The biology of gut feelings

Imagine you go to buy a new winter jacket. You’re on a budget and have already made a few decisions from an online search: the jacket must have a certain amount of padding, no silly drawstrings around the waist and preferably good inner pockets. You find jackets that meet all the criteria but can’t settle on any of them. Then, you try on a jacket that covers none of your conditions and buy it on the spot, because it just feels right, almost without making a conscious decision. You’ll probably be happy with the jacket. Your lifelong experience with wearing a winter jacket consists of large amounts of sensory information (how it feels over the shoulders, how easily your arms enter the sleeves, whether it gives you a silhouette you associate with feeling attractive, how it smells, how well its pocket depth suits your arms and hands) that combined are more crucial in making a good decision than those few conscious criteria.

You have made the decision based on a gut feeling. In situations like this one, gut feelings can lead to better decisions than cognitive analysis. Subconscious recognition can also, for example, help us judge a social situation as dangerous or threatening and make us flee, without really being able to explain why.

I’m not sure I’d go as far as claiming that gut feelings are trendy, or that the sixth sense is going through a renaissance – I get it, it sounds like saying that ears are in this year, or that upper arms are cooler than forearms. But there’s reason to believe that the concept of gut feelings is gaining traction and being demystified. In such cases people speak of precognition and intuition rather than gut feelings (and much less about a sixth sense). In the Swedish Radio show Kropp och själ (2021), cognition scholar Paul Hemerén describes precognition as subconscious perception that enables us to draw certain conclusions and make certain decisions in a split-second, without a proper thought process or conscious evaluation of the situation.

C. elegans, image by Heiti Paves via Wikimedia commons.

That precognition feeling

Experiments conducted on gut feelings, or decisions based on intuition rather than analysis, tend to find that prior experience is crucial to our ability to generate a decision equal to or better than one based on analysis. Organizational behaviour scholar Erik Dane conducted an experiment in 2012, whereby test subjects were tasked with discerning a real designer handbag from a well-made replica. Half the subjects were asked to analyze the bags thoroughly, half were instructed to ‘go with their first impression’. In another experiment from 2018, business administration scholar Vinod Vincent instructed test subjects to recruit the right person for a job position from a pool of applicants, half by thoroughly examining CVs and references, and half by making a quick decision based on their gut feeling. In both experiments, gut feelings generated better decisions than analysis for test subjects with extensive experience and knowledge of handbags or staff recruitment. However, for subjects who were novices and laymen within their respective fields, gut feelings returned worse results than analysis.

This demystified understanding of gut feelings as forms of subconscious, experience-based perception that lead to a type of precognition seems to be generally accepted today. But research barely touches on how precognition feels, or through what sort of mechanism it is expressed. Fiction provides examples, however. The two most well-known are Spider-Man’s spider-sense and Harry Potter’s scar. Spider-Man’s precognition has been described in detail throughout the years: it’s a prickling, tingling sensation the superhero seems to register through his skin – illustrated in comic books as wavy lines in the air surrounding him – that warns him of danger and enables him to see around corners and fight in the dark. Harry Potter’s scar similarly works as a kind of portal for precognition: it throbs and stings, functioning primarily as a Voldemort-alarm but is sometimes also activated from afar, and, more generally, when Potter’s enemies triumph or his friends suffer losses. Both examples describe precognition as a physical sensation expressed through the body.

We can assume that Spider-Man’s and Harry Potter’s creators made an aesthetic choice by not ascribing their precognition to the gut. It would have been a little ridiculous and intrusive to constantly reference their bowels, and difficult to avoid the final bathroom situation. But the connection of our guts to feelings is undisputed and recurring. We get ‘worried sick’ and ‘scared shitless’, get ‘butterflies in our stomach’ from love and stomachaches from anticipatory anxiety or acute longing. We vomit from horror, a shock separation or a panic attack. The covariation of IBS (irritable bowel syndrome) and depression or anxiety is well established in epidemiology, even if causality in which direction is, as yet, far from proven: whether stomach issues lead to depression, or depression leads to stomach issues, or depressed people just interpret regular gut issues as unhealthy, hasn’t been determined.

Serotonin of the gut

If you investigate the connections between cognition and sensory phenomena, particularly between the brain and the gut, it isn’t long before you come upon a famed little molecule: serotonin. Serotonin is the celebrity of neurotransmitters, well known to the public due to the breakthrough and widespread use of SSRIs (Selective Serotonin Reuptake Inhibitors). The first SSRI antidepressant of note, launched in 1988 under the name Prozac, was immortalized in Elizabeth Wurtzel’s best-selling autobiography, Prozac Nation. 

Approximately one in ten Swedes takes some sort of antidepressant, of which SSRIs are the most common1. The pills don’t contain serotonin but increase the effect of our body’s own serotonin by blocking the mechanisms that limit its impact duration. Cells interpret this as more serotonin. If there’s been skepticism towards ‘happy pills’ historically, serotonin has now established itself as exceptional for our wellbeing. It has been assigned such scientific radiance that we can buy beautiful silver jewelry in the shape of its structural formula.

What few laymen know, and what boggled me when I first learned about it in medical school, is that serotonin isn’t primarily a neurotransmitter of the brain but of the gut. Over 90% of our body’s serotonin can be found in our digestive system, and only a small fraction of it sloshes about in our brain. We don’t know exactly what all that serotonin is doing in our digestive system. So many complex, variable and sometimes contradictory functions have been scientifically proven that it’s difficult to discern which is the dominant one. So, biological scientists do what they usually do in the face of a perplexing mess: turn to a simpler organism – preferably a far, far simpler organism – and hope that clears things up. Serotonin is an ancient and widespread molecule, well preserved within evolutionary development. It exists in almost all living things, even in plants (many seeds, fruits and nuts contain relatively high concentrations of serotonin, the purpose of which is thought to be speeding up digestion in animals).

A famed nematode

The simple model organism that biologists use is the nematode, or roundworm, Caenorhabditis elegans. It’s a millimeter long, transparent and lives underground where it feeds primarily on bacteria from decomposition. You could say that C. elegans has no secrets left: all of its cells have been counted and described in detail. C. elegans was the first multicellular organism whose entire genome was sequenced, as early as 1998. Present-day knowledge of apoptosis, programmed cell death, stems largely from C. elegans research. The nematode has even been to space, multiple times. The primary scientific purpose of the space trips was to research zero gravity and its effects on muscle cells and aging. But what’s remembered most is that C. elegans survived the 2003 Columbia disaster when a space shuttle disintegrated on reentry and seven astronauts died.

The nematode’s contribution to molecular biology and to our understanding of life itself is almost immeasurable. It inspired Linda Gregerson to write Elegant, a kind of tributary poem to C. elegans. The poem’s simultaneous flow and complexity make it difficult to quote; any excerpt has an unavoidable amputated feel to it. But, since this description of apoptosis is so exceptionally distinct and beautiful, I must try:

its thousand and ninety invariant
cells of which
131 are always
the same
and always in a particular sequence are programmed
for extinction
[…]
Found
that death was not an afterthought. The genome
is a river too. And simpler, far
more elegant, to
keep the single system and discard the extra cells
it spawns.2

The little nematode has no brain. It has a small transparent body consisting of intersecting muscles that, through intricate cooperation, drive the worm forward as it wiggles around in dirt piles, decomposed forest and compost all over the planet. It searches for food, we say. But what does ‘search’ mean in this context? Its lack of brain, cerebral cortex and anything we might call eyes makes its search different than the fieldfare’s search for a buzzing mosquito or the wasp’s search for sweet windfallen fruit. It has no vision to identify food and no memory bank to recognize nutrient-rich environments. The reason why C. elegans doesn’t starve to death and seems able to distinguish and move between nutrient-poor and nutrient-rich microenvironments is likely down to none other than serotonin. The nematode’s muscular forward motion is seemingly constant but varies in intensity, enabling it to move at different speeds and slow its body down. This is crucial to its survival. It’s been proven multiple times, by biologist Elizabeth Sawin among others, that the presence of bacteria, the nematode’s main source of food, increases the production of serotonin in its digestive system. This, in turn, sends a signal leading to decreased muscle activity and a relative slow-down. The opposite is also true: an environment scarce in bacteria leads to an increase in body movement and speed.

As a result, C. elegans spends more time in microenvironments rich in bacteria, seemingly moving away from nutrient-poor to nutrient-rich environments. It does so without the ability to see, remember, or, as far as we know, willingly control its muscular activity. At the same time, this kind of livelihood cannot be described as fully passive or random. It uses a specific and very targeted mechanism, which is the result of behavioral responses to a certain stimulus (the presence of bacteria). Put in a way that molecular biologists tend to hate, the effects of this serotonin-transmitted signal could be described as: high serotonin = full stomach, sweet environment, we’re chilling, let’s stick around; low serotonin = empty stomach, poor environment, we have no future here, hurry up, migrate, migrate. 

An instruction like this is no little footnote in life but rather its lowest common denominator, the opening paragraph in its most essential handbook. To simplify the instruction even further and add to the indignation of patient molecular biologists who have mapped out thousands of receptors and ion channels to understand mechanisms like these, one could put it as: I like it here – stay; I don’t like it here – go. It doesn’t seem unreasonable to assume that this is the first gut feeling, primitive yet essential, the embryo of the complex precognition discussed and researched today. In contrast to the nematode’s gut feeling, human precognition is probably mediated by hundreds or thousands of biological relays and modulators, yet seemingly still conveys, like Harry Potter’s scar, the simple message: I don’t like it here – go. (There are those who might think, ‘but Harry Potter rarely leaves, more often he fights’. This is true. It is likely here that the complexity of our human cerebral cortex, as described in children’s literature, takes over and modifies the nematode’s simple go! signal into a plethora of possible outcomes.)

Arrested reactions

At this point we can no longer avoid the question of whether SSRIs, used by so many people, affect our gut feelings. Alert readers might have already asked themselves whether someone thought of feeding Prozac to C. elegans to see what happens. Biologist Elizabeth Sawin at MIT actually did ‘treat’ C. elegans with Prozac (fluoxetine) and the results were as expected: in the presence of bacteria, the starving nematode moved significantly slower with Prozac than without it (i.e., the ‘I like it here – stay’ signal was amplified).

It is not without reservations that I return to the SSRI example. It’s deceptively easy to contribute to disingenuous critiques of SSRIs by overapplying what we know about C. elegans with humans. Such reasoning assumes that antidepressants not only amplify the ‘I like it here – stay’ signal but also risks weakening the ‘I don’t like it here – go’ signal. Meaning that they silence signals imploring us to leave adverse, poor or even dangerous and destructive environments. From there it is no far stretch to see SSRIs as drugs of contentedness and inertia that, for example, lower our motivation to leave destructive relationships and generally increase our tolerance for bad circumstances, ultimately fiddling with our oldest and most important survival signal.

It makes sense to investigate the impacts of SSRIs on society. Skepticism and mistrust are reasonable reactions to a relatively new type of medication prescribed to such a large part of the population. But the assumption that SSRIs silence fundamental survival signals is incorrect for two reasons. First, it is impossible to directly transfer biological conclusions from a nematode with 302 nerve cells to a human with almost 100 billion nerve cells, all with significant variations and subcategories. That would be like drawing conclusions about the ocean based on a street puddle. Secondly, and much more importantly: readers of Sawin’s article will note that starving nematodes placed in completely nutrient-poor environments, without bacteria, weren’t at all affected by Prozac. They squirmed and wriggled just as much as the control group that wasn’t given Prozac. They had no serotonin signaling whatsoever, and since SSRIs don’t add any serotonin, the treatment didn’t affect behavior in extremely meager environments. The ‘I don’t like it here – go’ signal seems to work even under the influence of SSRIs, at least when circumstances are quite bad. In the end, we don’t even need to get into the prominent studies indicating that SSRIs have saved many lives (by decreasing the risk of suicide among depressed people), or the numerous testimonies from people describing how SSRIs helped them, to argue that biologically deterministic critiques of SSRIs lack merit.

Trust your experience

If you enter ‘gut feelings’ into a search engine, it’s likely to generate results about trusting your gut, or sometimes on how to ‘stop overthinking’. Our contemporary political climate is increasingly distrustful towards science and its claim on truth. So-called climate denial is one of many examples. That the US recently had a president who speculated live on air about drinking bleach to combat COVID-19 is another one, so grotesque it feels almost ludicrous to mention. More common in the Swedish political reality is assigning disproportionate weight to individual sentiments towards societal phenomena such as ‘perceived safety’.

In contemporary politics ‘low perceived safety’ is essentially considered as serious as an actual lack of safety. However, groups across the board who are statistically least at risk of violence (e.g., older women living outside big cities) are the most scared. It would nevertheless be political suicide – unlike 20 years ago – to try and calm those who perceive danger by arguing that their fear lacks reason. Today, ‘low perceived safety’ is used as justification to hire more security guards and other expensive safety personnel. Our society tells us, without major disclaimers, ‘trust your gut’. While no one says, ‘don’t trust official statistics’ – that’s the implied next step following ‘trust your gut’. As Dane’s and Vincent’s experiments show, the key disclaimer here should be ‘…if you have extensive experience in the matter’. If the tendency to privilege gut feelings over science continues without such an addition, we are heading toward an era of poor societal decisions.