I think it's just that I don't eat as much as I should -- my impression is that most people my height eat 3000+ calories a day, whereas I eat 1500-2500 calories a day last time I counted for a week or so. I'll count again this week to see if it has changed, since last I counted was a few years ago.

I don't know what a GI function is, how do I measure it? How do I measure whether I'm absorbing food?