[Dynseli]

Vegetarian with egg or dairy requires no adaptation, except expanding the vegetarian foods eaten. Some would argue that animal dairy required a recent adaptation, and we're not meant to eat it (ie lactose intolerance). Strict vegan hasn't been a part of the human diet, but perhaps an adaptation can be made, because there are people who live on it. I don't think we're (readily or easily) adapted to strict vegan, but humans have made adaptations from eating dairy to eating cooked meat. That vegetarian with eggs or dairy is suited for humans as much as omnivorism, is just my opinion. Others are entitled to favor veganism. The future of humankind is probably to eat only eggs and fruits, since these are bi-products, fruits made especially for human/animal consumption, that don't require killing.

B12 as far as they know is only made by some bacteria. Small amounts but not enough is made in the human gut. Cows make a lot of B12 in their gut because of their extensive digestive system. It is also made symbiotically in other animal products. Some herbivore animals sometimes eat dirt to get their B12. Also, animals are supplemented with B12.

Yeast by itself doesn't make B12, but I heard they fortify it. Maybe it has a relationship with bacteria to make it. Cereal is also fortified, maybe with B12, not sure.

The act of cooking meat causes it to have more carcinogens, than it may already have. Perhaps its the overheating, or not soaking it in wine beforehand.

Refined sugar also, depending on the methods, use animal bones to refine it. Not all refined sugar is made this way. Raw sugar is better all around anyways: no chemical residues, it has tiny amounts of other nutrients.

I'd like to disclaim, I'm not a vegetarian at all, but I'd like to eat less meat and eat more vegetarian foods (and dairy). Then try to stick with cruelty free. Eating a variety requires a garden, or a hefty budget.

@Jenniferlove - a pescatarian diet sounds healthy.