I don't think the fruit sugar from smoothies is more of an issue than from the whole fruits. Whether you purée the fruits or chew them, both releases pretty much all of the sugar contained in the fruits. The difference is how fast the sugars get released. Chewing whole fruits requires much more time than drinking a glass of smoothie. Given that the stomach acts as a puffer for the sugar, that difference shouldn't have any significance as long as you stick to 2-3 servings of fruits. Just take the amount of whole fruit you would eat at once as a sanity limit for your smoothie. This way you will allow your stomach to send satiety signals before you "overdrink" on smoothiefied fruits.
Personally, I limit my smoothies to 1-2 large fruits, e.g. one apple and one banana and drink them slowly. I prefer eating whole fruits, because I feel this is more appropriate for an adult person (smoothies are food for infants and theethless old geezers), but I like to make green smoothies containing lots wild greens and herbs which often don't taste very pleasant on their own, so smoothiefying them is a great way to incorporate them into your diet, and at the same time improve the biavailabilty of many phytochemicals they contain.
Edited by timar, 13 April 2015 - 10:31 AM.