I was very, very underweight while at High School and through the start of University. I'm 5'8", and I weighed less than 110lbs. Most of that, I attribute to anxiety. From age 13, I was unable to keep food down in the mornings at all, and had to have a very light lunch or I'd throw up. It wasn't intentional, it was just nerves (how much food I could keep down varied depending on how much stress I was under at the time).
For the first couple of years of university, while I put a little on (I was a lot less anxious, University was good to me), it wasn't much, then into my 3rd/4th year of Uni, I started drinking a lot more (lots of fat) and eating more fatty food, and I put on a lot more weight, peaking at around 185lbs. I lost that after graduating, but not a lot of it, and now I'm eating more healthily and exercising more, I'm around 135lbs. I'm still a little chubby - my body naturally doesn't carry a lot of muscle, but it's around about what seems like an ideal weight for me.
I expect a lot of the odd weights we're seeing here are due to anxiety in some form or another.
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