It all started when I read Scott Jurek's Eat and Run, he mentioned he's a vegetarian since animal's eat plants therefore instead of eating animals, eat the plants and cut out the middle man.
This resonated with me since my little daughter was just born. I thought, my daughter eats breast milk why eat that when she can eat my wife's diet and cut out the middle man, right?
But WAIT, a newborn lacks the gut microbiome to process the diet of an adult, she CAN'T eat what my wife eats (obvious-fugging-ly). As humans can not process the cow's diet due to lack of gut biome and the ruminant fermenting tank of an extra stomach.
Paul Jaminet's Perfect Health Diet then mentions ALL mammals eat the SAME macronutrient ratio ~20% carbs, 65% fat, and 15% protein by calories. But ruminant animals 'eat' grass or (in the case of a giraffe) Acacia leaves, where's the protein and fats.
The bottom line of this disconnect is that eating and ingesting are VERY different. A ruminant animal eats - through the mouth plants. These chewed plants enter a fermentation tank (stomach) where a biome of microrganisms who double in count every minute devour these stomach contents. When the quantity of these gut animals reach a critical mass they are then sent to the stomach-digestive tract that more resembles an omnivores (or carnivores) for final processing and finally absorbed into the blood stream. The final ratio of these microbes + plants is 20% carbs, 65% fat, and 15% protein by calories. See the pattern?
Assuming you have an omnivores short digestive tract and no ruminant stomach and can subsist from plants alone is flawed logic and needs careful supplementation just to survive. I love vegetarians and vegans that feel they're making ethical decisions, but the logic of physics is not fact-based.
Obey your biology and you'll be fine.