My brother, 21 yo, is like this. He’s ~180 cm tall and has never weighted more than 61 kg (during the 2020 covid lockdown, he made a big deal out of this since it was the first time he passed the 60 kg mark; after the lockdown was over he immediately dropped back to his perpetual weight of ~55kg). I live with him so I know his eating habits: he eats all the time, whatever he wants, whenever, how much he wants and simply doesn’t put on any weight. Is very energetic, no sick look / demeanour. It’s just fascinating to me cause I’m not like that at all, I have to watch what / how much / when I eat.
Why?
We lose weight by breathing. There are studies on this. If you are constantly moving you are breathing harder and expelling more CO2. https://www.bbc.com/news/health-30494009 https://journals.physiology.org/doi/full/10.1152/advan.00006.2021 https://theconversation.com/when-we-lose-weight-where-does-it-go-91594
Which makes even more sense when you ask “where does the fat go?”
Well where did that carbon you just exhaled come from?