While you can get all the nutrition you need from eating animals, you have to eat the organs as well. Also the stomach contents of herbivores even. Wolves eat the stomach contents of deer and such to get their green requirements, everyone needs some green in their diet. But liver, kidneys, most of the organs need to be consumed if you live on only animals.

The germanics basically did for a long time. They ate meat, with a little cheese and milk products and a few veggies thrown in there, a negligible amount of grains, for a long time, including the years after they just appeared in central europe around the 1st century BC or so, there were celts and gauls before that in those areas, both broad terms of many groups to be sure.

But they were all giants for the day, the men were all 6 foot and above, while the romans were more like 5’2" and subsisted mostly on lentils and grains, along with seafood, but very little meat, at least on campaigns. That extra size and bulk obviously didn’t do the germans much good in war, organization means more than brute strength ever could.