If the question is what *can* one eat then pretty much all animals are omnivores. A flea’s mouth is too specialized to eat anything but blood so they are sanguinivores. A butterfly can drink nothing but nectar so they are nectivores. Yet cats, cows, and humans are able to eat anything so all three would be omnivores. If we use the term to describe what we can eat and almost all animals can eat everything then the term doesn’t mean very much and, for this reason, it’s not scientific. Imagine a sociologist who describes humans by the language that they *can* speak. Broad labels don’t give much information.
Even then, keep in mind that “omni” means “all” and humans don’t eat everything. We don’t eat wood like termites do. Calling us “omnivore” is neither precise nor accurate.
I use the term to describe specialization as many scientists do—like D.J. Chivers who influenced my opinion on this matter. Thereby cats are carnivores (meat specialists), cows are graminivores (grass specialists), and humans are folio-frugivores.
Florence is a captive nurse shark who chooses to eat only plants. She is not a florivore (plant eater) because she is not anatomically specialized to eat plants. She is a pescivore (fish specialist) who chooses to eat plants. http://www.huffingtonpost.com/…/florence-shark…
Humans are folio-frugivores. Most humans choose to eat animals but that does not make them carnivores or omnivores.