Question

Using the science behind macro-nutrients, why marathon runners are so skinny?

Answer 1

This is because they train extremely hard in order to sustain stamina and endurance so, their bodies don't get the chance to build muscle because they burn more than they consume.

Answer 2

Professional marathon runners are also skinny because they train so hard to sustain endurance. This prevents their bodies from bulking up because they burn almost all the calories that they consume. Unlike sprinters, who need muscles, marathon runners don't need muscles at all.