All this is true, but there is a very interesting counter-argument put forth by a book supporting vegetarianism. How does the cow (or any other veggie animal) get protein? Not by eating other animals of course. So the notion that you have to be a carnivore to get protein is flawed. All plants have proteins.
Further, after 30, we only need a replacement level of protein as (most of us) do not grow any taller.
1. Homo sapiens are omninivore by the process of human evolution. They eat meat, vegetables and fruits. Just plant food alone can't provide the human animal all the amino acids needed, and humans can't biosynthesize all amino acids de novo. Which means some of the amino acids must come from other animals like cows.
In that sense, a pure vegetarian (Vegan) is not taking a balanced meal; sooner or later, their health will be affected.
2. Cows can biosynthesize all the amino acids, and they are largely herbivore, although as Mr. Nara pointed out, they can take animal feed mixed with their usual corn meal.
3. Some people ask if the Camels and Elephants live happily with pure plant food, why not humans?
Well, the biology of camels and elephants are quite different from humans.
4. Here is a surprise, even in the human body, neurons can't make glutamate - an important neurotransmitter and an amino acid - on their own, they have to get it from astroglial cells that surround them! Why?
Only Nature and Biological Evolution knows!
5. Most of the TBs are vegetarians, but they consume plenty of whole milk from cows (killing nearly 50% of the calves in this process in India)!
6. Vegetarianism came to India because of Jains and Buddhists... Hindus copied them very late, may be around 800 CE.... with a modification of consuming fish and/or cow's milk....that's the rest of the story.
Cheers.