The phrase "unnatural diet" refers to eating food that is not typically consumed or considered natural for a person or an animal. It could include processed or artificial food products instead of natural, whole foods. Full definition
These Salmon have been enclosed in pens and fed a very unnatural diet of corn meal, soy and even chicken feces pellets. (fitnessandpower.com)
They get trucked cross-country to pollinate vast crops, often while fed unnatural diets such as sugar water and soy flour. (scientificamerican.com)
Because farmed fish are raised on unnatural diets and in small enclosures they often breed disease, which can pass to wild populations. (wellnessmama.com)