John's jokes about the "carbo loading treat of the day" last week got me thinking about how, physiologically, carbohydrate loading - or "supercompensation" - works. The idea is that one can get muscles to store more glycogen if one eats a higher carbohydrate diet (while cutting back a bit on protein and fat), and tapers exercise. The first carbo-loading protocols had a depletion phase followed by a loading phase. In other words, eat low carb for a week, then high carb, and muscles will supercompensate for their carbohydrate-starved week by storing more glycogen. More recent studies have shown that the depletion phase is unnecessary, and that eating a high carb diet a week before a big race - along with tapering training - will result in higher levels of stored muscular glycogen. Liver glycogen too maybe, I'm not sure.

The question in my mind was this:
-Given that an excess of carbohydrate, or protein, or fat... are all stored as fat... how do you get muscles to store an excess of carbohydrate as glycogen, rather than fat?

Couldn't figure it out on my own so I emailed a couple other tri friends who have exercise science degrees. Finally got a good answer - copied and pasted below. The part I missed is that we are not always walking around with full glycogen stores - actually, with heavy training there is a chronic glycogen drain. I have a good graph in a textbook of muscle glycogen measured in runners who ran 10 mi every day for 3 days and ate normally (ie, not in an energy deficit, they were told to eat by hunger) and muscle glycogen level continues to drop. (sidenote: when you get up in the morning, eat, then go train, glycogen levels drop overnight too. And eat right after, glycogen resynthesis peaks in the first 15 min post workout).

As a note - even if you eat enough calories - you do not totally replenish muscle glycogen because "It's a matter of different rates of depletion and resynthesis. Even if you can improve glycogen resynthesis by eating CHO right after a workout, glycogen resynthesis does not only occur during a few hours after a workout, but it's a continuous process. It's not a "fast" process either, meaning that if meanwhile you workout again, then depletion occurs, preventing the levels from reaching their maximum value."

Carbohydrate loading then, and the "supercompensation" is not so super but merely letting chronically depleted glycogen levels get back to normal through reduced training and higher carbohydrate intake (though chances are if you're training lots, you're eating a high percentage of CHO already). You know how you feel like you've got more energy after a rest day? Glycogen stores should be a bit higher...

Below: copy/pasted email from my friend, about glycogen loading and how it works. This is pretty basic but it's interesting.

- endurance trained humans (and rats) demonstrate a capacity to store more muscle glycogen, without muscle hypertrophy, than untrained

- glycogen synthase activity might be higher in some muscle fibres (type II), as a result of endurance training (some conflicting studies on this)

-glycogenin content is higher in endurance-trained muscle

- endurance trained muscle has a higher concentration of GLUT4 transporters

Result: endurance-trained skeletal muscle can transport CHO into the cell at a higher rate, (possibly) catalyzes the conversion to glycogen faster, and has a higher glycogen storage capacity.

If your caloric intake exceeds expenditure, you can expect weight (fat) gain. But if you balance your caloric intake with caloric output, and simply increase the % of CHO consumed, you're still in energy balance. I think some people imagine that athletes walk around with full glycogen stores, and then somehow increase the size of the tank with a CHO loading protocol. It's more likely that most athletes are never topped-up during normal training, and only with a reduction in training load and a focus on increasing % CHO consumption do they approach their true glycogen storage capacity. Some recent work on CHO loading seems to show that glycogen stores can be maxed in as little as a day or two on a high CHO diet when coupled with a low training load (very short duration, with high intensity).