Updated: Jun 4, 2022
Herbivores have ruminant stomachs to ferment their food and derive nutrients.
In herbivores, the building blocks to construct their own biological cells are derived from the metabolization of ruminant gut microorganisms as the microorganisms are living and dying in countless millions of generations.
Herbivore nutrition, therefore, comes from the literal "deconstruction" of fermenting microorganisms in the stomach because plants themselves do not contain the fat-based lipid cellular membranes like those found in the fermenting microorganisms. Herbivores maintain their biological cells, like cells in skin or stomach lining, by using the raw material gathered from fermenting microorganisms.
Carnivores, however, do not have a fermentation chamber in their stomach, and so cannot derive essential nutrients from plants because they lack the microorganisms.
In order to construct their biological cells, carnivores must derive fatty nutrient complexes, like vitamin B12, directly from flesh, directly from the cells of their prey.
Necessary building blocks to construct the biological cells of animal bodies are contained in fatty complexes like B12, and their many constituents, found in both animals and fermenting microorganisms.
Because humans require vitamin B12 as an essential fatty nutrient, and because humans do not have a gut fermentation chamber to derive "cellular building blocks” from fermenting microorganisms, humans therefore must be, to the largest extent, obligate carnivores.