my guess is that this comes from the latest medical studies showing that carbs like pasta/high carb energy bars/etc. screw up your insulin/glycogen index and really do more harm than good. Your liver can only store so much "energy" for you, so woofing down carbs doesn't really do you much good and in fact probably slows you down. (The quote I recall from a personal trainer was "if you want to help your race with a high-carb energy bar, give it to your competitor.")
A book that goes into this in depth is "Protein Power" -- about $12 in paperback. I'm sure there are a lot of them, but I found this the most readable one. I also have some notes I recently typed up from a Nutritional Physiology class, if you'd be interested in them -- it goes into this somewhat. But that book really is the best source (I think).