Kitchen Mysteries is a weekly exploration of oddities surrounding cooking and food. They could be recipes that fail when they shouldn't, conflicting advice from different sources, or just plain weirdness. If it happens in a kitchen, and you're not sure why, send a tweet to The Food Geek to find out what's happening.
Becky Selengut asks via twitter:
and April F was nice enough to bring the post to my attention.
The short answer is that, yes, cast iron can absolutely increase the amount of iron in your diet, especially if you cook with acidic foods. Iron in and of itself is reactive with any number of substances, generally turning to rust if it meets up with oxygen. This caused the clever metallurgists to come up with stainless steel, which coats the steel with a film of chromium and nickel that will repel oxidizing substances. An older method of dealing with oxidation in pans is to "season" it with a coating of fat that keeps troublesome oxygen away.
When you cook in stainless steel or cast iron, acids will push aside the protective layer and dissolve not only the steel underneath, but also parts of the protective layer. Joel Kuligowski and Kopl M. Halperin did a study in 1992 for the Archives of Environmental Contamination and Toxicology where they verified that, not only does iron get added to your food (generally a good thing), but in the case of stainless steel, also chromium (also generally good) and nickel (potentially bad). For this reason, people who are nickel sensitive should avoid stainless steel cookware and find something else to cook with, especially if cooking acidic foods.
The nice thing about stainless steel is, except under specific circumstances, the protective film is self-healing and will protect the cookware from damage. For cast iron, with proper care you are re-seaoning it continuously, you can maintain it just about forever. It's very difficult to do permanent damage to a cast iron pan under normal kitchen use, so even if you get a little rust on it, a cleaning and seasoning process will make it great again.