'Due to genetics, some people will be able to get really skinny and lean and that is fine with them, others, like Pre, are not destined to be that skinny.'
It's not just genetics - there used to be plent of skinny people 100 years ago - it's called being starved. Some Ethiopian told me how difficult it was to run when they were younger and too hungry. We have it easy trust me. They had no money buy gave me dinner whenever I visited them and one of them gave me one of his only 2 Ethiopian vests. He's now a millionaire! (well married to one). I digress!
Part of the reason people are taller and bigger now is down to diet during upbringing. Sadly we have gone to far the other way. When you are talking runners though don't forget they are not people with a regular build. Even now plenty of runners - have that slim athletic build. On the other hand even Bekele has powerful thighs
Running a lot does naturally slim your build though and you have to make sure you maintain stength.
We still differ on protein though, but what would you consider the best form to take it. I would think some sort of white meat?