Answer: Wouldn't it be great if we could all go outside, and munch on some grass at lunchtime! But the human digestive tract is not set up for such a cellulose-rich diet. Most animals that consume grass and other plant material high in cellulose have complex stomachs designed for such a diet. Many are ruminants that chew their cud. We have evolved to consume an omnivore's diet--a mix of plant and/or animal material. Though the plant material we eat--fruits and vegetables--contains some cellulose, it is not as rich in the material as most tough grasses and leaves.
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