'It's 100% fruit juice, so it's healthy' is one of the most common nutrition misunderstandings. Whole fruit and its juice contain similar sugar, but your body treats them very differently — and the difference is fiber.
What juicing removes
When fruit is juiced, most of its fiber is stripped away, leaving concentrated sugar water plus some vitamins. An orange has fiber, bulk, and water that slow you down; a glass of orange juice packs the sugar of several oranges with the fiber gone, and it goes down in seconds.
The blood-sugar difference
Without fiber to slow absorption, juice sends sugar into the bloodstream quickly, causing a sharper blood-sugar spike than the same fruit eaten whole. This is why whole fruit is not linked to increased diabetes risk while sugary drinks — including juice — are. See sugar and type 2 diabetes.
The fullness difference
Whole fruit's fiber and volume make it filling — it's hard to overeat apples. Juice provides little satiety, so its calories add on top of your other food, the same problem as other liquid sugar. You'd never eat five oranges at once, but five oranges' worth of juice is an easy glass.
Is juice all bad?
Not exactly — 100% juice does provide vitamins and is better than soda, since its sugar is at least natural. But nutritionally it behaves much more like a sugary drink than like fruit. Guidance is to favor whole fruit, keep juice portions small, and treat it as an occasional drink rather than a health food. For kids, this matters even more — see kids and sugar.