When I was 36, trying to keep a beer gut at bay with unfocused exercise
I joined a local private 'body builders' gym
which focused on diet as well as weights.
I had to discipline myself to eating small amounts frequently.
Specific food groups, at set times of day.

All 'normal' healthy food.
No expensive dubious supplements or protein shakes.
No alcohol.

I had no inclination to be a body builder, just fitter and stronger for my age.
The strict diet was a pain in the arse to adhere to.
But I was allowed one cheat meal on saturday nights.

Between 37 to 41 I got my weight hovering around 13 stone,
with muscularity somewhere between a swimmer and middle weight boxer starting to go to seed.
ie, no well defined six pack, but no flab either.
Sort of like a Roman Gladiator with a healthy protective layer of fat.

Back then I never had my BMI measured.

Then boredom set in, and I lapsed for a few years back to drinking and takeaways..

I got nearly as fit again by my 50th birthday.
Perhaps even more muscular,
with a low BMI measure which I've forgotten,
but I think was in the lower 20s.

Again dietary discipline was a constant battle against temptation,
and hard weight training worked up a thirst for weekend cider binges..

After my 50th birthday, illnesses and the increasing stress of family commitments
put a complete stop to my healthy lifestyle regime...

I am now a fat very unfit weaker bloke in my early 60s.
But I know what it felt like to have a superior than average physical strength and fitness,
and will need some serious reassessment if I survive the covid era into my mid 60s..

Very recently, lifting heavy storage boxes to clear space in the house,
and failing to fit under the sink to fix a leaky pipe
like I used to be able to,
has left my back and knees aching and stiff..

So, I will take an interest in threads like this...