Liver glycogen is a major source of carbohydrates for your muscles to burn when you’re running. The glycogen molecule is just a big long chain of glucose molecules which is optimized for long-term storage.

When your body needs carbs during a run during a run, your liver can break down glycogen into glucose, shuttle it into the bloodstream, and send it to your active muscles to be burned for energy.

Stored glycogen in your liver is particularly important for running the marathon and ultra distance races, since the glycogen that’s locally in your muscles isn’t sufficient to get you to the finish line. But exactly how much glycogen do you store in your liver? And can training increase this amount?

As part of some broader marathon-focused research, I found myself pondering these exact questions. It turns out one of these questions has a short and simple answer, and the other sent me down a six-hour research rabbit hole.

Here’s the tl;dr: the average 150 lb runner can store about 65 grams of glycogen in their liver when fully fueled up; this drops to about 43 grams overnight (a 33-40% decrease).

That’s a loss of almost exactly one energy gel, or one banana, if you prefer) And unlike muscle glycogen, liver glycogen doesn’t increase with training.

Let’s take a look at where these numbers come from.

Can endurance training increase your liver glycogen stores?

Let’s tackle the second and seemingly more complicated question first. It turns out the answer is simple: no, training does not increase liver glycogen stores.

This review paper [1] pools data from multiple studies on liver glycogen concentrations in sedentary people, people with type 2 diabetes (who have impaired glucose regulation, and hence might have abnormal liver glycogen levels), and endurance athletes.

Conclusion? No differences! This stands in stark contrast to muscle glycogen, by the way, which is *way *higher in endurance athletes (and lower in people with type 2 diabetes [2]).

How much glycogen is stored in the liver?

Now let’s turn our attention to the initial and seemingly easier question. We want to know how much glycogen, in grams, the typical person’s liver can store. This is useful to know because if it runs out before you get to the finish line, you’re about to have a difficult final few miles of your marathon.

The internet is awash with sites–including Wikipedia–citing the number “100 grams.” I’m always suspicious of simple round numbers like this, and indeed, when you try tracking down the original sources, they are dead ends.

Wikipedia, for example, references the otherwise-great paper “Four grams of glucose,” which provides the 100 gram figure but does not cite any data supporting it [3]. Or the reference leads to a textbook, which again, doesn’t cite data.

Tracking down real data on liver glycogen content

So, back to Google Scholar we go, on the hunt for real experimental data. There’s quite a lot of research on the *concentration *of glycogen in the liver, and how it changes with exercise and diet (as you’d predict, liver glycogen stores go down after exercising, fasting overnight, or going on a low carb diet; they can be replenished within six hours or so after eating again [4]).

One useful finding to come out of this initial literature dive: after a 12-14 hour overnight fast, your liver glycogen concentration drops by one-third, and your muscle glycogen stores are virtually unchanged [5,6]. Translation: if you head out for a morning workout without eating any breakfast, your liver will be at about 66% capacity, assuming you had a carb-rich dinner the night before.

We’ve still got the same problem, though–66% capacity of what? What is the *total *amount of glycogen stored in the liver? It turns out there’s a good reason for the scant data on the absolute glycogen content of the liver: for many researchers, the only technique available is a liver biopsy.

The liver is underneath the rib cage, and to take a biopsy, you need to stick a pretty big needle *in between the ribs! *Needless to say, this is a pretty painful procedure, not to mention tough to justify to your human subjects research committee.

Plus, once you’ve done the biopsy, all you have is a little slice of the liver–it’s easy to measure the concentration of glycogen, but to know the total glycogen, you need to know the volume (or the mass and density) of the entire liver.

The situation improved in the 1990s with the advent of a special kind of MRI that quantifies the concentration of carbon-13.

With this technique, a painful biopsy was no longer necessary. Even so, it still only gives a concentration of glycogen, not an absolute amount. You might wonder–while they’re in the MRI tube, why not measure the liver volume while you’re at it? One study did, but didn’t report the absolute glycogen content! What we need is the volume (or the mass and the density) of the liver.

We could do what some older studies did, which is to assume a normal-sized liver of (circa 1.3 liters) and go from there. But we can do better–a 1979 study using the then-newly-developed CT scanner provides raw data on height, weight, and liver volume for 22 healthy adults [7].

Not surprisingly, larger people have bigger livers, and the relationship holds similarly for men and women, and we can use this fact to predict more accurately the range of liver volumes we’d see for a runner of a given size.

So, what we’ll aim to do is get a good estimate of liver glycogen concentration, then predict liver volume using the regression line and prediction intervals in the plot below.

The dashed lines show the range that 90% of people at a given body weight would fall into (i.e. it is the 90% prediction interval).

Aside: some useful data on liver density and the molecular mass of glycogen

(This part is technical and boring; you can skip it and miss nothing important. I’m just putting this info here so I know where to find it in the future).

Some studies quantify liver glycogen in grams per kg, others use mmol (millimoles) per liter. Converting liver volume to liver density is easy–the liver is a very homogenous organ with a density very close to that of water; these two references peg its density at 1.03 or 1.05 g/L [8,9]; I used the latter figure.

The molecular mass is a little more complicated–glycogen itself is a long chain of glucose molecules, so its true molar mass is all over the place. Liver glycogen research actually quantifies glycogen concentration using the effective molar mass of the “glycosyl monomer,” i.e. the molar mass of one glucose unit inside the glycogen chain. Confusingly, this *is not *the same as the molar mass of glucose itself.

When glucose molecules link up to form glycogen, they drop a water molecule, so each glucose unit in the chain is a bit lighter than free glucose. Free glucose has a molar mass of 180.15 mol/g; one glucose in a glycogen molecule has an effective molar mass of 162.14 mol/g. Some papers use 165, others use 160 to account for slight variation in the branching of the glycogen molecule [10]. Either way it doesn’t make a huge difference.

Because we’re calculating the effective molar mass of glycogen, and not its literal weight, we don’t (yet) have to worry about the fact that each gram of stored glycogen brings 2.4 grams of water along for the ride, which is liberated and excreted in urine as glycogen is broken down [11]. But that’s a useful fact to know.

If we have an estimate of liver volume (by drawing simulations from the regression equation above), we can estimate total liver glycogen content using the density and molar mass figures here. Caveat: we are assuming liver glycogen concentration is uncorrelated with liver volume; as far as I’ve seen no study has looked at this directly.

Liver glycogen content after a meal and after waking up in the morning

After falling deep down the reference rabbit hole, I ended up with estimates of liver glycogen concentrations in over 200 healthy adults from 12 different studies.

I pooled both the averages and the standard deviations of liver concentration to get a better idea of the actual variability in stored liver glycogen across individuals.

I combined these estimates with the model of liver size from above to estimate the range of typical values for stored liver glycogen a few hours after a carbohydrate-rich meal, for runners of different body weights.

Since I had a lot of simulations, I was able to model both the average and the range that 90% of people would fall into. Here are the results:

And results in table form:

Body size	Average glycogen (grams)	90% range
Small runner (100 lbs)	50.0	24.9 - 77.7
Medium runner (150 lbs)	64.9	32.4 - 99.2
Large runner (200 lbs)	79.9	40.2 - 120.9

The upshot? Even for large runners, the oft-quoted “100 grams of glycogen” is an overestimate for the majority of runners.

My analysis does confirm the cited figures above regarding overnight losses–among the studies I found that examined liver glycogen after a meal, and again after a fast of 10-12 hours, glycogen losses amounted to 33-40% (or ~22-26 grams of carbohydrates).

You need about twice the carbs to replace overnight liver glycogen losses

Because of the physiology of carbohydrate intake, you probably need to consume about double the amount of carbs lost overnight to completely restore your liver glycogen levels.

When you take an energy gel, or eat a banana (both of which are about 50/50 glucose and fructose), about a third of the glucose goes to your liver, and (probably) half two two-thirds of the fructose eventually ends up getting incorporated into new liver glycogen [12,13].

Complete replacement is not so much the goal of a pre-workout or pre-race breakfast, though, so don’t feel like you must absolutely replace 100% of the glycogen you lose overnight.

Another aside: How accurate is a back-of-the-envelope calculation?

Interestingly, the above estimates of overnight glycogen losses are only a bit lower than a back-of-the-envelope calculation I did a while back to guesstimate overnight carb losses. The argument goes like this:

An average-sized completely sedentary person burns ~1500 calories per day, and since you are (hopefully) sedentary overnight, you’d burn around 1500/24*10 = 625 calories in the ten hours from evening bedtime to morning run.

At rest, about 30-40% of your energy comes from carbs, and there are about 4 calories in a gram of carbohydrates, so that works out to 47-62 grams of carbs. Barely off by a factor of two, so not too bad! This oversimplification ignores the fact that you can also make glucose de novo out of fat and protein, especially at rest, so that helps to explain the overestimate.

Why training doesn’t increase liver glycogen storage

As for *why *training doesn’t change your liver glycogen capacity, I’m genuinely puzzled–many other basic physiological systems, like your blood volume, heart size, and mitochondrial enzyme expression, massively increase with training.

It might be that normal day-to-day variation in liver glycogen is already big enough that the additional depletion that happens with training is not much of a stimulus to the liver, or maybe the human body already stores as much glycogen as it possibly can in the liver, even without training.

Now, it is worth pointing out that the study above didn’t technically show that training doesn’t increase liver glycogen–it just showed that liver glycogen concentration in endurance-trained athletes is not greater, on average, than liver glycogen concentration in healthy normal people.

It’s possible that the liver itself gets bigger with training (it does shrink when it’s glycogen-depleted, but that comes along with a drop in glycogen concentration). It doesn’t seem that plausible, though–the increase in size would have to be pretty big, and you’d think someone would’ve noticed by now if endurance training made your liver ten or 20% bigger.

For now, it’s a mystery. I’d love to see a longitudinal study on liver glycogen concentration over the course of, say, a high school cross country runner’s four-year career, or over the course of a six-month marathon training cycle. (If you’re a researcher and you want to do this, send me an email! I do specialize in modern longitudinal data analysis methods…)

Summary: Liver glycogen and running on an empty stomach

Liver glycogen stores are quite variable across runners: a typical 150 lb runner might be able to store a maximum of anywhere between 32 and 99 grams of glycogen in their liver, with the average being 65 grams. Runners with a greater body weight tend to be able to store more liver glycogen, but not by much–a 200 lb runner averages about 15 grams more liver glycogen storage than a 150 lb runner.

**Overnight, you lose the equivalent of about one energy gel (or one banana) worth of glycogen. **You might need to eat about twice this amount of carbs (e.g. one gel *and *one banana) to fully replace this glycogen, because only around half of the carbs you eat end up as liver glycogen. Still, replacing your overnight glycogen losses doesn’t require an enormous breakfast. That’s great news for runners with a sensitive stomach!

If possible, you should still eat something, since having some carbohydrates coming into your system helps “spare” liver glycogen. So, if you run on a completely empty stomach, your liver glycogen stores will be only around two-thirds full, *and *you’ll be burning through your liver glycogen stores more quickly.

You probably don’t need to worry about loss of liver glycogen overnight for typical training sessions, but it does start to become a concern for very long or very fast runs, marathon-specific workouts, and marathon or ultramarathon races.

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