Just to add to the anecdotal data, I've had the same experience upping animal fat and being able to be productive (mentally and physically) even without eating, and I work a physically demanding job at night, either of which alone can induce fatigue. I eat mostly butter, egg yolks, cream, coconut oil, and fatty cuts of meat like pork belly and fatty ground beef (epsom salts, mineral water and magnesium supplements take care of any muscle soreness).