here is my position on this:
stick to watery fruits which are made up of almost all water and fiber. the sugar in whole fruit like watermelon is decieving.. now we can eat a steak and have the body produce sugar or we can eat watery fruits that contain glucose and fructose perfectly ready to use.
now here is where i make all the 'book nerds trainers' get butt hurt. there is something called the glycemic index. watermelon (using it as an example) has a GI of 72 which is pretty damn high. that is as high as donuts and bagels! so you should stay away right? WRONG!! GI tells us how fast watermelon gives up carbs to the body, not the impact on blood sugar. that is the glycemic LOAD. and watermelon and other fruits like berries and watery fruits are very low.. <5
of course doctors will tell people to stay away from fruit cause it has sugar!! but have no problem telling them to eat other trash that actually spikes blood sugar.
now you are asking about specifically FRUIT JUICE. if you were to juice something and drink it right away it would be fine, but you are still putting it through a processing phase. if you buy juice from the store it is awful and pasteurized and loaded with extra sugars and shit. so the lesson is EAT WHOLE FRESH FRUIT.
if you do drink fruit juice, drink a little and make sure it is fresh. and also avoid dried fruit
and by all means go to town on fresh watery fruits like watermelon, papaya, mango, cantaloupes, peaches, apples, berries, cucumbers, and watery veggies like lettuce. anyone who says you will get fat eating any of those foods is an idiot IMHO. nobody gets fat eating watermelon lol. eat less fruits like bananas which are lower in water.