Pandas (Ailuropoda melanoleuca) are among the pickiest eaters in the animal world. In the wild, they eat more than 12 kilograms of bamboo each day--and little else.
They have to eat so much because, although bamboo contains proteins, sugars and fats among other nutrients, most of its calories are locked in hard-to-digest cellulose fibers that make up plant cell walls. A 1982 study of two pandas, Ling-Ling and Hsing-Hsing in the Smithsonian National Zoological Park in Washington DC, found that 92% of the cellulose and 73% of the hemicellulose (a chemically similar fiber) in the bamboo they ate passed right through their digestive tracts and ended up in their feces.
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Article source: Nature - "Microbes help giant pandas overcome meat-eating heritage" (http://www.nature.com/news/2011/111017/full/news.2011.596.html)
