I subscribe to a thing called Metabolic typing. Basically metabolic typing says that each person processes food differently. Person A may need lots of Fats and Oils to function optimally while person B may need certain veggies and grains to function optimally. If you feed Person A a diet of ONLY veggies and grains, he will lose energy, get fat, and get sick, while on that same diet Person B feels great loses weight and stays healthy. This is why the cookie cutter low fat low calorie diets do not have lasting results for everyone. (The FDA is a bunch of corrupt jerkbags, but that is a whole different story.)I USED to think this was true, but now I've found this argument completely untrue for vegetarianism
There are various ways to obtain nutrients such as Iron, Protien and Vitamin B12 aside from eating meat, fish and fowls
If you have a protein heavy metabolic type, vegetarianism will make you sick, and unhealthy. For many, you cannot get sufficient amounts of protein, vitamin B12, vitamin A and vitamin D from a vegetarian diet. It would require supplements which totally ruin your caloric intake. The neat thing about food is that the calories you get from eating are more than enough to process the nutrients in the food. Supplements do not give those calories and vegetarianism makes it difficult to make up for it.
Vegetarianism can work, but you have to be a person who can optimally work on non-meat fuel. I would also HIGHLY recommend any vegetarian eat organic vegetables. Plants become healthy foods by the nutrients they gain from the LIVING soil. By living I mean the micro organisms found in soil. Pesticides kill these micro organisms and basically make fruits and vegetables nutritionally useless.