Originally Posted by verklingen

As the meat of a dead animal sits (including in the intestines), it begins to rot and releases toxic gases. These gases do not bother carnivores because they have a short digestive system (only about 3 times their body length) which allows them to quickly eliminate waste from animal flesh. Herbivores (including humans) are generally built with a very long digestive system (about 12 times their body length) which helps them break up the coarse fibers of plant matter and extract its nutrients. So when we eat flesh, it rots within our bodies for a very long time, and this can cause many complications.

Aside from this, there are other clues indicative of the human's natural status as an herbivore. Our teeth are built for crushing and mashing plant matter; a carnivore's teeth are built for grabbing and ripping flesh free from a body. Our jaw is built to move side to side in order to grind plant matter into a fine pulp; a carnivore's jaw simply opens and closes, chewing up and down and swallowing what cannot be pulverized. Our neck is built for extending forward and up; a cornivore's neck is built for reaching down and twisting flesh free. Even beyond all these points, our stomach acid is not equal to the task of properly breaking down animal flesh to make it easily digestible.

The Greek thinker Pythagoras is quoted as saying:

“Oh, my fellow men, do not defile your bodies with sinful foods. We have corn, we have apples bending down the branches with their weight, and grapes swelling on the vines. There are sweet-flavored herbs, and vegetables which can be cooked and softened over the fire, nor are you denied milk or thyme-scented honey. The earth affords a lavish supply of riches, of innocent foods, and offers you banquets that involve no bloodshed or slaughter; only beasts satisfy their hunger with flesh, and not even all of those, because horses, cattle, and sheep live on grass.”

The Roman Plutarch replies thus:

“Can you really ask what reason Pythagorus had for abstinence from flesh? For my part I rather wonder both by what accident and in what state of mind the first man touched his mouth to gore and brought his lips to the flesh of a dead creature, set forth tables of dead, stale bodies, and ventured to call food and nourishment the parts that had a little before bellowed and cried, moved and lived. How could eyes endure the slaughter when throats were slit and hides flayed and limbs torn from limb? How could his nose endure the stench? How was it that the pollution did not turn away his taste, which made contact with sores of others and sucked juices and serums from mortal wounds? It is certainly not lions or wolves that we eat out of self-defense; on the contrary, we ignore these and slaughter harmless, tame creatures without stings or teeth to harm us. For the sake of a little flesh we deprive them of sun, of light, of the duration of life to which they are entitled by birth and being.

“If you declare that you are naturally designed for such a diet, then first kill for yourself what you want to eat. Do it, however, only through your own resources, unaided by cleaver or cudgel or any kind of ax.”

Plutarch's last point is very illuminating. If you want to eat like a tiger, get your food like a tiger, using only those assets with which you were born. How can it be said that killing animals is good because we need them to eat? It's madness, if you ask me.