"In order to reconstruct the diet of Neandertals, scientists have analyzed the stable isotopes of nitrogen and carbon in Neandertals’ bones. Identify the group of animals with which Neandertals share the closest levels, indicating that they had a similar diet."

Neanderthal had a diet based in different food, for example, there are some evidence that they were feed on nuts, mushroom, and moss. The quantity of veggies were different depending the season and the region of the planet the neanderthals were. Of course, most of their diet were based in meat, basically of big animals because the evidence is that they had a digestive system appropriate to eat lean protein.

Studies of the bones and the coprolites showed a ratio of different elements as nitrogen and carbon, the ratio of the heavier element to the lighter, show the relationship among the meat/vegetable diet.

Hope this info is useful.