I'm trying to roughly match my carbohydrate intake to my rate of glycogen depletion with the goal of replenishing without overdoing. Is there an established way to estimate how much glycogen an activity utilizes? In my case I am doing 1.5hr sessions of olympic weightlifting that are fairly intense but most sets are fairly low rep (ie 1 to 3). I've heard/read contradictory statements about this from basically very little to >200g.
To answer your basic question, other than utilizing gas analysis to measure CO2 used along with blood and muscle sampling during exercise, there really is no way to tell how much glycogen is being used for a specific activity on a person to person basis. Additionally, your efficiency increases the more you do an exercise and the fitter you get.
However, I don't really think it should be that big a concern for you in regards to weightlifting performance. The average individual has enough glycogen to fuel 90ish minutes of continuous exercise, much longer if you are doing intermittent exercise (Such as the weightlifting).
The primary fuel for muscle contraction is ATP. In short burst, near maximal efforts (30 seconds or less), your replenishment is going to come from Phosphocreatine and anaerobic glycolysis. For longer efforts, aerobic glycolysis starts coming into play.
Refueling during your workout is kind of like doing situps to reduce belly fat. The body will replenish where it feels it needs to first, so there is no guarantee that what you are taking in is refueling the muscles that you are specifically using.
Eat a well balanced diet, spaced out for your specific workouts, have an energy bar handy if you feel just kind of "bleh" towards the end of a workout, but there isn't really any way to specifically tell how much glycogen was used during a workout session.
I was still a bit unsatisfied with the answers so far so spent a couple of hours doing research on this. I found a key review of glycogen usage in resistance training that has some useful numbers (Astorino, T., & Kravitz, L. (2000). Glycogen and resistance training. IDEA Personal Trainer, 11(7), 21-23.). The studies reviewed were typically small and narrow but it looks like they are all we have to extrapolate from in terms of weight training (there seem to be many more studies related to endurance training).
In one study 6 sets of leg extensions done at 70% of 1RM were found to reduce glycogen in the used muscles by 39%. In the other key study a 30 min workout consisting of 5 sets each of front squats, back squats, leg presses and leg extensions resulted in a 28% decrease in muscle glycogen (and also 30% decrease in intramuscular triglycerides).
Some additional Googling gives some estimates on typical glycogen storage in humans. These estimates seem to be all over the place but the general consensus is that glycogen stores in muscle mass total around 250-400g depending on the amount of training and muscle the individual has.
So for me, I'm going to guess I have 300g of stored muscle glycogen based on just a conservative look at this. Might be higher.
I'm going to justify using a higher glycogen rate of depletion than the 28% quoted due to the length of the workout (mine is 90mins, not the 30 used in the study).
On the other hand I suspect, because the training I do is focused on Olympic weightlifting, there are muscle groups that have much greater rates of depletion than others. The most heavily worked muscle groups like the quads and posterior chain are clearly the largest muscles overall but nonetheless some other significant muscles get very little work (like the chest).
For frame of reference, my training typically involves some warm-up, some lighter sets of compound lifts or complexes to work on technique (say 6-8 sets of 3-4 reps of 2 or 3 movements), some heavier sets (say 6-8 sets each near 90% of snatches and clean & jerks), some pulling from the floor movements (i.e. 4 sets of 3 reps of clean or snatch pulls or deadlifts or similar at 80-90% max) and then 30 mins of squats (typically 6-8 sets of 5-8 reps at 60 - 80%). I'd guess this heavily uses maybe 85% of my muscle groups with only the chest and biceps being really left out. I can assure you those do not account for much mass in my case.
So if I take 85% of the 300g of stores that leaves me with ~255g of glycogen in the active muscles. The assumption here is that the storage allocation is fairly constant across the different muscles. Thinking about it this is probably false - lifters with an Olympic focus probably have higher relative stores in the heavily trained muscles of the legs, but less in the muscles of chest for instance. There is no data I could find on this so we'll keep it simple and use our arbitrary number for this.
If I assume a total 35% depletion after a 1.5 hrs training session (and there are admittedly some big assumptions in this), and roughly 255g of active glycogen storage, that would give me a depletion of 89g of stored glycogen. If we widen the ranges a bit so that we are assuming a glycogen depletion rate of 28-39% and total glycogen stores in the 300-350g range (255-298g in worked muscles), we would have an estimated amount of glycogen burned of 71-116g.
This seems to fit pretty closely with my personal experience. I find myself wiped and useless for the next workout if I don't have at least a small amount of nice dense starchy forms of carbohydrate in the time between workouts. Particularly when I get to the squats. Spending most of my time eating relatively low-ish carb (typically less than 50-75g a day), I think adding a sweet potato (30-40g carbohydrate) or two in afterwards and/or in the next day will likely get me better replenished and ready to roll for the next session.
Your body burns the most glycogen when you perform anaerobic exercises, such as sprinting, heavy weight lifting and other short bursts of intense exercise. Aerobic exercise also burns glycogen, although at lower percentages. For example, if you walk, jog or bicycle at 70 percent to 75 percent of your maximum heart rate, 50 percent to 65 percent of the calories you burn come from glycogen. The remainder come from stored fat.
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