# How does the human body know which substances are toxic (and need excreting) and which are not? **URL:** \ **Category:** Factual Questions\ **Created:** [July 7, 2024, 8:02pm UTC](https://boards.straightdope.com/t/how-does-the-human-body-know-which-substances-are-toxic-and-need-excreting-and-which-are-not/1003844 "2024-07-07T20:02:43Z")\ **Posts on this page:** 18\ **Page:** 2
**Author:** ![Broomstick](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/broomstick/32/246_2.png) [@Broomstick](https://boards.straightdope.com/u/Broomstick)\ **Post date:** [July 8, 2024, 1:20pm UTC](https://boards.straightdope.com/t/how-does-the-human-body-know-which-substances-are-toxic-and-need-excreting-and-which-are-not/1003844/21 "2024-07-08T13:20:52Z")
> [@Francis\_Vaughan](#): > > Many insect sprays are basically neurotoxins. Not far removed from nerve gasses. They kill insects and not us because we have the ability to break them down fast enough that they don’t kill us. That’s more of a instance of dose vs. body mass. A spray of insecticide kills insects in part because such a spray delivers a lethal sized dose to a small insect but it’s not enough poison to kill a human. Insecticides certainly _are_ lethal to humans if they get a large enough dose. They just need a much larger dose than a bedbug or wasp. ---
**Author:** ![Francis\_Vaughan](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/francis_vaughan/32/3093_2.png) [@Francis\_Vaughan](https://boards.straightdope.com/u/Francis_Vaughan)\ **Post date:** [July 8, 2024, 1:36pm UTC](https://boards.straightdope.com/t/how-does-the-human-body-know-which-substances-are-toxic-and-need-excreting-and-which-are-not/1003844/22 "2024-07-08T13:36:22Z")
> [@Chronos](#): > > theobromine is only a little more dangerous to dogs than to humans I have seen it as somewhere between two to three times as bad for dogs as humans. That probably does count as only a little, but it is enough to get worried. The rate of elimination is different, but doesn’t totally explain the difference. ---
**Author:** ![Francis\_Vaughan](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/francis_vaughan/32/3093_2.png) [@Francis\_Vaughan](https://boards.straightdope.com/u/Francis_Vaughan)\ **Post date:** [July 8, 2024, 1:56pm UTC](https://boards.straightdope.com/t/how-does-the-human-body-know-which-substances-are-toxic-and-need-excreting-and-which-are-not/1003844/23 "2024-07-08T13:56:28Z")
> [@Broomstick](#): > > That’s more of a instance of dose vs. body mass. A spray of insecticide kills insects in part because such a spray delivers a lethal sized dose to a small insect but it’s not enough poison to kill a human. Looking at Permethrin, which has some interesting traits. [http://npic.orst.edu/factsheets/archive/Permtech.html](http://npic.orst.edu/factsheets/archive/Permtech.html) The topical LC50 for honeybees is 0.029 μg/bee - That is topical, not ingested. Honeybees mass about 0.1 grams. So that translates to 0.29mg/kg. Animals fed on a diet of Permethrin with an equivalent dose a thousand times higher showed some minor effects over time. None died. Birds are pretty tolerant. > [@](#): > > Oral dose LD50s for chickens, mallard ducks, and Japanese quail are \>3000, \>9800, and \>13,500 mg/kg body weight, respectively. You can’t make some of this up: > [@](#): > > A study was conducted involving 196 women who had applied a single, full-body, dermal dose of 4% permethrin as a scabies treatment during their second or third trimesters of pregnancy. Researchers found no evidence that exposure to permethrin affected the outcome of the participants’ pregnancies Aquatic life is ruinously sensitive. nanograms per litre in the water is deadly. ---
**Author:** ![gnoitall](https://avatars.discourse-cdn.com/v4/letter/g/bb73d2/32.png) [@gnoitall](https://boards.straightdope.com/u/gnoitall)\ **Post date:** [July 8, 2024, 2:09pm UTC](https://boards.straightdope.com/t/how-does-the-human-body-know-which-substances-are-toxic-and-need-excreting-and-which-are-not/1003844/24 "2024-07-08T14:09:35Z")
> [@Chronos](#): > > but a 180 pound human wouldn’t eat 30 pounds. ![](https://media1.tenor.com/m/jZVZa853eNEAAAAC/gif.gif) ---
**Author:** ![LSLGuy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lslguy/32/5813_2.png) [@LSLGuy](https://boards.straightdope.com/u/LSLGuy)\ **Post date:** [July 8, 2024, 2:23pm UTC](https://boards.straightdope.com/t/how-does-the-human-body-know-which-substances-are-toxic-and-need-excreting-and-which-are-not/1003844/25 "2024-07-08T14:23:42Z")
> [@Whack-a-Mole](#): > > I would think, stated simply, it is clear our bodies do not know how to rid itself of any toxic substance. Poisons exist. Venom exists. These will make us very sick or dead because our bodies cannot rid itself of these things. I would substitute “every” for “any”, although that may just be you and I writing / reading different forms of “any” 's many meanings. Some bad things the body can handle. Some bad things it cannot. > [@Velocity](#): > > I thought it was that the body DOES know to excrete these things, but unfortunately, they usually kill before the body has enough time to do so. Like snake venom killing you in 1 hour when your body might have been able to excrete it in 24 hours (if given enough living time to do so.) You are right to add the time element to all this. Some fast-acting substances do great harm before they can be adequately handled. I’ll quibble a bit with “excrete”. In most cases the body doesn’t eject bad stuff as-is. Which is what “excrete” implies. Instead it chemically disassembles = “metabolizes” the bad stuff into less-harmful or fully harmless constitutent components. Which in turn may be further metabolized and eventually excreted in the simplified harm-reduced forms. Given time and metabolic capacity. As someone pointed out upthread, your system has X capacity to perform the Y → Z+A+B chemical transformation. If you’re taking in more Y than you can disassemble per unit time, you’re on a trajectory to become overloaded with Y even as you’re still successfully converting some of it to Z+A+B. ---
**Author:** ![Darren\_Garrison](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/darren_garrison/32/92_2.png) [@Darren\_Garrison](https://boards.straightdope.com/u/Darren_Garrison)\ **Post date:** [July 8, 2024, 4:12pm UTC](https://boards.straightdope.com/t/how-does-the-human-body-know-which-substances-are-toxic-and-need-excreting-and-which-are-not/1003844/26 "2024-07-08T16:12:27Z")
Some things aren’t even _directly_ toxic. Like ethylene glycol. It harms you because a metabolite of a metabolite of it is toxic. ---
**Author:** ![LSLGuy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lslguy/32/5813_2.png) [@LSLGuy](https://boards.straightdope.com/u/LSLGuy)\ **Post date:** [July 8, 2024, 4:45pm UTC](https://boards.straightdope.com/t/how-does-the-human-body-know-which-substances-are-toxic-and-need-excreting-and-which-are-not/1003844/27 "2024-07-08T16:45:10Z")
There are also pro-drugs where the material you ingest doesn’t do anything useful in the body itself as-is. But once the body partially disassembles it, some of the resulting daughter products _are_ effective at doing whatever the medicine is intended to do. This whole branch of how drugs (or anything else) are processed once inside the body is called > **[Pharmacokinetics](https://en.wikipedia.org/wiki/Pharmacokinetics)** > > Pharmacokinetics (from Ancient Greek pharmakon "drug" and kinetikos "moving, putting in motion"; see chemical kinetics), sometimes abbreviated as PK, is a branch of pharmacology dedicated to describing how the body affects a specific substance after administration. The substances of interest include any chemical xenobiotic such as pharmaceutical drugs, pesticides, food additives, cosmetics, etc. It attempts to analyze chemical metabolism and to discover the fate of a chemical from the moment Ph... The OP might enjoy reading some of this article or other related ones. ---
**Author:** ![Machine\_Elf](https://avatars.discourse-cdn.com/v4/letter/m/82dd89/32.png) [@Machine\_Elf](https://boards.straightdope.com/u/Machine_Elf)\ **Post date:** [July 8, 2024, 5:05pm UTC](https://boards.straightdope.com/t/how-does-the-human-body-know-which-substances-are-toxic-and-need-excreting-and-which-are-not/1003844/28 "2024-07-08T17:05:16Z")
> [@LSLGuy](#): > > You are right to add the time element to all this. Also dosage. I can shotgun a 12-ounce beer containing an ounce of ethanol, and I’ll be pretty much back to normal in an hour. If I shotgun 12 ounces of ethanol, I’m guessing I’ll end up dead (or close to it). There are also toxins for which, if the fatal effects are temporary and can be treated with supportive measures, the body will metabolize the toxin and you’ll recover. > **[Tetrodotoxin | Symptoms and treatment](https://en.wikipedia.org/wiki/Tetrodotoxin#Symptoms_and_treatment)** > > The diagnosis of pufferfish poisoning is based on the observed symptomatology and recent dietary history. > Symptoms typically develop within 30 minutes of ingestion, but may be delayed by up to four hours; however, if the dose is fatal, symptoms are usually present within 17 minutes of ingestion. Having pins and needles of the lips and tongue is followed by developing it in the extremities, hypersalivation, sweating, headache, weakness, lethargy, incoordination, tremor, paralysis, bluish skin, lo... > Therapy is supportive and based on symptoms, with aggressive early airway management.[[51]](https://en.wikipedia.org/wiki/Tetrodotoxin#cite_note-uhm-52) If consumed, treatment can consist of emptying the stomach, feeding the victim [activated charcoal](https://en.wikipedia.org/wiki/Activated_charcoal) to bind the toxin, and **taking standard life-support measures to keep the victim alive until the effect of the poison has worn off**. Bolding mine. ---
**Author:** ![gnoitall](https://avatars.discourse-cdn.com/v4/letter/g/bb73d2/32.png) [@gnoitall](https://boards.straightdope.com/u/gnoitall)\ **Post date:** [July 8, 2024, 5:06pm UTC](https://boards.straightdope.com/t/how-does-the-human-body-know-which-substances-are-toxic-and-need-excreting-and-which-are-not/1003844/29 "2024-07-08T17:06:07Z")
> [@Darren\_Garrison](#): > > It harms you because a metabolite of a metabolite of it is toxic. There’s quite a lot of that, in fact. The liver (or other detoxifying organ) doing its job can create the actual toxic substance. For instance, acetaminophen/paracetamol is decomposed in the liver into a toxic substance that it can usually detoxify quite rapidly in turn, but that second stage requires a chemical that can run out if too much is taken… so acetaminophen toxicity is based on an overdose creating a toxic metabolite that the liver runs out of the ability to detox. > **[NAPQI](https://en.wikipedia.org/wiki/NAPQI)** > > NAPQI, also known as NAPBQI or N-acetyl-p-benzoquinone imine, is a toxic byproduct produced during the xenobiotic metabolism of the analgesic paracetamol (acetaminophen). It is normally produced only in small amounts, and then almost immediately detoxified in the liver. > However, under some conditions in which NAPQI is not effectively detoxified (usually in the case of paracetamol overdose), it causes severe damage to the liver. This becomes apparent 3–4 days after ingestion and may result in dea... ---
**Author:** ![Whack-a-Mole](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/whack-a-mole/32/141_2.png) [@Whack-a-Mole](https://boards.straightdope.com/u/Whack-a-Mole)\ **Post date:** [July 8, 2024, 5:17pm UTC](https://boards.straightdope.com/t/how-does-the-human-body-know-which-substances-are-toxic-and-need-excreting-and-which-are-not/1003844/30 "2024-07-08T17:17:37Z")
> [@LSLGuy](#): > > I would substitute “every” for “any”… Fair correction. I like your language better. And as you and @Velocity noted, a timely manner is important for this. Although, some bad things for us do seem to linger and accumulate over time. There is no metabolizing some things (like mercury). ETA: I may be wrong about metabolizing mercury. It seems it is more complicated than I thought. ---
**Author:** ![jjakucyk](https://avatars.discourse-cdn.com/v4/letter/j/3d9bf3/32.png) [@jjakucyk](https://boards.straightdope.com/u/jjakucyk)\ **Post date:** [July 8, 2024, 10:27pm UTC](https://boards.straightdope.com/t/how-does-the-human-body-know-which-substances-are-toxic-and-need-excreting-and-which-are-not/1003844/31 "2024-07-08T22:27:47Z")
Here’s the way I like to think about evolutionary questions like this. It’s not that our bodies “learned” which substances are toxic and need expelling, it’s that our ancestors who didn’t expel or avoid those substances, for whatever reason, didn’t survive to pass on their genes. Subtle genetic variation and/or mutations mean that those ancestors who were less turned off by bitter or otherwise foul smelling and tasting substances were more likely to eat or drink something poisonous and die. Similarly, we didn’t “learn” to be creeped out by bugs and spiders. Our ancestors who were less likely to swat at itchy/ticklish feelings on their skin were more likely to get stung or bitten and thus more likely to die either directly by venom or indirectly by being incapacitated and less able to gather food and water. Iterate this over millions of years and generations and here we are. It’s similar to the hypothesis of why we tend to like shiny things. Shiny things are generally safer because they’re cleaner. Water is shiny/sparkly in the light, and water is critical to our survival. So our ancestors who liked shiny things more often noticed the glint of some water through the trees and were less likely to die of dehydration. ---
**Author:** ![Napier](https://avatars.discourse-cdn.com/v4/letter/n/ce73a5/32.png) [@Napier](https://boards.straightdope.com/u/Napier)\ **Post date:** [July 9, 2024, 11:35am UTC](https://boards.straightdope.com/t/how-does-the-human-body-know-which-substances-are-toxic-and-need-excreting-and-which-are-not/1003844/32 "2024-07-09T11:35:58Z")
I think many mechanisms for coping with toxins evolved as separate little details in our makeup. A possible example I find fascinating is the sensitivity of the inner ear to some toxic substances, which makes us dizzy when we consume them. We have a nausea response to dizziness, which tends to make us vomit. I’m tempted to see this nausea response as an evolved mechanism for ejecting the toxins. On the other hand, we are awfully good at noticing relationships that aren’t actually there, so… ---
**Author:** ![LSLGuy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lslguy/32/5813_2.png) [@LSLGuy](https://boards.straightdope.com/u/LSLGuy)\ **Post date:** [July 10, 2024, 2:59pm UTC](https://boards.straightdope.com/t/how-does-the-human-body-know-which-substances-are-toxic-and-need-excreting-and-which-are-not/1003844/33 "2024-07-10T14:59:48Z")
> [@Francis\_Vaughan](#): > > > [@Chronos](#): > > > > And sometimes the solution to that problem is to just give large enough doses, > > Which swiftly gets us to a big fraction of drug interactions. The liver has a finite ability to degrade stuff, and if the degradation pathway is already working hard to break down drug X, adding drug Y that also has the same degradation pathway to the mix can result in both drugs rising in concentration. > Lots of drugs are processed by the family of Cytochrome P-450 heamproteins. These are not just in the liver. > > A famous example of interaction is how grapefruit juice interferes with a P450 pathway in the small intestine resulting in higher than intended doses of a drug making it into the bloodstream. > > Monoamine oxidase inhibitors are another great example of stuff that goes wrong. Monoamine oxidase degrades another class of stuff we eat, but also plays a part in regulation of neurotransmitters, degrading them at a constant-ish rate leading to a stable concentration. Treating depression with inhibitors of MAO increases levels of the transmitters, which can be beneficial, but reduces the amount in the gut that is making stuff safe to eat. So foods with lots of monoamines get past - with potentially life threatening results. Here is a blogger I follow who today happened to speak to exactly this idea: **[The Next Paxlovid | Science](https://www.science.org/content/blog-post/next-paxlovid)**. In this case, in the rush to get Paxlovid to market they formulated it as a combination of the actual active substance that attacks COVID (nirmatrelvir), and a different drug substance (ritonavir) that does nothing whatsoever to COVID, but keeps the liver so busy tearing up the second decoy drug that the first one sneaks through in sufficient quantity to attack your circulating COVID virus. Very clever drug design. Biology in general and the human body in particular is very very _very_ complicated and the experts readily admit we are just barely, barely scratching the surface of understanding how this all works. Every time somebody thinks they’ve peeled the onion all the way, somebody else discovers a brand new umpteen-layer deep onion hiding inside. ---
**Author:** ![Francis\_Vaughan](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/francis_vaughan/32/3093_2.png) [@Francis\_Vaughan](https://boards.straightdope.com/u/Francis_Vaughan)\ **Post date:** [July 10, 2024, 3:02pm UTC](https://boards.straightdope.com/t/how-does-the-human-body-know-which-substances-are-toxic-and-need-excreting-and-which-are-not/1003844/34 "2024-07-10T15:02:56Z")
Heh. I finishing reading that exact blog only a couple of minutes ago. Derek is always worth reading. ---
**Author:** ![LSLGuy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lslguy/32/5813_2.png) [@LSLGuy](https://boards.straightdope.com/u/LSLGuy)\ **Post date:** [July 10, 2024, 3:04pm UTC](https://boards.straightdope.com/t/how-does-the-human-body-know-which-substances-are-toxic-and-need-excreting-and-which-are-not/1003844/35 "2024-07-10T15:04:21Z")
Totally not my field of expertise, but I really enjoy his skill and attitude. Like with your writing here, listening to an expert speak skillfully to laymen about his own wheelhouse is always a fulfilling journey of discovery. ---
**Author:** ![Al128](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/al128/32/16541_2.png) [@Al128](https://boards.straightdope.com/u/Al128)\ **Post date:** [July 12, 2024, 8:40pm UTC](https://boards.straightdope.com/t/how-does-the-human-body-know-which-substances-are-toxic-and-need-excreting-and-which-are-not/1003844/36 "2024-07-12T20:40:20Z")
> [@Chronos](#): > > And sometimes the solution to that problem is to just give large enough doses, frequently enough, that you can keep up with the liver. any alcoholic knows that on a visceral level… ---
**Author:** ![Al128](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/al128/32/16541_2.png) [@Al128](https://boards.straightdope.com/u/Al128)\ **Post date:** [July 12, 2024, 11:01pm UTC](https://boards.straightdope.com/t/how-does-the-human-body-know-which-substances-are-toxic-and-need-excreting-and-which-are-not/1003844/37 "2024-07-12T23:01:09Z")
> [@Francis\_Vaughan](#): > > Derek is always worth reading. just mentioning Derek in this context - is enough to put a smile on many faces … he is def. one of those (nowadays) rare internet nuggets beneath a sea of influencer-crap that might actually lower the reader’s IQ. Always 10 min. well spent with Derek - and the igually great comment section! ---
**Author:** ![nearwildheaven](https://avatars.discourse-cdn.com/v4/letter/n/90db22/32.png) [@nearwildheaven](https://boards.straightdope.com/u/nearwildheaven)\ **Post date:** [July 13, 2024, 3:38am UTC](https://boards.straightdope.com/t/how-does-the-human-body-know-which-substances-are-toxic-and-need-excreting-and-which-are-not/1003844/38 "2024-07-13T03:38:09Z")
One of my professors (early 1990s) had his office door completely covered, on both sides, with science-oriented cartoons. (He probably spends his retirement time on xkcd!) My favorite had no caption, but was just a big group of people walking down a street while holding up a sign that said “BAN P450!” [Previous page](https://boards.straightdope.com/t/how-does-the-human-body-know-which-substances-are-toxic-and-need-excreting-and-which-are-not/1003844.md?page=1)