I have been using a 4/1 carb/protein mix during exercise, and it has been "generally" effective I'd say (I run 25 mi/week with 1-2 hard runs per week, + 2 days cycling/wk, + 2 days/wk weights for upper body if i'm lucky...), although i sometimes feel that i get weird hypoglycemic effects. In general my experience is that if you are adapted to low carb, don't take extra carbs unless you need them, but with high intensity stuff, I think you do need them. I ran 3 miles of tempo with no carbs on board today as an experiment, and was 10 seconds/mi slower than last week and got a side stitch as well. I've been trying to figure this issue out as well, because I want to lose 10 more lbs, and can't seem to be able to lose when I am liberal with the carb intake (by that I mean about 150grams/day). I myself am going to get that book mentioned above and see if it helps me, it's hard to find info on this topic!