I know a young lady who is like this. She eats and eats but doesn’t gain. In her case there is a reverse discrimination. People accuse her of being bulimic. This is unfair. She has tried so hard to gain. Being too slim causes their own problems. It makes it harder to become pregnant quite often because the body generally is looking for enough fat to allow for the pregnancy process. It’s about making sure the fetus, then the baby is safe.
Having said that anorexics and very thin bulimics, or bulimarexics will most likely have difficulty in getting pregnant as well.
But the later can correct this. The ones naturally with overly active metabolism can’t.
Next time you see a skinny person, don’t judge too quickly. You don’t know which issue they are struggling with – the first or the second.