“Whatever is left over of the meat, that bury in a hole, for there is none in heaven and earth, among the Samanas or Brahmanas, among gods or men, by whom such food can be eaten without hurt to himself, save alone the Tagatha.”
-Buddha, speaking to Chunda shortly before his death
The Buddha was most likely speaking of the karmic harm of eating meat, which is tainted with death and thus breaks the code of non-harming (common to Buddhism and yoga). He could as truly have been speaking about the physiological and environmental effects of eating meat. The most convincing reason (to me) is one given by Nature herself: when fruits and vegetables are ready to eat, they turn colors or otherwise show their readiness to be eaten. If we do not eat them, they fall to the ground, giving themselves to the animals, that they might live. Animals on the other hand, run away from us when we try to eat them, calling for their mothers.