Before I say anything, all the info Im presenting comes straight out of http://home.hia.no/ ~stephens/mustrn.htm. This webpage gives a very thorough look at slow twitch/fast twitch muscle fibers.
Slow twitch, or type I fibers, have no glycogen stores, or negligible at that. Type IIa fibers have inermediate glycogen stores. Type IIb have high glycogen stores.
This means those who are mostly slow twitch would have little carb tolerance - their bodies simply can't either store OR use glycogen effectively. Just think of a marathon runner with 80% slow twitch fibers. In this scenario, only 20 % of his musclefibers could use glycogen effectively at all! On the same token, this means that those who are mostly type IIb would have a much higher carb tolerance - their bodies are simply designed to not only store, but use carbs as their primary source of energy. These same individuals would have very little oxidative capacity and would likely have a hard time shedding those extra pounds of fat (your typical endomorph? these individuals would initially have to train for endurance to force type IIb fibers to develop an oxidative capacity by creating mitochondria, which will allow them to burn more fat - in other words turn typeIIb into typeIIa fibers).
Just think of a powerlifter with 80% fast twitch fibers, with higher ration of fiber IIB.
Those with mostly type IIa would be in the middle - they would have good oxidative capacity, AND good glycogen storage and use capacity. They can use both glycogen and fats for energy very efficiently and will have a relatively easy time building muscle AND burning fat. These sound just like your typical mesomorph.
This seems to support Poliquin's views on diet. He pointed out that only one fifth of the whole population can tolerate a high carb diet and those are the same individuals that incidentally are mostly fast twitch. He recommended if you dont fall into this category, you will use fats for energy much more effectively, and will store less fat as well. These would be your slow twitch types. All those in between would tolerate relatively higher ammounts of carbs, but not as much as high twitch individuals. I am not physiologist, have no extensive knowledge of anatomy,and im not a medical student. But this seems to strongy support what Poliquin has said. That's it!!!!!!!! Do with it as you will (read: reply and tell me what you think!).