# What's the advantage of fatigue poisons?

**URL:** <https://boards.straightdope.com/t/whats-the-advantage-of-fatigue-poisons/334250>\
**Category:** Factual Questions\
**Created:** [December 5, 2005, 9:57pm UTC](https://boards.straightdope.com/t/whats-the-advantage-of-fatigue-poisons/334250 "2005-12-05T21:57:07Z")\
**Posts on this page:** 14\
**Page:** 1

<div class="post-metadata">

**Author:** ![Love\_Rhombus](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/love_rhombus/32/10694_2.png) [@Love\_Rhombus](https://boards.straightdope.com/u/Love_Rhombus)\
**Post date:** [December 5, 2005, 9:57pm UTC](https://boards.straightdope.com/t/whats-the-advantage-of-fatigue-poisons/334250/1 "2005-12-05T21:57:07Z")

</div>

Or, in other words, suppose there was someone who just didn’t get tired. Sleepy, yes, but he could run and run and not feel it. Would that person be in danger, assuming he was in otherwise great health?

---

<div class="post-metadata">

**Author:** ![bordelond](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/bordelond/32/150_2.png) [@bordelond](https://boards.straightdope.com/u/bordelond)\
**Post date:** [December 5, 2005, 10:06pm UTC](https://boards.straightdope.com/t/whats-the-advantage-of-fatigue-poisons/334250/2 "2005-12-05T22:06:08Z")

</div>

> [@Love Rhombus](#):
>
> Or, in other words, suppose there was someone who just didn’t get tired. Sleepy, yes, but he could run and run and not feel it. Would that person be in danger, assuming he was in otherwise great health?

I’m not grokking the difference between “sleepy” and “tired”.

---

<div class="post-metadata">

**Author:** ![Fish](https://avatars.discourse-cdn.com/v4/letter/f/bc8723/32.png) [@Fish](https://boards.straightdope.com/u/Fish)\
**Post date:** [December 5, 2005, 10:13pm UTC](https://boards.straightdope.com/t/whats-the-advantage-of-fatigue-poisons/334250/3 "2005-12-05T22:13:50Z")

</div>

Are you speaking of lactic acid buildup in muscles when someone exercises for long periods?

I don’t precisely think that the acid itself is an advantage, but the fact that we can _create_ the acid is. It’s generally made when the body isn’t getting enough oxygen during exercise, and it’s a chemical way to continue to process fuels without it.

If we didn’t have any lactic acid buildup, we’d be more like an internal combustion engine, or a fire: without oxygen, the engine stalls or the fire goes out. Your theoretical person who can run without becoming tired would need some other means to process fuel, or a better way to increase oxygen intake, or something.

---

<div class="post-metadata">

**Author:** ![Colibri](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/colibri/32/1841_2.png) [@Colibri](https://boards.straightdope.com/u/Colibri)\
**Post date:** [December 5, 2005, 10:16pm UTC](https://boards.straightdope.com/t/whats-the-advantage-of-fatigue-poisons/334250/4 "2005-12-05T22:16:28Z")

</div>

Why should there be an “advantage” to “fatigue poisons”? You get fatigued because you are using up energy in your tissues faster than it can be restored, and because lactic acid is building up due to insufficient aerobic metabolism. Your question is something like asking if a car would be able to run forever if it didn’t run out of gas.

---

<div class="post-metadata">

**Author:** ![Mangetout](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mangetout/32/19_2.png) [@Mangetout](https://boards.straightdope.com/u/Mangetout)\
**Post date:** [December 5, 2005, 10:29pm UTC](https://boards.straightdope.com/t/whats-the-advantage-of-fatigue-poisons/334250/5 "2005-12-05T22:29:03Z")

</div>

Fatigue poisons aren’t advantageous in themselves, they just happen to be the metabolic results of _normal processes that are advantageous_ - typically sustained activity.

Take the fatigue poisons your muscles produce as a result of respiring anaerobically - when you have to exert yourself harder and/or for longer than the oxygen supply to your muscles would normally allow; rather than just shutting down, your body starts to metabolise its energy reserves in a different way - sort of like an emergency override mode - the waste product of this emergency metabolism is lactic acid - when you stop exerting yourself, your body still needs to work hard and process loads of oxygen to rid itself of the lactic acid - this is called the ‘oxygen debt’ and is the reson that you have to keep puffing and panting even after you stop running.

Your body typically responds to these kind of poisons with sensations of pain and weariness (although some of this is not pure sensation of the poisons, it’s sensation of their actual toxic effect on various systems such as your brain and heart) - but if you were for some reason unable to percieve this buildup of toxins, then you would presumably be at risk of continuing beyond dangerously toxic levels and death would be a possible consequence.  
However, this isn’t likely to happen, because there isn’t just a single, simple ‘poison detector’ process at work - they are perceived as a result of their effect on many parts of the whole system.

---

<div class="post-metadata">

**Author:** ![Love\_Rhombus](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/love_rhombus/32/10694_2.png) [@Love\_Rhombus](https://boards.straightdope.com/u/Love_Rhombus)\
**Post date:** [December 5, 2005, 10:30pm UTC](https://boards.straightdope.com/t/whats-the-advantage-of-fatigue-poisons/334250/6 "2005-12-05T22:30:37Z")

</div>

My apoligies, I miswrote. I didn’t intend to imply that there was an advantage, but more along the lines of “why do we have them.” Why didn’t I put that as the thread title? Because I’m an idiot who was trying to sound intelligent in a subject he is clueless about. Sorry about that! My fault! I’m a :wally

Yes I’m speaking of lactic acid buildup. So what you’re saying is, the acid allows us to keep going longer than if we didn’t have it? I may be missing something here; pretend I’m an idiot, as stated above.

---

<div class="post-metadata">

**Author:** ![Mangetout](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mangetout/32/19_2.png) [@Mangetout](https://boards.straightdope.com/u/Mangetout)\
**Post date:** [December 5, 2005, 10:31pm UTC](https://boards.straightdope.com/t/whats-the-advantage-of-fatigue-poisons/334250/7 "2005-12-05T22:31:21Z")

</div>

I meant to emphasise/reiterate in summary that - it isn’t advantageous that running fast generates fatigue poisons; it’s just advantageous to be able to run fast (or at least this was substantially the case for our ancestors)

---

<div class="post-metadata">

**Author:** ![bordelond](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/bordelond/32/150_2.png) [@bordelond](https://boards.straightdope.com/u/bordelond)\
**Post date:** [December 5, 2005, 10:40pm UTC](https://boards.straightdope.com/t/whats-the-advantage-of-fatigue-poisons/334250/8 "2005-12-05T22:40:59Z")

</div>

> [@Love Rhombus](#):
>
> Yes I’m speaking of lactic acid buildup.

Yikes. Your OP title made me think you were actually asking about poisons … like some bizzare potion that you could slip to someone that would make them never feel tired.

Never mind me. :o

---

<div class="post-metadata">

**Author:** ![Telemark](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/telemark/32/372_2.png) [@Telemark](https://boards.straightdope.com/u/Telemark)\
**Post date:** [December 5, 2005, 10:43pm UTC](https://boards.straightdope.com/t/whats-the-advantage-of-fatigue-poisons/334250/9 "2005-12-05T22:43:56Z")

</div>

> [@Love Rhombus](#):
>
> So what you’re saying is, the acid allows us to keep going longer than if we didn’t have it? I may be missing something here; pretend I’m an idiot, as stated above.

The acid is a byproduct of the chemical process that allows us to run fast. It’s not like we can have something for nothing, evolution works with the building blocks available. In order to draw on our energy reserves stored in fat and muscle, lactic acid is produced. It’d be wonderful if the process didn’t work that way, but chemistry doesn’t give us much else to work with.

Burning fuel produces byproducts. Your body must get rid of these byproducts somehow, it’s not free.

---

<div class="post-metadata">

**Author:** ![Mangetout](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mangetout/32/19_2.png) [@Mangetout](https://boards.straightdope.com/u/Mangetout)\
**Post date:** [December 5, 2005, 10:44pm UTC](https://boards.straightdope.com/t/whats-the-advantage-of-fatigue-poisons/334250/10 "2005-12-05T22:44:01Z")

</div>

Think of it like those scenes in _Star Trek_; they’re being hotly pursued by some alien energy-based amorphous lifeform and kirk demands Warp Factor Ten; Scotty cries “Och, the engines cannae take it cap’n!”, but Kirk just barks “Give me all you’ve got Mr Scott, that’s an order!”. So they escape(just), but at the expense of abusing the drive system - so while Kirk is snogging the green-skinned alien princess, Scotty has to work hard at repolarising the dilithium crystals

---

<div class="post-metadata">

**Author:** ![Excalibre](https://avatars.discourse-cdn.com/v4/letter/e/898d66/32.png) [@Excalibre](https://boards.straightdope.com/u/Excalibre)\
**Post date:** [December 6, 2005, 4:49am UTC](https://boards.straightdope.com/t/whats-the-advantage-of-fatigue-poisons/334250/11 "2005-12-06T04:49:09Z")

</div>

I’m going to try to remember the specifics of my high school biology class here. There’s two different types of metabolism at work here. First, glucose is broken down to produce smaller three-carbon molecules; this process is called _anaerobic respiration_, and it doesn’t require oxygen. But only a small amount of energy is produced. Next, these three-carbon molecules undergo further breakdown called _aerobic respiration_, which produces substantially more energy, but requires oxygen to work. Eventually, what results from the breakdown of the glucose is carbon dioxide, which you breathe out.

When you run fast for a long period of time, your circulatory and respiratory systems simply can’t work fast enough to get sufficient oxygen to your tissues to support the high rate of metabolism you’re forcing your muscles to do. So aerobic respiration stops, while anaerobic respiration continues. This is inefficient, and you can’t get nearly as much energy out of a glucose molecule, but it allows your body to continue to exert energy even when you can’t breathe fast enough to keep up with the oxygen demand.

So the three-carbon molecules (and I can’t remember what the name of the chemical is) are instead converted into lactic acid. Large amounts of lactic acid in your muscles signal fatigue, reminding you to stop running as soon as you can. Once you’ve returned to a more normal rate of exertion, you continue to huff and puff to get oxygen into your tissues, which allows the lactic acid breakdown to proceed to completion, and you again exhale the final product as carbon dioxide.

So lactic acid builds up because you simply can’t get enough oxygen into your muscles to support the amount of energy they require. It’s not advantageous in and of itself, but it _is_ useful to have the ability, since otherwise you simply couldn’t exert yourself beyond the level supported by your ability to move oxygen into your muscles.

Your question, then, is sort of like asking, “Why is it advantageous for a person to pay credit card interest?” It’s not that lactic acid is somehow useful in itself. But the ability to run harder than you would be able to otherwise is useful. It’s like being able to buy groceries on a credit card in an emergency. You have to pay interest to the bank for the privilege, but otherwise you’d be stuck and couldn’t buy groceries at all.

---

<div class="post-metadata">

**Author:** ![AskNott](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/asknott/32/5790_2.png) [@AskNott](https://boards.straightdope.com/u/AskNott)\
**Post date:** [December 6, 2005, 4:59am UTC](https://boards.straightdope.com/t/whats-the-advantage-of-fatigue-poisons/334250/12 "2005-12-06T04:59:39Z")

</div>

We get threads like this occasionally, and I’m puzzled/irked when it happens. “What is the purpose/advantage of (some body part/natural function?” Usually the question assumes Intelligent Design is valid. If so, how the heck should we know, we’re not the intelligent designer. Or, if you’re not assuming ID, why should there be any purpose/function?

Why is the human shoulder designed so darned poorly? I wish I knew.

---

<div class="post-metadata">

**Author:** ![Quercus](https://avatars.discourse-cdn.com/v4/letter/q/7ab992/32.png) [@Quercus](https://boards.straightdope.com/u/Quercus)\
**Post date:** [December 6, 2005, 2:53pm UTC](https://boards.straightdope.com/t/whats-the-advantage-of-fatigue-poisons/334250/13 "2005-12-06T14:53:38Z")

</div>

> [@AskNott](#):
>
> We get threads like this occasionally, and I’m puzzled/irked when it happens. “What is the purpose/advantage of (some body part/natural function?” Usually the question assumes Intelligent Design is valid. If so, how the heck should we know, we’re not the intelligent designer. Or, if you’re not assuming ID, why should there be any purpose/function?

Actually, asking ‘What is the advantage of \<body feature\>?’ doesn’t have to assume Intelligent Design. It could be a shorter version of “What is the survival and reproductive advantage of \<feature\> that has led to it becoming universal in the gene pool of the organism?”

Similarly, asking “What is the purpose…” could be shorthand for the same evolution-oriented question.

---

<div class="post-metadata">

**Author:** ![AskNott](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/asknott/32/5790_2.png) [@AskNott](https://boards.straightdope.com/u/AskNott)\
**Post date:** [December 6, 2005, 6:18pm UTC](https://boards.straightdope.com/t/whats-the-advantage-of-fatigue-poisons/334250/14 "2005-12-06T18:18:14Z")

</div>

> [@Quercus](#):
>
> Actually, asking ‘What is the advantage of \<body feature\>?’ doesn’t have to assume Intelligent Design. It could be a shorter version of “What is the survival and reproductive advantage of \<feature\> that has led to it becoming universal in the gene pool of the organism?”
> 
> Similarly, asking “What is the purpose…” could be shorthand for the same evolution-oriented question.

I think I covered that in my post, too. What I may not have made clear is that we earth creatures often have parts and functions that have no apparent function or advantage (bushy eyebrows or the ability to roll your tongue in a tube, for example.) For us to wildly guess what evolutionary advantage Bob Knight’s eyebrows have over the rest of us, it’s just a parlor game. It’s amusing, but it’s not good science.