Humans are omnivores, as during their evolution they required the ability to eat a broad diet in order to survive. Carnivores develop in species that are swift and voracious, but only where a continuous and ample supply of prey exist. Imagine a tiger with nothing but grass to eat. The tiger may be willing to eat almost anything when the hunger pangs get strong enough, but his digestive system wont process anything but meat. Carnivores die when their prey dies out. Species that are vegetarians also have specialized digestive tracts, designed to break down the tough fiber that is intrinsic to plant life. They have multiple stomachs in many cases, digesting in stages. All species that eat plants consume insects as a matter of course, as insects are scattered throughout the plants they munch on, and thus are always part of the intake.
Thus, species that evolve are either carnivores, vegetarian, or omnivores. The omnivore, of which mankind is a member, evolve to meet wildly swinging cycles of food availability. Early humans, being land animals and highly mobile, could travel during drought to areas lush with vegetation. Strictly vegetarian animals do this likewise, but as their digestive tracts digest fiber effectively, they can nibble on dried vegetation on the way. Humans, evolved from apes which were adjusted to eating fruits and insects as well as vegetation, do not have the apparatus to digest fiber. Thus, while on the road during droughts, they would have starved unless able to kill and eat meat. They have dual digestive systems, in effect.
Humans are designed, due to the influence of food availability during evolution, to eat either vegetables and fruits or meat, but not both at the same time. This is a fact not widely recognized or understood by humans, and thus they do themselves damage by eating both foods at meals, routinely. Imagine the cave man on the road, traveling to lush fields of vegetation where fruits and grains and tubers could be located with ease. The troop kills a deer or elephant, and feasts on nothing but meat and blood for days, consuming the entire kill before it can spoil. They do this repeatedly while on the road. When they arrive at their destination, they find they no longer need to take the physical risks that hunting invariably presents - flailing hooves and charging frightened beasts. They become vegetarians.
Modern man misunderstands what the cave man ate while lolling about during their vegetarian periods. They did not live strictly on vegetables and fruits and grains. They ate any and everything that was handy, and this included numerous insects and slow moving life forms such as mollusks and possums. They ate less meat, but the diet was highly varied and included occasional small bites from sources other than plants. Thus, those modern humans who try to live what they interpret to be a strictly vegetarian life suffer from malnutrition - poor immunity, anemia, lack of strength, and inability to deal with stress. Man was not designed to live by vegetables alone, and must accommodate their body with protein sources from living creatures other than plants, or suffer the consequences.