Interesting idea. I'm very serious about improving gut health myself, for a multitude of reasons.
I do think there is a difference between people who can actually eat "anything" and "not gain weight" vs. people who just look like they do. I used to think my husband was one of the former, but when I learned to distinguish between grains/sugar and the rest of food, I realized that he just naturally eats only meats, veggies, fruits, dairy, & eggs w/ little grains, b/c that's what he's always naturally leaned to. I used to curse him (figuratively, lol) for eating steak & butter & bacon and never gaining weight, imagining he was eating huge quantities of "unhealthy" food, but now I can see that he eats decent man-sized (ha!) portions of only real foods, and *that's* why he never gains weight. He could never eat anywhere close to the amount of steak to compare to the amount of grains & sweets I used to consume.
If I could be that wrong about how my dh eats, I imagine a lot of naturally lean & strong folk are eating in a similar manner and we just don't realize it. I think the real fundamental difference between me & him is that he just isn't inclined to eat grains/sugar. Why, I still haven't figured out entirely, but have some theories.
5'4" 39yo mother to five sweeties & married to their AMAZING DaddyGrok
Deadlift: 225lb x 4
Bench: 125 x 2
Back Squat: 155 x 2
Overhead Press: 80 x 1
Determined to beat binge-eating disorder no matter what it takes!