I share an office with my collegue and have been for nearly a year now. I have never seen him eat anything, apart from a piece of cake once when my boss gave it to him cos it was her birthday, so not be rude I think.
I find it baffling that he can go through the whole day without lunch. I've asked him and he says he doesn't feel hungry if he is busy at work. But however busy I am I always get hungry every 2/3hours or so at work.
I've offered him a biscuit on occassion and he always says no. I eat my breakfast at my desk too, plus a morning snack and an afternoon snack, plus lunch too (which I might eat elsewhere sometimes), so I feel like I am always eating! Now my BMI is about 19, so it's not a problem anyway.
My collegue is overweight I'd say (not fat), so the calories must come from somewhere and they certainly don't come from 9-5pm during the day!
He even says if he gets in late from work (he has a very long commute, 2hrs each way) he is too tired to eat dinner 
AIBU to be really puzzled by this?