On how the body uses energy during a race, why runners hit the wall in a marathon, and what can be done about it

A key fundamental issue with the marathon is that the distance is farther than the human body can capably race in one go without consuming fuel during the race.

Long story short, aka I’m about to paraphrase a ton of science without citing any sources:

Running utilizes glycogen from the lower body muscles used in running. Glycogen is produced from carbohydrate and stored in your muscles and organs for later use.

Glycogen is not transferrable: For example, glycogen in the arms is only used when those arm muscles are used, and cannot transfer its glycogen to the core or legs. Once the quads use up their glycogen stores, they are tapped out, even if the glycogen stores in your lats or liver or biceps etc are full. And you’re certainly not going to compensate for that by speed-crawling or anything similar.

This is why runners “hit the wall” in marathons, i.e. reach a point where their energy precipitously drops and running becomes much more of a struggle. Their lower body muscles run out of glycogen well short of the finish line, and it’s too late to add enough glycogen to restore their previous pace.

Most people hit the wall because they go out harder than their bodies are capable of maintaining for the whole distance, and they don’t take in sufficient fuel to replenish their body and keep the pace.

Even then, the body can only digest and utilize so much consumed fuel. You can’t drink a gallon of Gatorade at the start line and expect all of it to convert to glycogen. If you don’t die trying, you’ll probably just piss or vomit most of it back out.

In theory, it’s entirely possible to travel the whole 26.2 mile distance without fuel… if you possess enough general stamina and travel slowly enough to spread out those glycogen stores and use fat.

Yes, fat. Your body does use some percentage of fat in exercise. But what ratio of fat to glycogen it uses depends entirely on the intensity and difficulty of your exercise. The more intense your effort, the higher the ratio of glycogen used to fat. You can’t just program your body to give an 80% max effort but use 100% fat. Your body does not work that way.

Fat also gets processed for energy very, very slowly compared to glycogen. Glycogen is spurred to action almost immediately during intense exercise, while fat takes a lot more time.

It’s akin to glycogen being like a phone call, while fat is more like sending a traditional fax. Both modes of communication can easily work, but one is a lot quicker and easier to complete.

Meanwhile, you also have anaerobic power, which is like a rocket booster. This comes into play during very intense effort, like a sprint. It works super quick and provides a ton of power. If glycogen is like a phone call, anaerobic power is like yelling through the other person’s window.

The most common application of anaerobic power for most people comes when they lift heavy stuff. Weightlifting is pretty much all anaerobic power used in very brief bursts.

Every runner, however, also uses some anaerobic power during running, mostly when accelerating from a standing or walking start to a full run. The strength required to change your velocity comes from anaerobic power, which gives way to aerobic conditioning and glycogen/fat once you’re up to speed.

However, anaerobic power runs out very, very quickly. It’s essentially a sort of adrenaline, rather than calorie-based, and only serves to more quickly activate your glycogen stores. If your muscles’ glycogen stores are like a vehicle’s gas tank, think of your anaerobic tank as a shot-bottle of 5 Hour Energy in comparison. It pretty much lasts less than a few minutes of constant effort, if that.

This is why you can’t sprint a marathon. Usain Bolt can try, but even he will slow to a panting jog within a mile, and good luck finishing the other 25.2. And again, anaerobic power is not used in lieu of glycogen: It simply serves to quickly activate your use of glycogen, and that glycogen still gets burned. A runner’s energy needs come from glycogen and fat.


So, what is the ratio of glycogen used to fat when running? There is no exact number since everybody is different, even people who weigh the same and have the same height. But science has given us a scattered assembly of approximations that I will distill into a roughly accurate number for some key activity:

The human body is pretty consistent about the number of calories burned per mile. For example, given my nearly 40 year old, shrinking but still sizable 160 lb body, it takes me about 122-125 calories to move one mile.

This rate will change as my size and basal metabolic rate changes, but the resulting energy I need to move a mile on my two feet will pretty much remain the same whether I’m walking that mile or sprinting it. How long you take to do so obviously varies (though obv doing it quicker means you have more left over time to burn more calories overall in a given day). And in turn how much of that energy comes from glycogen vs fat can vary based on the intensity of your effort.

Easy walking roughly uses 80-85% fat, 15-20% glycogen. A real easy run is more like 45-50% fat, 50-55% glycogen. A tempo run that gets you breathing hard uses about 80% glycogen and 20% fat. Running a mile as fast as you can uses about 90% glycogen to 10% fat, and running any faster than that pretty much uses nothing but glycogen.

Jonathan Savage offers a formula based on research that can approximate the percentage of glycogen used based, if known, on the percentage of your VO2max effort you’re using.

% of Glycogen used (as a whole number) = 0.0021x2 + 0.7896x – 21.031

(where x equals the percentage, as a whole number, of your VO2max being used)

But, even if you test for and know your VO2max, or you use the complex charts of Daniels Running Formula with your known race results to get an approximation based on his VDOT metric… knowing exactly how much of your VO2max you’re using at any given time is little more than swings at a pinata in a wind storm. I have certainly crunched my own numbers and come up with some estimates, but admittedly they’re little more than educated guesses.

The Hanson Brothers have offered more refined approximations for a marathon-pace effort in their book Hanson Marathon Method. They approximate that most runners giving a serious race-effort or the distance will use 65-75% glycogen and 25-35% fat. Along with this, the Hansons’ book offers formula estimates for the amount of glycogen you have stored in your relevant lower body muscles, plus the total calories you will need to finish the marathon to offer an estimated number of calories you need to consume during the race to not “hit the wall”.

Using myself as an example, I will burn approximately 3250 calories running the Chicago Marathon. If I manage to run at my optimal marathon-pace effort the entire way, let’s split the different between 65-75% and say I will burn 70% of those calories from glycogen. My lower body will require a total of 2275 calories of glycogen to complete the race entirely at that pace, about 86.7 calories of glycogen per mile.

Now, I won’t go into details but according to the Hansons’ approximate formulas, my relevant lower body muscles hold about 1280 calories worth of glycogen at full capacity. You’ll notice that number is well short of 2275. If I gave a marathon-grade effort with no other fuel, I should be able to go a maximum of 14.75 miles before I “hit the wall”. If I take in no fuel before or during the race, those last 11.47 miles will be very miserable ones. But I’m not stupid.

(By the way, you’ll notice I’m not paying any mind to how much fat I’m storing. A pound of fat contains 3500 calories. I’m carrying about 24-25 pounds of fat. Do the math. Don’t worry about getting enough energy from fat.)

If I absolutely commit to that 70% glycogen effort, I will need to consume an additional 995 calories of glycogen early enough for it to enter my bloodstream, make its way to my lower body and subsequently be absorbed and used by my legs while it still matters.

Eating or drinking anything at mile 26 won’t help my race if it won’t kick in until 30-60 minutes after I cross the finish line. So it must be consumed either shortly before the race, where it won’t be digested and in my bloodstream until after I’ve began tapping my glycogen stores aka started the race… or during a portion of the race where it will be converted to glycogen for use before the end of the race. Whatever I consume must be consumed well before the finish, so that it will be available during the race and not after.

And, most importantly, bear in mind that this is 995 required calories of glycogen, which must be carbohydrate.

For example, I can’t just eat 995 calories worth of protein bars during the marathon, because (never mind that the volume of sugar alcohols will wreck your intestines and lead to a very bad time) not all 995 of those calories are carbohydrates. Some are protein and some are fat. While both are useful, those are not carbohydrates and will not be used as glycogen.

Using a Clif Bar as an example, its 240 calories contain about 71.5% carbs. Eat 995 calories worth of this (ignoring the corresponding very bad time for now) and you’ll get about 711 calories (about 178g) of carbohydrate.

Plus, not all of that is quickly and usefully digestible: Some of it is soluble fiber, great for staying regular but not great for immediately fueling the last miles of a marathon. Also, we don’t know how much of it your body will actually digest. Ideally your stomach is a well oiled digestion machine and everything gets used like it should. But naturally some of it will pass through unused and get pissed or crapped out later.

(And of course, your entire body needs and processes glycogen, so not all of it will go directly to your lower body where it’s needed most. But for this example let’s be super generous and assume that during the heat of a race it essentially will. Your organs and bloodstream may kick in a modicum of additional glycogen during the race, so we can assume it’ll even out.)

Still, my stomach by mile 10 is probably all-hands-on-deck trying to milk whatever comes in, so let’s be a bit optimistic and say only the 5g per bar of dietary fiber (about 20.3g total) doesn’t get used.

Every gram of carbohydrate supplies about 4 calories of energy, so that leaves me with about 616 calories worth of glycogen. Even if all of it is perfectly used, I’m still short 379 calories of glycogen.

This is one key reason runners swear by Gatorade. It’s literally all engineered-sugar, nothing but fiber-free carbohydrate that is easily converted to glycogen.

So then why not drink Gatorade? It also contains water, which you need. Most marathons offer it at the aid stations anyway, right?

A big 32 oz bottle of factory-produced Gatorade contains about 240 calories of carbohydrate. At minimum, I would have to consume 133 oz of Gatorade. A gallon contains 128 oz of fluid.

Never mind whether or not I can pound a gallon+ of Gatorade without puking/pissing/diarrheing it back out (hint: super unlikely). Every cup of Gatorade offered at an aid station contains on average 4 oz of fluid. If you took one cup at every single aid station, that’s “only” 80 oz (600 calories). Even if it all goes down fine, you’re short 395 calories.

Plus, the Gatorade at aid stations is usually mixed on-site from a concentrate powder. So, unlike manufactured Gatorade, you don’t know the consistency of a given cup. A race station cup of Gatorade usually is very watered down. So you’re not even getting a full 4 oz of Gatorade much of the time, though you’re at least getting necessary hydration and some electrolytes

Yes, marathons offer bananas later in the course (about 100-110 calories per banana of mostly glycogen, though again with soluble fiber). Yes, you could bring additional food or calorie-enriched drink. Yes, there are workarounds, and clearly thousands of people finish a marathon in style every year, so we’re not talking about an impossible task here.


Let’s go back to a previous point I paraphrased from Jonathan Savage: It is possible for anyone to finish a marathon if they go slow enough. There is no requirement that you give a moderate-paced effort requiring 65-75% glycogen. I could just run slower than my desired pace, at something closer to my easy running pace, and even walk part of the way. Physically finishing the marathon is not the challenge, though doing it the slower way obviously takes a lot longer.

If you give an easy effort that requires 55% glycogen instead of 70%, your glycogen requirements go DOWN. More of your consumed energy comes from fat because you’ve lowered the intensity enough to allow more of that fat to be used in the race.

If I were to run the marathon at said easy pace, then I’d only need 1788 calories of glycogen. With 1280 calories in the proverbial tank, I would then only need 508 calories of spare glycogen. That’s still a considerable amount (and eating that much in Clif Bars would still lead to a bad time in an on-course port-a-potty), but lopping 487 calories off your in-race needs saves a ton of trouble and digestion.

Running at that pace, if I drank diluted Gatorade at every aid station, that alone would probably provide enough glycogen no matter how badly watered down it was. Eat a mid-race banana or two for good measure, and I’d have no problem crossing the finish line with dignity, even if it took me 5 hours to do it.

The key to this working of course is to START at that intensity and commit to it. If you go out at M-pace and then slow down later, you’re still burning the extra glycogen up front and you still run the risk of bonking even if you slow to an easy pace later. The good news is that I’ve been training to run a marathon pace that has gotten gradually easier, and chances are I stick to the game plan.


There is your highly scientific summary of how your muscles use energy for running, why runners hit the wall, and how to use this information to fuel a marathon. I eagerly await the arrival of my Nobel Prize in the mail.

Meanwhile, I have my own hypotheses as to what to do with this info, but I’ll wait until I finish Chicago and see how useful they end up being.

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