"Although we think we are one, and we act as if we are one, human beings are not natural carnivores. When we kill animals to eat them, they end up killing us because their flesh, which contains cholesterol and saturated fat, was never intended for human beings who are natural herbivores." - William C. Roberts, M.D., editor, American Journal of Cardiology
Being vegan (more accurately, eating a whole foods plant based diet) since August 2006, I commonly talk to misinformed people who defend the common Western Diet (based on meat, eggs, milk, poultry, etc.) as healthy. The question I am asked most often is how do I get protein - as if it is not possible to have protein in a plant-based diet. I now realize it is a common misconception.
Instead of debating which diet is most healthy, let's look at the comparative anatomy of Carnivores and Herbivores. It is a scientifically demonstrable fact that biological form usually defines function.
Here we go:
The above demonstrates that humans have absolutely no anatomical characteristics in common with Carnivores. Wow. It looks like our Creator, or evolution (depending on your beliefs) designed humans to be Herbivores. Furthermore, consuming a diet that we are not designed to consume (ie Western diet, animal products) results in sickness and disease that can easily be avoided by eating a plant based whole foods diet.
How many carnivores/omnivores on the planet have to cook their food (meat) before they eat it? How many would get sick an die if they do not? It seems that carnivores & omnivores are designed so that meat does not make them sick and die, unfortunately for us it has the opposite effect.
Notes: For a more detailed summary table that compares the properties of carnivores, herbivores, and omnivores side by side, see this page: Comparative Anatomy & Taxonomy, also see an article by Dr. John McDougall and one by Dr. Milton R. Mills (both M.D.'s), this page for a similar take, or this page for a different perspective on it.