Dr Robert Lustig (Fat chance, the hidden truth about sugar, obesity and disease) he explains that dietary fibre which is found in fruits, vegetables, whole grains and legumes is undigestible by the human gut. There are 2 types of fibre, soluble and insoluble. Soluble(lentils, apples, oranges, nuts, flaxseeds, dried peas, blueberries, carrots, oatmeal)slow the digestion and absorption. Insoluble(wholewheat, wholegrains,nuts, couscous,seeds,brown rice, fruit, dark leafy vegetables etc). Together they work as a team, the insoluble fibre forms a lattice and soluble sits in the gaps to slow the rate of flux from the intestine to the blood stream, which allows the liver to meabolize what is coming in, so there is no overflow.
He suggests that although fruit contains fructose, eating it whole, the fibre matches the fructose content. However by using a blender on fruit, the action of the blades destroys the insoluble fibre of the fruit. So without the lattice to slow down the absorption of the sugar, it is absorbed quickly. Fibre is responsible for maintaining both the dose and flux of carbohydrates.

I knew about the high sugar in the fruit juices which are often used as a base for smoothies, but I was surprised at the function of the fibre in the fruit too. I also thought about those people who think that lots of fruit is good and so put loads of fruit in a blender and consume the contents which would be far more than they would eat at one sitting.