Health: If one looks at the alimentary canal of humans and compares it with herbivorous, omnivorous and carnivorous, it can be easily understood that our digestive system is far more closer to that of herbivorous animals. Lets look at some of the facts:
* our saliva is alkaline which is similar to Herbivorous animals, we are supposed to chew the food thoroughly, as much to mix it with saliva as to break it up in small chunks, as saliva helps begin the digestion process with the food in the mouth before we swallow.
* Our intestines are relatively long like that of herbivorous animals, compared to carnivorous animals, which is basically designed to absorb nutrients from vegetables and fruits for a longer period of time.Meat putrefies quickly and staying in the intestines for a longer period will only lead to all sorts of diseases in the long run. That's why carnivorous animals have small intestines which make them to excrete the flesh faster.
* The upper and lower jaws of humans are designed to munch, chew both in vertical and horizontal directions. That is not the case for carnivorous and omnivorous animals. In case of carnivorous animals they are designed to tear apart the flesh, bite out chunks, and then swallow wholly.
* Acid in herbivorous animals are weaker than that of carnivorous animals. Humans have weaker acid compared to that of tigers or lions, etc which also means we are very similar to herbivorous animals. Since carnivorous animals cannot chew their stomach acid is evolved to be stronger than that or herbivorous animals.
* Our body is mostly alkaline in nature except stomach and couple of other parts. Eating acidic foods like milk, meat, fish, eggs increases the acidic nature in the blood thus leads to all sorts of problems.
The Indian subcontinent was predominantly vegetarian some decades ago, with very little consumption of milk, but this has changed. Predominantly meat was earlier consumed by western cultures, because of the scarcity of vegetation during winter, and desert conditions elsewhere. Since these empires were involved in conquest, the conquered peoples started eating meat. One of the primary reason to eat meat is because kings, and other elite upper classes ate meat, and non elites considered eating meat as a sign of social status and began eating meat, abandoning all the good things that comes from being vegetarian. Now in India, after-wedding parties which do not serve meat are considered low class, lacking in status.
Looking at the tribal people of Africa and Amazon regions, indigenous people hunt an animal once in few days or week, and the consumption of meat by any individual person in that tribe is very minimum and not to the level we now see in modern society. They rely on a variety of berries, nuts, wild honey and other plants for their overall nutrition.
The fallacy which the food industry of western society tends to portray via bad coverage in the media, is that obesity, cancer, diabetes, are due to eating a high carb diet like pasta, rice, wheat, sugar, etc. These carbs are called simple carbs, lacking in fiber, providing a burst of glucose when consumed jacking up insulin levels and then stored as fat. These foods are not that great and should be easily replaced by millets, vegetables and fruits with a good amount of fiber that does not increase insulin levels excessively and produce constant energy all through the day.
Also since majority of the vegetables are from argiculture eventually the soil is depleted of minerals which are essential for human body as part of eating vegetables/fruits. Since lot of farmers do not care for such minerals in the soil, meat industry shows another reason why vegetables lack nutrition in the modern day. So it is important to replenish the soil with essential minerals.
Another bad example is given to vegetarian is because lot of pesticides are used in growing vegetables consumng them without properly washed will lead to all sorts of chronic illness. To avoid such it is highly recommended to consume organic foods thus avoiding pesticides.
There can be a case to be made that purely vegetarian can cause deficiency of B12, however B12 vitamin deficiency is also present with non-vegetarians. Vitamin B12 comes out of bacteria so they can be easily found in nature.
All of the minerals, vitamins, fats, carbs, proteins and other essential and non essential nutrients are found in greater abundance in plant life than in meat. It's not to say completely avoid meat however, it is recommended to reduce it greatly. So for optimum performance of mind and body for better health vegetarian food is the best.