this is a very good topic because in America it's hard to find anybody who doesn't have a gut
It wasn't the case in the 70s and mobster and I have talked about on the podcast what changed which is several things but mostly the ability to eat what we want when we want and the way food is marketed as a treat by food companies and pushed by celebrities/athletes.
I was always the skinny-fast kid growing up. couldn't do a pullup to save my life but i was always one of the fastest in PE. got into weights at 14/15 and fell in love with it and got stronger and muscular. i was ripped down to 5% body fat with an 8 pack and muscular at my peak as a teenager. i ate solid and training my balls off 6x a week. during college i was so busy that i broke my dedication and was only training 1-2x per week and was eating a lot of shit food cause thats all they had there.
After college I stopped exercising almost entirely and I started to eat what I wanted when I wanted. Pizza at least twice a week lots of sugar lots of milk, lots of food including McDonald's and restaurant food killed me as well. at my biggest i was probably 22% body fat which isn't that bad for american standards but bad when you were skinny all your life.
a couple things changed my life
First when I went for a jog around my neighborhood I got back home and dry heaved and realized how I went from being a fast runner my whole life who would run miles to train for soccer and make weight for weight lifting to someone who couldn't even run around the block.
2nd a chick dumped me due to my insecurities and i wanted to show her up
3rd i was getting nasty heartburn everyday and the doctors put me through a bunch of tests which cost thousands and they couldn't find anything wrong, so one doctor simply said 'you have to lose weight'
so i set a goal to get into the best shape of my life. i boxed up all the junk food in my house and donated it, and went to a mostly vegetarian diet + chicken breast baked plain with lemon only. no eating out, no fast food, not processed food, time restricted eating. i also got back into working out. in 1 month i went from 20%+ body fat down to 10%. heartburn was gone, i had crazy good energy and felt great. and that chick that dumped me tried to get back with me but i turned her down

i've been a health freak since and love it. this is why i am always pushing a natural whole foods diet, its what the body wants and its the healthy way