|MadSci Network: Zoology|
Lions and other felines can theoretically survive short-term on a vegetarian diet, but would need extensive supplementation to survive long-term. Felines have a very high requirement for certain amino acids (the building blocks of protein) that are only found in meats. If these were supplemented, the chances of long-term survival would increase. Their ability to digest some plant materials is also limited because of a lack of the correct enzyme in the intestinal tract.
Try the links in the MadSci Library for more information on Zoology.