Even whole fruit can be a little contentious in the Paleo community. It’s generally safe, especially in small quantities, but juice is another matter.
Fruit vs. Juice
Whole fruit has varying amounts of fiber which help to both lower the glycemic load of the fruit and serve as a prebiotic (meaning it feeds the beneficial bacteria in our gut). Juice doesn’t have any of this fiber, and because you need more than a handful of fruit to fill a glass with juice, you’re consuming much more sugar than you would if you simply ate an apple or an orange. Add to that the fact that most store-bought juice has sugar added to it and you’ve basically got a cup full of insulin.
Don’t Follow the Crowd
Mark Sisson has a list of reasons not to make juicing a habit.