How did Lanza and Atchison get so thin?

Title, do we know?

Lanza was a hardcore anorexic for a very long time, so his BMI was obviously very low, to the point of his hair falling out.
No idea with Atchison. He constantly said he was dying of some unspecified illness that he had apparently been diagnosed with (though he never went to any doctors so he was lying). He apparently wanted to gain weight, and was trying to, but died with a BMI only slightly above Lanza so he was either failing spectacularly or was lying about his attempts. He was cripplingly depressed to the point of lacking an appetite but I feel there had to have been something else as well. No idea what though. He seemed to exhibit borderline munchie behavior when it came to his “illness”, but it wasn’t for attention so no idea why he bothered.