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Counterintuitive Bodily Responses, and Why They Happen

A sports scientist explains why our bodies seem to be working against us at times.

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Our earthly temples are designed for self-preservation, hard-wired for survival, with innate responses that kick in automatically when things get inconsistent with, you know, life. Sometimes those responses are inconvenient — spiking a blazing fever, for example — and unreasonable, given that you’re on a once-in-a-lifetime trek in Bhutan. But even through the delirium you can appreciate that your body is actually saving your hide. Short term, you feel like you’ve been trampled by yaks, but ultimately, infection is not fatal.  

Of course, these physiological responses were written into the human program long before we had marathons or even ran for recreation. I’ve encountered some bodily responses, either through observation or personal experience, that have all the hideously graphic short-term downsides, but seemingly no redeeming upside. They make no sense. It’s almost as if your body was actively making the situation worse.

Example? Vomiting and diarrhea as a response to dehydration. Wouldn’t the old Central Liquids Department, in times of drought, quash any reaction so flagrantly wasteful of precious fluids? If the water level is already low, why on earth would the natural response be to pump the remaining ounces out in explosive fashion as a means of self-preservation?

Other things I put in the “counterintuitive responses” category are sleeplessness as a response to hard training, and inflammation that double dips as both a response to and a cause of injury. I dumped these perplexing problems out on Ross Tucker’s desk and said, Explain. He’s the sport scientist of website and podcast Science of Sport fame, and particularly skilled at speaking high-level physiology using short, easy words. I supplied the dubious theories — things I heard from a friend of a friend of some guy —  and the head scratching questions; Tucker supplied the clear, smart, white-coated answers. That’s the division of labor you will observe in the following examination of counterintuitive responses.

Spoiler: our bodies are indeed very clever, and user error may play a role in bad outcomes.

Counterintuitive Response #1: Vomiting and Diarrhea in Hot Weather

Green porta potty line.
Photo: Getty Images

As anyone who has run or watched a hot weather race can probably attest to, vomiting and diarrhea do happen. “A good number of those cases are wrongly attributed to dehydration, when they’re more likely acute overhydration, combined with exertion,” Tucker stepped right into it, via email from his home in South Africa.

If you’ve got enough liquid to hurl, or poop firehose fashion, you’re not dehydrated, according to Tucker. Humans are designed to lose fluid over prolonged exercise without harm.

“On average, people doing endurance exercise will lose weight [technically dehydrated], and be perfectly fine,” Tucker said. “We know this because only about 0.8% to 1% of runners end up needing medical attention. But 80% to 95% of the runners lose weight. They are dehydrated with zero consequences. That’s because the body is not regulating weight, it’s regulating sodium concentration, and it’s quite happy for a short term increase in salt levels caused by dehydration, provided it’s not ridiculous, because we are able to regulate sodium in much more complex ways than just fluid.”

Clinical dehydration happens of course, Tucker says, just not in exercise scenarios, and certainly not during a race where there’s a drinks station every five kilometers. In fact, it’s those frequent drinks stations that may cause vomiting and diarrhea.

“It’s known that exercise can make us nauseous,” Tucker said. “That’s probably a result of the metabolic processes and changes that happen because of exercise — changes in pH levels, salt levels, and blood pressure being the main culprits. We send blood away from the gut during exercise, because our bodies are smart — they know we don’t need to absorb nutrients and fluids while we are exercising. It’s not a priority. So, when we force fluid on our guts, they just aren’t prepared or capable of dealing with it.” Ergo, rejection.

Just to muddy the water a bit, while pouring gels or liquids into a nauseous stomach is to blame, dehydration may be a factor in the nausea. Tucker explained.

“The exertion of exercise plus mild to moderate dehydration causes a rise in blood sodium levels, a condition called hypernatremia.  It’s important to note that hypernatremia is the normal response to exercise, particularly prolonged exercise. [Again, a condition that’s not dangerous unless the athlete went days completely without fluid, e.g. getting lost in the Sahara.] I raise this because people think that sweating a lot causes us to lose salt and thus, salt levels would decrease. But that doesn’t happen because, very importantly, our sweat has a lower sodium concentration than our plasma. So we can only ever drive the sodium content of our plasma up as a result of sweating. We are always going to lose relatively more water than salt as a result of sweating.  

“In any event, this hypernatremia might, in some people, contribute to nausea, that develops over the course of a marathon or long race. Then, when that person ingests too much fluid, say a single drink of 500ml or more, or the 40 fluid ounces that some people still foolishly recommend during exercise, the body, already nauseous thanks to the hypernatremia, rejects the fluid and they throw up. 

“That’s not dehydration causing the vomiting, per se, but it kind of contributes to it because the dehydration, which is not a bad thing, might cause hypernatremia which contributes to nausea, in which fluid can’t be tolerated.”

Whew. So, vomiting and diarrhea are not a response to dehydration, but rather, a response to over-hydration + exercise-induced nausea. “Our bodies are plenty smart. We’re foolish for trying to make them do something unnecessary.”

In other words, user error.

Counterintuitive Response #2: Training-Induced Insomnia

Woman unable to sleep looking at phone in bed.
Photo: Getty Images

Now for the second counterintuitive response: why is the inability to sleep a response to hard training? It seems that you would sleep more than usual to keep up with increased training. 

First off, Tucker explains there’s a difference between being fatigued and being sleepy. In the case of training-induced insomnia, your body is fatigued, but not sleepy.

“When we train hard, the signal that makes us ‘sleepier’ is the cytokines that are part of the inflammatory response to training. Sleepiness, however, is complex, and those cytokines are but one contributor. You might have that signal in abundance, but other things are overwhelming the sleep mechanisms. For instance, pain. Chronic pain especially, from the muscles and joints can prevent sleep. The main one, though, is the persistent heightening of the sympathetic nervous system, because we’ve trained ourselves to the point of being stuck in a persistent sympathetic response.”

Sympathetic response — increased heart rate, increased blood pressure, awash in cortisol, fight-or-flight preparation — is designed for short-term use. 

“The cortisol levels are high, we are trying to control inflammation, trying to restore the body to some kind of resting balanced state, and the prolonged stress of doing that is what then prevents sleep. In simplified terms, we’re over-aroused and can’t sleep.”

Sleepiness is the body’s answer to the damage that exercise incurs, but we can override this regulatory response. Tucker provided an anecdote from our knuckle-dragging ancestors to explain why this ability to override sleep signals came about.

“Let’s say we were migrating, or dealing with extreme weather situations plus predators, etc. We’d need to be alert and have readiness to survive pretty much all the time. Being excessively sleepy might have been disastrous. So we can shut off sleep for the sake of survival.

“Now our survival very rarely depends on our ability to be awake more, but the physiology underpinning it is the same. That sleep-deprivation capacity we had, mediated by the sympathetic nervous system, was never meant to be a prolonged one. It was a short duration, just until we could escape the situation. But things like persistent hard training keep us in that physiological state.”

So sleeplessness is your body’s way of indicating it’s blown past normal sleepiness, and is in survival mode, something you may not have realized. It’s a waving red flag. A precursor, Tucker said, to Force Quit: “At some point the body will begin to shut down or cut out the processes essential to life in an attempt to help us recover.” 

Again, user error.

Counterintuitive Response #3: Inflammation

Inflamed knee joint.
Photo: Getty Images

Finally, the third counterintuitive response — inflammation. Inflammation double dips in that it’s the body’s response to injury. But once present, florid and bloated and loud, it starts pressing on boundaries and causing friction and, in general, further bollixing up the situation it was originally called in to fix. Helpful? I think not.

Tucker sorts it out.

“So, inflammation is healing. As you know, inflammation involves a number of processes whose ultimate goal is to remove damage, then repair damage, then restore normal function. And it works beautifully most of the time. Even in animals, injuries are not fatal, because of inflammation. It’s effective and elegant 99% of the time, if not more.”  

Just so you get the message in no uncertain terms, Tucker said, after injury, chemicals that sensitize nerve endings flood into the site and then inflammation/swelling presses on the already sensitized nerve endings. Ouch. Ideally, you heed the pain, stop doing the thing that caused the injury, and inflammation moves on to phase II of the healing process.

“However, if we don’t respond, in either a behavioral sense, or at a metabolic level, and the inflammation is either excessive in intensity or duration, then the very things that help us begin to harm us. I always use the analogy of a city struck by an earthquake. The first thing that should be done afterward is to clear out the rubble, maybe knock down a few damaged buildings, in order to clear the way for reconstruction. But imagine something goes wrong and the wrecking balls and the dump trucks are too aggressive, or stay there too long. The earthquake damage [primary injury] is added to by the inflammation damage [secondary injury].

“If we continue to cause damage, the signal that brought those dump trucks and wrecking balls there in the first place is never removed. We keep signaling them to work, we keep the pressure on our immune systems to clear out the damage, and so we get persistent inflammation. Then a mild injury becomes a moderate one, then a severe one, like in the case of shin splints, or a muscle strain that starts out as a little twinge and ends up a chronic tendinopathy.”

In Summary 

And there you have it. The physiological programming, it turns out, is not haywire. There may not be an upside to vomiting or insomnia or shin splints. They are indeed making a bad situation worse, and that’s to purpose. Because some ninny has not read the user manual.

Tucker defends the physiological programming: “We have regulation of normal physiology, even for very stressful situations — prolonged exercise without fluid, acute or chronic injury, or very hard exercise that makes us hungry for sleep. But we kind of force things on ourselves — drinking more than needed, or training despite lack of sleep, or continuing to train despite injury and pain — and responses designed to help us instead cause problems. Our stupidity, ironically, turns the body’s ‘cleverness’ into a disadvantage.”