Found this in today's Globe.
[Y]our muscles really do need more carbohydrate during exercise lasting longer than two hours. Even the most quickly absorbed carbohydrates (glucose and maltodextrin) can only pass from your intestine into your blood stream at a rate of about 60 grams (a little more than 200 calories) an hour – not enough to keep up with demand during intense exercise.
To get around this limit, Dr. Jeukendrup realized he could mix different carbohydrates that are absorbed from the intestine through separate mechanisms. While fructose is only absorbed at 30 grams an hour, it doesn’t interfere with glucose absorption. That means glucose and fructose mixed in a 2:1 ratio can be absorbed as quickly as 90 grams an hour.
For Ironman athletes such as Ms. McGlone, the ability to absorb up to 90 grams an hour of carbohydrate is crucial, though it takes practice to tolerate such a high intake without stomach upset. More generally, Dr. Jeukendrup says, the glucose-fructose mix is important only for sustained exercise lasting more than about three hours. But if you’re trying to avoid bonking during a marathon or long bike ride, it could be the key.