I wonder whether you are all over thinking this? I really do not want to offend anyone but by following the old adages mentioned below I am, at 60 around the same weight and fitness as I was I my 20’s. How? By eating home made food, not manufactured (even if you spend one day a week cooking for when you don’t have time) and taking exercise each day, as simple as taking a walk or cleaning the house from top to bottom. I did manage, in my early 20’s to put on a lot of weight by not following these rules, moving back home and only eating what my Mum prepared helped me see the light. Good, uncomplicated/manufactured food and moderate exercise, not rocket science.

https://optimisingnutrition.com/brea...like-a-pauper/

Some of these old adages seem to make sense.

“A little of what you fancy does you good”

Not in the least bit offended .. I know you the rules inside out, I could be a nutritionist.. I just decide not to follow the rules, I like cheese and curry and bacon and beer and steak waaaay too much ..

Being happy with what one does counts for a lot.

You are 100% right if I consider last generations who did generally outdoor work were rarely out of shape even if diet may not have been healthiest at times according to modern thinking.

Even people today that have healthy diet and exercise regularly are mostly in shape.

I think the issue is people who are out of shape and bodies are aclimitized to bad habits, then it’s a hard graft and often impossible but it can be done.

Generally people have many bad habits not just one, they try to give up all the bad stuff in one go and crash. It must be a gradual process give up one bad thing at a time. It’s often not just willpower as our brains are conditioned to sugar, alcohol etc and this affects mental aspects not just physical,