The Hiccup Mystery: Exploring the Scientific Reasons Behind an Annoying Reflex

Weird fun fact about your author - I am exceptionally prone to hiccups. I’m talking about daily, unwarranted, random hiccups that plague me for a couple of minutes at a time, and it’s been like that since I was young. It’s become a weird joke amongst friends - I hiccup on a schedule. To make it worse, for some reason, the other folks around me don’t ever hiccup! Frustrated, and sick of that nasty twitchy feeling in my chest, I set to the internet to figure out what on earth is up with my unfortunate, glitchy diaphragm.

To finally discover the reason for this often-occurring affliction, let’s dive into the science of hiccups.

What Happens When You Hiccup?

Hiccups are caused by an involuntary spasm in your diaphragm, that sensation is more than apparent whenever it is occurring. It’s sharp and sudden, and occasionally painful directly below the breastbone - making it a very acute way to sense where one’s diaphragm is placed. The muscle twitch causes your vocal cords to briefly close mere milliseconds after you feel it in your diaphragm, which produces a “hic” sound as air is rapidly passed through it.

But, what causes our body to do such a strange action? This odd little symptom pops up for a wide variety of causes. From stress to too much alcohol, to chewing gum, and in more serious cases, from tumors and metabolic issues, to nervous system disorders, hiccups happen commonly and, often, unreliably at the most inopportune moments.

Though nearly everyone experiences them, scientists and doctors know very little about them. Medical researchers do know that several irritations can trigger the symptom, namely, vagus and phrenic nerve stimulation, stomach distention, autonomic nervous system overstimulation, or irritation of the diaphragm itself can lead to it, but the precise mechanism is still not grasped.

Why Do Humans Hiccup, Then?

As you’ve likely surmised by now, we don’t know why it happens - or even what the purpose of hiccups is. The concept remains one of science’s many extant mysteries in every facet. Usually, an understanding of mechanisms at play leads to a better guess at the “why” part. So, the fact that we haven’t even unearthed that portion is telling.

We do know, however, that hiccups happen most frequently in utero, and most humans decrease their average frequency of hiccups as they grow into adulthood. This fact can potentially hint at a potential answer.

We can start our investigation then, with the fetus. The diaphragm completes its formation by week 10 of pregnancy. But, notably, fetal hiccups begin near the end of the second trimester (10 whole weeks after the diaphragm is done forming). This fact lines up neatly with some potential causes, as we can see what else is forming when hiccups begin. Baby’s first hiccup happens at the same time as several key developments - the brain begins developing the ability to sense, the lungs finish developing, and the body starts moving in a coordinated way in the weeks following the first hiccups. The first instance of diaphragm spasming in humans happens when the lungs are practicing movement (without air) and the nervous system is being harnessed!

Based on this evidence, you’ll likely be able to guess some of the more popular ideas:

1. It’s a passive body function administered by your breathing muscles to ensure that your system is functioning well or to return it to homeostasis. (Science Direct)
2. It’s a weird human glitch that’s an evolutionary remnant of a past ancestor. (NIH)
3. It’s a form of fetuses strengthening the muscles involved in respiration that we must endure as adults despite no use. (CNN)
4. It’s a method to remove air from the stomach rapidly. (NIH)
5. It’s a reflex to move food trapped in the esophagus. (Healthline)

Of these, two are currently standing out in the scientific community as the closest approximations of truth: #2 and #4, due to a significant amount of growing evidence respectively: a wide collection of autonomic nervous system stimuli causing the action without any apparent benefit, and a good portion of organs responding as if the reflex was intended to affect the stomach.

But neither of these theories is perfect. The air removal theory doesn’t quite explain why fetuses would begin this function before the air is in their stomachs, and the full-glitch theory doesn’t properly account for some of the recorded benefits of instances of hiccups (e.g. counteracting overdistention).

Do Other Animals Hiccup Too?

After meeting a dead-end in the world of human development, I broadened my search to the animal kingdom and discovered that most mammals get hiccups - it seems to be an unfortunate side effect of having a diaphragm.

Reptiles, amphibians, and birds, consequently do not get hiccups - as diaphragms are a mammal-specific organ. But, there are documented cases of a wide swath of mammals getting hiccups - and even more anecdotes of various mammalian pets and creatures spasm-ing in the classic hiccup way.

Since hiccups are exclusive to us lucky mammals, we can postulate potential reasons for their purpose with common traits of our in-group. This is a key puzzle piece in the mystery, as it significantly narrows down potential causes. We can narrow our search to two sectors: what makes mammalian lungs special, and what makes their stomachs special.

Let’s focus, then, on what makes our lungs special. They move because of our dear friend, the hiccup-causing diaphragm. Whereas reptiles move the muscles around the jaw and lungs to take in air, birds use bellow-like organs to pull in air from the atmosphere through a one-way, looping route, and insects rely on tubes and air sacs all about their thoraxes and abdomens that connect about their body, mammals are special in their ability to use an organ to pull air back and forth into our system. If this is indeed the only important system to the purpose of the diaphragm, it may very well be that this structure has the unfortunate side-effect of the hiccup glitch.

There isn’t quite as much of a striking physical difference between animal digestive systems, however. True, there are indeed differing chambers along the way, and some species employ chambers of mechanical digestion along with the chemical sort we know and love, but they all mostly sort of work the same way, as long as there’s an input and output tract (Food goes in, gets ground and fermented, gets absorbed, then gets expelled). But, it may be that the key difference in this system isn’t exactly visually obvious - it could very well be a result of gut bacteria content. As of writing this, the research on the human gut microbiome is ongoing, and there’s much complexity still to unravel. I surmise that, if the hiccup reflex has any functional use for the stomach, this exciting sub-field of medical research may help lead us to an answer.

Why Haven’t We Figured Out Hiccups Yet?

Despite a lack of a full understanding of this part of human anatomy, there isn’t as much active interest in pursuing this in medical or scientific studies as one might expect. In short, it’s a difficult field to get involved with. Part of the difficulty in these studies is that people cannot hiccup on command (Though I must admit, I’ve been hiccupping much more this week after thinking about it so much). As most bouts of hiccups are random, short-lived, and unreliable, studies are strikingly hard to administer on them, except on the unfortunate folks who have long-term hiccups. As such, much of the research done has been biased towards individuals with chronic hiccups, which is most certainly not representative of the vast majority of hiccups that occur benignly on any given day.

To truly begin to understand the purpose of hiccups, we must first obtain a better understanding of the muscles and organs, digestive bacteria, and how to better manipulate both of them. Should we unearth the methodology to artificially cause hiccups, exploring such questions will become much more practical and feasible. Until then, we might be stuck in postulation. It’s, frankly, remarkable how well scientists have been able to narrow down likely causes with the limited data that’s available on the subject.

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Hiccups are still a mystery, but we’re just a couple of breakthroughs away from understanding them deeply enough to know their purpose - if they do indeed have one. In the coming years, our impending domino of medical breakthroughs may also affect this sillier subsection of medical science. No matter the results, I’m certain the answer will be rather fascinating.

As it stands, hiccups are an excellent example of how much there is still to discover in the world of science and how much there is still to learn about us and the way our bodies work. There is still so much understanding to unearth in this strange, collective experience we’re sharing.