When we eat a can of tuna, we're ingesting a portion of the yellowfin variety of this oily fish, usually large. The other two ingredients we're ingesting are oil, which can be either olive oil (in rare cases, extra virgin) or sunflower oil, and salt. In fact, the proportions of salt can be high because it's the ingredient used to preserve the tuna in the can. For this reason, Dr. Pere Gascón states that "a can of tuna doesn't replace one of the three recommended weekly servings of fish." The doctor also warns about the origin of the canned fish. "Often they're not locally caught fish, but quite the opposite," he says.