Even if you put brown rice in a smoothie, if you blend it enough to break the husk of the rice, it changes from a low GI carb to a high GI carb because your body can absorb the sugar much quicker.
Breaking fruits down into juice definitely changes the GI index of the carb. Check out the difference between eating a whole apple and drinking apple juice. Apple juice is almost like pure sugar, where the fiber in the whole apple causes your body to digest it much slower.
I don't quite buy this. You're still getting all the fiber in a smoothie, it's just pre-chewed. By the time I swallow a chunk of apple, it's pretty well juiced. The only difference is that actual juice has the solids removed.
It is essential to understand GI not as the contents of an ingredient, but how your blood sugar reacts to you ingesting it. If it is pre-chewed, your body will have an easier time consuming it, and your blood sugar levels will change in a different way then if it is not pre-chewed.
My non-expert undestandung is that glycemic index does refer to the total simple sugar content or food by volume. Glycemic load, aka glycemic load index, is a more recent metric that measures the rate of absorption into the blood stream. This is the largest factor causing large blod sugar shifts, which can lead to fat conversion and lack of concentration.
Breaking fruits down into juice definitely changes the GI index of the carb. Check out the difference between eating a whole apple and drinking apple juice. Apple juice is almost like pure sugar, where the fiber in the whole apple causes your body to digest it much slower.