thank you for your brilliant advice. I'm a nerd and bookworm as long as I remember. So yeah, I used to spend my time curled up and read for hours, the only exercise I had is walking to the library or doing some charity work.  But I went to doctor 2 months ago because my eating habit got worse, I keep skipping meals during exam because I'm too busy studying and forgot to eat because I don't feels hungry at all. The doc told me my insulin sensitivity is high, that's why I can't eat a lot because I feels full easily. fortunately my metabolism is fine, the doc told me to increase my physical exercise.  So I went to gym 3 times a week and actually I feel better. now I can feels really hungry when I got home from the gym, I eat a lot now, I make sure its only healthy foods tough. it's been 2 months and I gain weight, it's slow but steady..