A research team from Germany and Spain looked closely at known remains of teeth from the so-called Deninger's bear, a direct ancestor of the cave bear. The new analysis used sophisticated statistical methods, called geometric morphometrics, to compare the shapes of the mandibles and skulls of both bears. "The analyses showed that Deninger's bear had very similarly shaped mandibles and skull to the classic cave bear," explained Anneke van Heteren, lead-author of the study. So at the very least, Deninger's bear would already have been adapted to a vegetarian diet before the cave bear descended from them.
This is just another example of the reality that a body can be large and strong with a vegetarian diet.