Although it is true that many people do well on vegetarianism, it depends on the individual as everyone has different genetics. Some people require more animal products than other.
There have been many cases where people after years of vegetarianism eventually ran into health challenges which was remedy by adding in animal products.
On the Livin La Vida Low Carb Show, Denise Minger talked about her years of being a vegetarian until she encounter health issues. Her health improved when she added back animal products. She then started learning about nutrition and gave the talk “How to Win an Argument with a Vegetarian“. Some people believes that eating purely plant product is healthier. Her talk debunks some of those myths.
Similarly, Chris Masterjohn talked with Dr. Mercola about similar situation of running into health issue when Chris was a vegetarian. Again these problems disappeared after adding back animal products. He believes that everyone can benefit with at least some animal products. Chris Masterjohn is now studying nutritional science as an PhD candidate.
Others that believe in the health benefits of animal products includes the Paleolithic diet advocates and the Weston A Price Foundation.
For those who want to avoid land animal meats, consider eating fish. If not, consider bivalves (such as scallops and muscles) which do not have brains and can not feel pain. If not, at least consider eggs. Eggs are very healthy and consists of complete high quality protein as well as vitamin B12 and other nutrients.