Then why replace one imprecise term with another? Fiber is a carbohydrate. Humans use close to nothing from its energy. (Though it plays another important role in the digesive system.)
Try eating 100g of grass per hour during a marathon and you will see. That's the metabolic edge horses have over humans.
Horses don't eat during races (and aren't evolutionarily disposed to marathons, anyway). No edge there; it takes quite a while for their symbiotic gut flora to downconvert fodder to glucose.
Try eating 100g of grass per hour during a marathon and you will see. That's the metabolic edge horses have over humans.