One thing to think about is that for Indians back in those days fat was hard to come by as wild animals are very lean and eating just lean meat can lead to protein poisoning or "rabbit starvation". The people who killed bison for their tongues were probably after the fat in them....their bodies were probably craving it. Also the organs were probably prized for the same reason. Sugar and carbs had to be a mostly seasonal thing and what fruit and berries they had were not sweet like our hybridized versions. Fat is the other concentrated source of energy to run a human body and brain.
I think I remember accounts of Lewis and Clark's men struggling to do hard labor on a diet of deer and elk and were glad to get beaver, other rodents, and dogs from the Indians because of the fat.
