That's a completely different thing.

Honey, for instance, has a high glycemic index. Watermelon has a high glycemic index. They will not get you fat the same way cane sugar and watermelon juice will.

A juice made of celery, kale, and spinach, will still have a low glycemic index. But it's still nowhere near as good for you as unprocessed celery, kale, and spinach.