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Dear Mark: Glycogen

Dear Mark,

I’m trying to understand how glucose that’s created by proteins and fats is used and stored. Is that ~200g of glycogen stored in the muscles to be used for exercise, or is it stored in the liver and used to fuel the brain and “day-to-day” functions? Also, if muscle glycogen is depleted, will ingested carbohydrates be used first to replenish muscle glycogen and then to fuel other daily functions, or are they used the other way around? I’d like to be able to use ketones to fuel my daily activity, but still have enough muscle glycogen stores to fuel intense exercise.

I always appreciate comments and questions that spur more explanation and discussion about the body’s functioning. Let’s take this one apart and have a look. Fat is always the best fuel to use at low levels of effort. We evolved to be very efficient fat-burners and even those of us at single-digit body fat levels have plenty to spare. Glycogen, as you remember, is stored glucose and is the body’s first-line energy stockpile of fuel for harder physical efforts and keeping specific systems (brain, red blood cells, kidney cells) running efficiently all day. It’s stored primarily in the liver and muscles; however, (of the two) only the glycogen stores of the liver can be used by the rest of the body. The muscles can’t share their stores. Selfish, you might say, but that’s how it works. As you suspect, the glycogen stored in the muscles (provided you aren’t taking in other glucose) is used along with fats to fuel physical activity throughout the day like your work out, your daily walk to the train, walk to the fridge, etc. The glycogen in the liver, while it can contribute circulating glucose to working muscles, generally serves up energy (reconverted glucose) to other parts of the body – the brain for example – as it sees fit.

As to the question of whether ingested carbs will go first to the muscles or to other functioning, it depends on lifestyle and how much you rely on carbs. If you are training hard every day and depending on a high-carb diet, the muscles will probably do a better job of storing those first incoming carbs from a meal eaten right after the workout. On the other hand, in the case of a low-carb strategy, the 200 grams a day you refer to are made in the liver via gluconeogenesis and mostly stored there to provide energy for vital functions. Yes, some will go to muscles recovering from a Primal-style brief-but-intense effort [7], but the rest stays in the liver and provides glycogen/glucose for the brain and red blood cells, etc.

Ultimately, though, this line caught my eye the most: I’d like to be able to use ketones to fuel my daily activity, but still have enough muscle glycogen stores to fuel intense exercise. I’d suggest looking at this kind of goal from a different angle. You really can’t have it both ways. Because you can’t override your body organs’ pecking order, it becomes a choice of either doing a ketogenic diet, which puts all your systems on an alternative fuel burning plan, or doing a moderately low carb diet that provides just enough glucose for your brain’s needs and for intense exercise bouts of an hour or less (centerpieces of the Primal Blueprint [8]). If you’re looking to lose weight (fat), the ketogenic diet will be the most dramatically and swiftly successful. If you’re happy with the weight or are OK with a more gradual move toward your “ideal” body composition, the moderate-low path may be the way to go.

Finally, if it’s a question of fueling longer aerobic workouts (long distance running, for example), I’d suggest what I’ve said in the past. Endurance athletes, while they don’t need to indulge in the worst of the traditional carb loading, will need to make compromises to the Primal style. If they are “classically trained” in doing long workouts at 75-90% VO2 max, their bodies will simply require more glucose (and, hence, muscle glycogen) to accomplish the feats asked of them. In that case, ketones can’t play much of a role. On the other hand, it is possible – over a long period of time – to redirect gene expression to favor fat and ketone metabolism even at higher workloads. But this requires that the training be done at much lower heart rates for longer periods of time. That becomes the only way to recommend endurance athletes adapt their training and diet regimens to maximize fat burning efficiency. Check out Primal Compromises for Athletes [9]for more on that discussion.

As always, thanks for your questions and keep ‘em coming!

Bob.Fornal [10] Flickr Photo (CC)

Further Reading:

The Definitive Guide to the Primal Eating Plan [11]

What Happens to Your Body When… You CARB BINGE? [12]

The Context of Calories – 200 Calories is 200 Calories. Right? [13]

Dear Mark: Did Our Bodies Evolve to Run on Ketones? [14]

The Entire Definitive Guide Series [15]