I figured the veganism thing was going to come up sooner or later.
Pure carnivores feed only on meat. Their teeth are designed for killing and slicing flesh, not for grazing and grinding up plants, and their digestive system is designed for processing flesh - it cannot break down the cellulose found in plant material. This is why cats vomit when they eat grass, which they do deliberately to help them bring up fur they have swallowed whilst grooming.
Cellulose is difficult to digest, and requires a large stomach in which to 'stew' the food, symbiotic bacteria to help break it down, and a long gut to give the nutrition time to be absorbed. Some animals effectively eat their food twice in order to gain the best nutrition from this low-quality food - some, like cattle, regurgitate what they have eaten and chew it a second time ('chewing the cud'), before it goes to another stomach chamber for further processing. Rabbits eat their droppings, allowing the food to pass through the digestive system twice.
Carnivores do not have these problems, as meat is a high-quality food. They have short digestive systems, since meat is easier to digest and contains more nutrients than plant matter, and lack the symbiotic bacteria required to break down cellulose. In other words, even if you could get a lion to eat plant matter, it would not be able to digest it, and would eventually die of starvation or malnutrition.
In humans, we have a combination of traits from both digestive systems. We have long intestinal tract for processing plant food, a smaller stomach with more powerful acids than most other creatures for processing meats and plant matter. We are designed for ripping apart flesh and plant matter, and processing both, for obvious reasons. To ignore nature is to smack her in the face. Veganism is a smack in the face to nature's design. Nature made you this way for a reason. You need other meats to survive. Synthesized sources from plants (just like minerals and vitamins found in supplement pills) are not as effective or as healthy for you as eating it directly from a source that contains it. In fact, soy milk is shown to inhibit protein processing. Oops, so much for keeping you healthy.
Remember the vegan parents that gave their baby a vegan diet and killed it? If vegan diets are not complete for growing babies, what makes you think it's good for a full-grown human? Babies need the EXACT same things as adults to grow and survive. If it won't support a baby, what would make one think it's going to support a fully-grown human in a proper fashion?
It won't. I have a vegan friend with MS. Our little circle of friends, plus our circle of doctors have tried to get her to eat meat instead of going to soy for protein and using vitamin supplements. She refuses and deteriorates faster than any MS patient that eats a regular diet with meat and eggs and fish. The doctors have told her it is specifically her diet causing this. It breaks my heart every day to look at her, who used to be a beautiful busty woman, and see what looks like a man, now.
That "B12 myth" isn't a myth at all. Vitamin B12, whether in supplements, fortified foods, or animal products, comes from micro-organisms. A vegan diet would strictly forbid this as it is an animal product by definition. As stated before, supplements are not as effective as finding the natural source and consuming it.
I'm a member of a community full of vegans and other free-minded hippy people, known as Radical Faeries. I've seen thousands of vegans - most with pretty bad health problems that obviously stem from a poor and improperly-balanced diet.
