That’s the whole thing right there !
Some people can eat anything and stay thin and healthy while others struggle eating healthy and exercising only to be told there near death doors and have to improve themselves.
I had a co worked like that. He would only eat lettuce and eggs for breakfast and lunch. He lived to tell me everything I’ve done and am doing wrong siting his own constraints as law for everyone. He would shout across the shop floor obscenities and ridicule me for eating chicken, yes! Eating grilled chicken on bread. Unbelievable. It got bad and I had to have him spoken to by the bosses.