Should I Feel Stupid FOR Running or Just AFTER Running?

Runners in the Zurich Marathon of 2008. Click Image for credit.

An infrequent but enjoyable commenter (Black Sheep) recently asked a question that I thought I would answer with a post. The relevant portion of the comment is:

A friend and I are always puzzled by the way our bodies, or rather minds, react after a run. This really only tends to happen when we do longer runs, like when we were training for a half marathon. After we finish, whether it be a race or just a training run, we feel completely stupid…why the diminished mental capacity?

Some might argue that a person who wants to do such a long run is actually starting out with a diminished mental capacity. However, there is actually a very good reason for why you can feel stupid after a long run, even if you started with a keen mind. You will find the answer below the fold.

Your body is an amazingly efficient biological machine, and like any machine, it needs a source of energy. Most machines designed by humans utilize only one source of energy, but your body is better designed than anything built by people. Your body can utilize three different energy sources: carbohydrates, fats, and proteins. These energy sources are collectively called the macronutrients, and each one has its own purpose in the body.

Carbohydrates are the “catchall” fuel for the body. They are digested down to simple sugars, which are generally converted into a specific simple sugar known as glucose. All cells in the body can use glucose as a fuel. When your body has excess glucose, it strings those glucose molecules together to make a very long carbohydrate known as glycogen. This stores the glucose for later use.

While carbohydrates are an excellent all-around fuel, they do not store energy as efficiently as the body would like it to be stored. Thus, the body uses glycogen only as a short-term means of energy storage. When you eat a large excess of food (something I am quite fond of doing), your body converts a lot of it into fat. Fat can store more than twice as much energy per gram as carbohydrates, so your body uses fat for long-term energy storage.

Of course, you can eat fat, too. Your body breaks down the fat you eat into smaller units called fatty acids. If your body needs fuel right away, it can burn those fatty acids. Otherwise, it will take those fatty acids and use them to make more fat for long-term energy storage. Once the fat is stored, it takes time for the body to convert it back into fatty acids that can be burned for fuel. As a result, your body will use its fat stores only once it has run out of other readily available sources of energy.

The third macronutrient, protein, can be used for fuel, but that’s not its primary purpose. When you eat protein, your body breaks it down into smaller units called amino acids, and those amino acids get shipped to all the cells in your body. Every cell has to make a host of proteins in order to live and do its job, so each cell uses those amino acids to build the proteins it needs. The main reason you eat proteins, then, is to get their building blocks (amino acids) so your cells can make their own proteins.

In general, your body will only burn proteins for energy when you have a large excess of them, or when they are the only thing left. For example, muscle is composed almost entirely of protein. When you start working your muscles hard, they might run out of carbohydrates and fatty acid molecules. When that happens, they will burn proteins for energy until they are given more fatty acids or carbohydrates.

While I have glossed over many details, that is the general scheme for how most of the cells in your body use fuel so they can do their jobs. However, this discussion doesn’t apply to the cells in your brain, which are called neurons. Neurons are very sensitive cells that use a lot of energy, but they can only use glucose as a fuel. When the body runs out of glucose, then, the neurons cannot function well.

When you start out on a run or other workout, your body first burns the macronutrients that are readily available in your cells and in your blood. After that, it is forced to start drawing on its reserves. Because glycogen can be easily converted back to glucose, your body relies heavily on its glycogen storage. However, remember that glycogen is only a short-term storage solution. As a result, on a long run, you will eventually deplete all of your glycogen reserves.

There comes a point, then, when all your stored carbohydrates are gone. Your muscles can still work, however, because they can burn their proteins for fuel, and they can burn fatty acids that your body pulls from its fat storage. Your neurons, however, can’t burn either of those fuels.

Does that mean your neurons starve to death once you run out of glycogen reserves? No, because your body is cleverly designed. It has a last resort mechanism. It can convert proteins into glucose. This takes time, however, so your neurons don’t get all the energy they need. Instead, they get a slow, steady flow. To conserve energy, then, many neurons that are not involved in the absolute essentials of life (like running your circulatory and respiratory systems) reduce their activity. This reduces your ability to think. If you really push the limits of your body’s metabolism, you can actually reduce the activity of enough neurons that you go into a sort of “waking dream” state where you lose most of your ability to think rationally, and you even start hallucinating.

So in the end, you can feel stupid after a long run because you have run through all your body’s carbohydrates, and the only way your brain cells can get the fuel they need is through the relatively slow process of turning proteins into glucose. This requires your brain to reduce the activity of many neurons, making it harder for you to think clearly.

You can mitigate this problem by making sure you have maximized the amount of glycogen stored in your body to begin with. This is typically done by eating a carbohydrate-rich meal a couple of hours before you run. You can also take some carbohydrate gels to eat while you run. You can’t get too many carbohydrates that way, however, because it is difficult for your digestive system to digest a lot of food while you are running. However, it can still help reduce the amount of “brain drain” that comes from a very long run.

8 thoughts on “Should I Feel Stupid FOR Running or Just AFTER Running?”

  1. That’s rather interesting. I’ve gotten nearly to that “brain drain” state when I was shoveling the TCF Stadium last month. Would it follow that it’s just a matter of energy output that matters and not necessarily a matter of aerobic versus anaerobic work? It took a lot of effort climbing flights of stairs and shoveling snow, so even though it isn’t as high wattage as rapidly transporting oneself it is still an output which drains the stored chemical energy from one’s body in the form of heat and motion.

    1. Ben, you can get to that point whether your workout is aerobic or anaerobic, but there is more to it than straight energy output. Anaerobic workouts burn carbohydrates less efficiently than aerobic workouts. So for the same energy output, anaerobic workouts will burn through your glycogen reserves sooner.

  2. Oh Dr. Wile. How should one react when called an “enjoyable commenter” but it’s suggested that they have a “diminished mental capacity” all in one blog post?!?!?

    I do thank you for your response. It’s actually quite interesting. I certainly use the techniques you mentioned, carb-loading and gels, but as you said, it seems I can’t get enough in my body while actually running to stop from feeling… well.. stupid.

    Though part of me thinks to a certain extent this isn’t so awful for a runner during a race. Obviously, I want to keep those neuron-dudes you mentioned fed well enough that I don’t start hallucinating, but a certain amount of numbness in the brain isn’t really so bad. The best runs for me are when I can just zone out and run.

    I do find it funny that you say, “Some might argue that a person who wants to do such a long run is actually starting out with a diminished mental capacity” when I happen to know that you have participated in at least one half marathon, and I believe are currently registered for another. Just an observation.

    1. Black Sheep, I happily admit that in some ways, I have a severely diminished mental capacity. Look at how well I make business decisions, for example…

      In terms of carb-loading, it is common for runners to do that the night before a race. While that helps, your body does burn through some glycogen overnight. Thus, it is important to get some carbs in your body just a couple of hours before you race.

  3. Hi Jay, I just found your blog! Nice article – very interesting! As a distance runner, my friends and I have experienced this phenomena so often, that I’ve dubbed it PRSS – post race stupidity syndrome. It often manifests itself at marathon finish lines, where one literally isn’t quite sure what to do next … thank goodness for helpful race volunteers! (just keep moving forward …)

    We also see it during very long runs. I (a chemist) run with two CPAs, an HR director, and other competent professionals, and we literally have trouble doing mental math — let’s see, if we run down to the sign and back, that’s .7 miles times two, added to 18.3, that gives us our 21 … no, wait, that’s not right … huh?

    Now we all have Garmins!

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