[teiferer]

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.

[IAmBroom]

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.

[teiferer]

My gut flora won't do that, so they do have an edge. (Not during a marathon, but that wasn't my point anyway.)