I've been reading about nutrition to help me adapt to a vegetarian and mostly vegan diet, and I'll put my conclusions here. My main sources are looking at the results of scientific studies with large sample sizes, websites especially
Vegan Health Home Page, and a book on vegan nutrition.
Becoming Vegan: Express Edition by Brenda Davis, Vesanto Melina
1. The argument for vegetarianism and veganism should primarily be about animal rights, animal welfare and the environment, not which diet is more healthy.
2. There is not much difference between human health overall on diets with and without meat.
3. If a diet is carefully planned from a nutrition stand point, vegetarian and vegan diets are probably slightly superior. If a diet is carelessly planned, including meat or diary is probably slightly superior.
4. Vs a diet with a large amount of meat, especially red meat, vegan or vegetarian diet is probably better. Vs a diet with a modest amount of meat there is not much difference.
5. For overweight people (and especially older overweight people with cholesterol build up) a vegan diet would be particularly good.
6. Diets with plenty of meat are characterized by an excess of certain things, especially fat and cholesterol. Vegetarian and more so vegan diets the concern is the opposite - a deficiency of things, with Vitamin B12 at the top of the list and then a few others. (Iodine could be one to watch.)