Also, for the most part, we're just borrowing the atoms in our food. The carbs are turned over pretty quickly, except for a small part which ends up in our DNA. Fats and oils the same, though some end up in odd places, after being modified and turned into more us. Proteins may stick around for longer, depending on which proteins and diet.

The only atoms we really hang on to are metals: iron, manganese, copper. They're relatively rare in nature, even in food, so we've evolved uptake and scavenging systems that try to keep them around as long as possible. People with adequate intake (that is, taking mineral supplements or having a very metal-rich diet) might hang on to some of their metal atoms for life.