Not a heck of a lot on an atomic scale. Iron is Iron. Metallic iron is a large amount of iron atoms arranged in a crystalline lattice. Dietary iron is usually one iron atom coordinated with a biological molecule. An example would be hemoglobin.
If you swallowed a piece of iron metal, some of it would react with the acid in your stomach to form an iron "coordination compound" (inorganic chem term) such as FeH6 or FeCl6 etc. This iron would eventually be incorporated into a biological molecule via enzymatic reaction or it would be eliminated from your body by the liver or kidneys. The majority of a piece of iron metal would pass through the alimentary canal.
If you swallowed a piece of iron metal, some of it would react with the acid in your stomach to form an iron "coordination compound" (inorganic chem term) such as FeH6 or FeCl6 etc. This iron would eventually be incorporated into a biological molecule via enzymatic reaction or it would be eliminated from your body by the liver or kidneys. The majority of a piece of iron metal would pass through the alimentary canal.