shamloo et. al., 2024). This can be a good or a bad thing depending on both the minerals released and whether the person eating the meal is already meeting the recommended daily allowance levels for those minerals. 

Iron is one of the minerals that many people may be deficient in, especially those in demographics who require higher iron intake levels (children, teens, women of childbearing age, those with certain chronic diseases) or those whose dietary patterns make it more challenging to meet the recommended daily allowance for iron (vegetarians, vegans). The rumor that cooking in cast iron cookware can increase the iron in your food does appear to have some solid science behind it. 

The results of multiple studies indicate that foods cooked in cast iron contain a higher iron content than those cooked in other forms of cookware. Researchers have found that cooking in cast iron increased the iron content of pea paste (Xing et.al., 2018), apple sauce and spaghetti sauce (Cheng & Brittin, 1991). While both of the previously mentioned studies looked at the iron increase in non-heme iron containing foods (plant-based) other research has examined the iron content AND bioavailability of heme iron containing foods (animal-based), and found that the iron content of meat and absorption of that iron in the body increased more than that of legume and vegetables when cooked in cast iron (Sharma et. a., 2021). Foods cooked in cast iron which are also high in vitamin C, such as fruits and even lemon water, have been found to have elevated iron which is absorbed in the body at a substantially increased efficiently (Sharma et. a., 2021), something that may be good news for folks following a plant-based diet who want to increase their efficiency of iron absorption from a meal.

Bottom line: Cooking foods in cast iron cookware appears to increase the iron content of food and its absorption. This is especially true of foods which contain heme iron (animal-based) or foods high is vitamin C.