How old are you? My father was thin until he turned 27. He was never huge, but at 27 he was 170lbs (6'2") and at his peak, around age 65, he was a little over 200. He warned me about that. At 27, I was also 170 lbs (6'2") (I am nearly a clone of my dad), and then slowly gained weight. I briefly ballooned to 215 around age 31, but hammered that back down to 185, but since then it has slowly climbed to 200 (at age 60). I am back down to 196. In my early twenties I could eat a large 'special' pizza by myself in about 45 minutes. Metabolism changes with age. I dearly miss being able to eat anything I wanted at any time with no consequences.

When I was in my mid-20s I could also eat anything I wanted, and a big challenge for me was finding pants that were long enough (36” inseam) but also had a narrow enough waist. This was pre-internet, so the options were limited (I discovered early that Big & Tall stores required you to be both).

Fast-forward 20-ish years, and I’m still just as tall but I don’t have any problem finding pants that fit me. Sigh.

Start lifting :)

45 now.

Nonsense. Eat 4000 calories a day, every day, and lift heavy weights. You will quickly gain lots of weight.

Confirm. Went from 165 lbs running 55km races to 235 lbs with a 1,405 lbs powerlifting total in 5 years. I started this month to reverse the process, going to run a 50 miler next time this year.

So that would be close to double what I usually eat. I doubt I would find that easy, and it would probably involve lots of unhealthy crap.

I think the same applies principles apply as with bigger people saying they can't lose weight. You need to change your habits, and that is hard.

https://thefitness.wiki/faq/why-cant-i-gain-weight/