Well, it's not
just good for you. I have problems with sugar, and occasionally people look aghast at me when I decline a glass of fruit juice saying "But it's natural sugar!" as if that made some kind of difference. The fact is that a large glass of fruit juice probably has all of the energy from several pieces of fruit, but with none of the fibre or other non-soluble goodness in it.
In fact, according to
recent research children who drink more than two glasses of fruit juice a day are more likely to be overweight/obese. And
this article from 2005 indicates that fructose might be
worse than normal sugar.