Good questions. I'm partial to Jaminet's view that mammals share a ratio of circulating aminos/lipids/glucose between meals and in their breastmilk, implying a common genome for energy management. The digestive input differs sharply but the output does not--e.g. ruminants get their lipids from excess fiber carbs, and predators get their glucose from excess protein. Human conversions are pretty lackluster so we should eat closer to what we utilize.
But I wish these issues wouldn't collapse into silly absolutes. No mammal eats zero lipid or zero glucose. Even wolves will eat windfall fruit.