That's a good question that I've been wondering about, too. The only "20%" that I allow myself are the 2 cubes of sugar I put in my morning coffee. I calculated that if this is the only sugar I use, in one year I will have consumed 4 lbs of sugar. That was a little scary, until I learned the average American consumes about 2 to 3 lbs. per week.
Still, I'm wondering about the effect of even a little sugar, outside of the caloric question.