Fruits contain fibers that are beneficial for your digestive system and control the spike of sugar level in you body. If you put them in a blender, you destroy much of the fibers and one of the major benefit of eating fruit.
Apparently the lowered glycemic index for foods
of a smaller particle size is only for starchy
foods (smaller particles hydrolyze easier,
making them higher GI). So while my statement
may have held true for starches, it wouldn't for
fruits. I have no idea about smoothies! It's out
of the scope of the paper, though, so all we can
say is that the study suggests that juicing
increases diabetic risk and whole fruits lower
it.
http://www.slate.com/articles/double_x/the_kids/2013/03/pure...