Herbivore vs Omnivore.

In this day and age vegetarianism is believed to hold more health benefits than staying an omnivore.
However, the human species was evolved with the ability to consume both plant matter and animal matter. Ancient ancestors - the hunter/gatherer type suits this omnivore lifestyle. This is due to meat being higher in protein and energy which would have to be required to hunt the animal in the first place (as for hunters). But sometimes suitable prey is not always found and so it is easier to look for fruits and edible plant matter (as for gatherers).

Basically you expend more energy and as a result can obtain more energy at a time. This can be seen in nature with herbivores and carnivores. Many Herbivores spend the whole day eating - a constant energy gain and release (moving around slowly looking for food) while carnivores can chase after prey and gain a large amount of energy while expending alot of energy to chase their prey.

However, in today's so called 'civilized society' generally, we gain alot more energy than we expend. Of course this results in many health problems such as obesity and cancers or other illnesses. Eating meat without hunting for it in the first place is a great way to get obese - unless you have a way to use the energy obtained (physical exercise etc). Because plant matter has less energy that can be obtained than animal matter, vegetarianism is much more suitable for people who dont expend as much energy as ancestral hunters which is the majority of society now. The rise of obesity is expected as more people are less active due to the convenience of technology and simply there being too much food available in certain parts of the world.
If humans were to return to ancient ancestral ways, obesity would not be a problem anymore and the people that survive will be more fit and healthy that many people of today (although if today's society suddenly had to resort to hunter/gatherer tendencies i think half the worlds population would probably die - just goes to show how weak the human race has become).

I am aware that survival rate of hunters/gatherers is probably somewhere around 50-70% but at least the weak are left out of the gene development so that a stronger human race can continue to develop.