I’ve read years ago that those skinny lumberjacks needed/required 10k calories per day to work and not starve to death. Most of those calories were beef fat from boiled beef and drippings mopped up with bread.
Not 10k
The US Army determined “heavy exertion” needs 4,500 calories. Something like that. And when they classify “heavy” exertion, they know what they are talking about. The USDA 2000 calorie RDA is intended for moderately active individuals, most people can easily gain weight with those nimbers, especially elderly or sedentary.
That number would not surprise me. There’s hardly any protein in beef fat, though. It would take a lot of beef protein to keep muscles going.
Did you see the lumbering movie in the Pacific Northwest called “Train Dreams”? Interesting look at the life of timber crews back in that era. It’s about an “everyman” eking out a living, falling love, and tragedy.