I have to agree with Kestrel, I realized that I may not be as 'cardio" as gymee, but I can climb the same mountain passes now that I did as a 28 year old, back then was stopping every hour or so to have "trail mix" or "gorp" full of sugars, raisins, m & m's, etc. Now at almost 60 I pass the spots I used to snack at, don't stop, keep going on fat and protein. I cannot agree that God would have created us with a metabolism that only functions best with a substance (carbohydrate/sugar) that is unknown in some parts of the world, seasonal in others. Obviously, if gymee switched, he'd fall on his face, as he says. But with enough adjustment, he'd be back to his old self. And If that turned out NOT be true, that he really couldn't do his workouts on a fat/protein based diet, then I'd say that's nature's way of telling him what he's doing is unnatural, kind of like steroids. Cardio workout, two IV's, glucose in one arm, steroids in the other, look at me go.