It's pretty simple. Sit down with some insanely difficult physics problems and before you know it eight hours have passed without you having had a bite. Then you quickly make a small sandwich before going to bed and voilà, you've only taken in 10% of what you ought to take in for that period of time. Repeat and you'll lose around a kilogram per day.
I'm talking from unintentional experience. Luckily I've managed to balance it out with candy so now I'm in the 55 - 60 kg range.