The old saying “trust your gut” is often meant as a reminder to trust our inner intuition. The rumbling discomfort in our stomach right before a big interview or the loss of appetite after watching a gory scene on TV are all too familiar “gut feelings.” However, these sensations are not mere intuition, but rather the result of precise communication between the brain and gut through the autonomic nervous system (ANS). Through the use of different chemical signals, this channel maintains the bidirectional communication of the gut-brain axis. This creates a direct link for emotions or feelings, which originate in specific regions of the brain, to manifest as physiological “gut feelings” in the gastrointestinal (GI) tract. On a molecular level, this occurs through neural, hormone, fluid, or immune signals. Faulty communication along the gut-brain axis, however, can lead to GI disorders linked to cognitive stress, such as irritable bowel syndrome or inflammatory bowel disease. Understanding this connection might be key to finding a way to tackle and develop treatments for GI disorders by addressing both players in the game: the brain and gut.
These symptoms are likely mediated through the function of the ANS, which deals with the involuntary movements in the body including regulating heartbeat and digestion. It plays an integral role in maintaining homeostasis, or the internal balance of our various organ systems and functions. To do so, messages sent by the brain can activate two specific components of the ANS: the parasympathetic and sympathetic nervous system. The sympathetic system is involved in activating “fight or flight” responses whereas the parasympathetic system is for “rest and digest.” The opposite nature of these two ANS divisions is what allows the body to together maintain homeostasis. A “fight or flight” response system can be triggered in response to not only perceived physical threats or emergencies, but also psychological stressors as well. Considered an acute stress response, a recent study has identified that gut motility, or the internal muscle movement of food through the GI, can be inhibited when Glucagon, a stress hormone, is released. During a fight or flight response, the body prioritizes survival and will divert resources to select bodily functions. This can occur even if the situation isn’t life threatening, such as a job interview or family difficulties. The stress and negative emotions an individual may experience with these events are enough to trigger a stress response in the body. In contrast, the “rest and digest” is the metaphorical brake pedal of the body, with the purpose of conserving the body’s energy consumption. When an individual feels relaxed, there is increased motility and release of gastric liquids in the GI tract to facilitate the digestion of food and promote hunger sensations. Thus, the overlap behind the concept of hunger, the gut, and emotions highlights the psyche’s power to disturb the body’s internal balance.
Despite the integral functions of the ANS and its potential link to gut disorders, there is limited research focusing on the link between emotions and GI responses. Existing literature often only focuses on the gut microbiota and the role it plays in the gut-brain axis. This gap in the scientific literature drove UC San Diego undergraduate student researcher Suchita Rao to devise her own experiment. She proposed the following research question: To what extent do our emotions affect hunger levels and how are our brain and gut intertwined?
Unlike most studies in the field, Rao’s approach was unique as it involved stimulation of emotions via videos to produce a physiological response. Participants were first asked to identify what makes them feel relaxed, pleasure (i.e. happiness), and discomfort via a questionnaire. Rao then prepared ten minute videos for each participant, based on their answers, to evoke these particular emotions in participants. For example, a video of slithering snakes and jumpscares might evoke discomfort in a participant who identified snakes as a source of fear. The aim was to produce a strong emotional response in the participants that could be recorded by resulting ANS activity.
To monitor the activation of the ANS and the gut, Rao measured heart activity through a respiration belt and gut activity through an electrogastrogram (EGG) belt. She specifically recorded the heart rate variability (HRV), which are the fluctuations in time between consecutive heartbeats. This variable can give a direct and objective measure to activation of the sympathetic or parasympathetic activity. A lower measurement is interpreted as sympathetic stress driven “fight or flight” activity because heartbeats are more rapid and frequent while a higher HRV would indicate more time in between heartbeats and the relaxed “rest and digest” parasympathetic response. A respiration belt records HRV by measuring the changes in heart rate following inhales and exhales. This measurement worked in conjunction with her second measurement tool, an EGG. An EGG, placed in the stomach region, can measure gut activity through electrical signals reflecting the gut’s muscle movements as it moves food through the GI track. A stronger EGG measurement means greater gut muscle to indicate the digestion of foods through the GI track. Therefore, a stronger EGG reading would mean greater muscle movement of the gut and higher hunger sensations and vice versa. Collecting these two data points, while the participants experience specific emotions, can give direct measures of ANS activity as it relates to hunger levels. Together, the measurements serve to not only confirm which of the two systems of the ANS is activated, but to show how hunger levels are impacted by different emotions.
Prior to the experiment, Rao had the participants fast for 12-13 hours and not drink water the morning before the data collection period to obtain an accurate baseline gut activity for each participant. She additionally had all participants listen to white noise for ten minutes to lower their heart rates to another baseline she could collect. Obtaining a baseline was an important point of comparison to determine if the participants in Rao’s experiment experienced any changes in gut activity associated with HRV-measured emotional changes she evoked. This step is crucial to ensure that Rao could ascertain if the changes she observed in gut activity was due to the experiment’s influence on the participants’ emotional state and not prior digestion. A 10 minute video of white noise was used to calm participants to obtain accurate baselines. To invoke the three emotions, as mentioned above, participants were shown 10 minute emotion stimulating videos followed by 2 minute rest periods. This process of video and rest periods was repeated for the rest of the emotions.
To analyze the collected data, Rao compared the EGG and HRV by plotting the readings taken every 60 seconds while watching the video to analyze potential trends. To determine the average EGG reading, which is also referred to as EGG power, Rao looked at the highest power reading at each time point. She found that her study’s results aligned with her initial predictions: pleasure emotions, where participants felt no stress or anxiety, had the greatest EGG power. This finding is also consistent with the “rest and digest” functions of the parasympathetic nervous system, suggesting its activation when experiencing pleasure emotions. The opposite was observed for the fear and discomfort videos, where low EGG power reflects the activation of the sympathetic nervous system in the “fight or flight” response. The average heart rates were relatively low for all three focus emotions. However, pleasure emotions had greater heart rate variability compared to discomfort and fear. In particular, the activation of the sympathetic division of the ANS caused the heart rate variability to drop. The more fear the participant felt, the lower their HRV dropped below the average baseline that was taken prior to the experiment. This data measurement further confirms which autonomic response was activated per each emotion and its corresponding effect on gut activity.
The results of Rao’s research demonstrate the influence that our emotions can have on our GI activity and heart rate. The feelings we experience are not simply fleeting, but can leave footprints that result in physiological changes in our bodies. In the future, Rao hopes to expand this study to include additional emotions that she did not have a chance to explore here. Expanding the scope of emotions included in the study can help to gain a better understanding of how different emotion categories or trends may contribute to gastrointestinal conditions like IBS. A comprehensive review of the physiological changes that impact the gut, heart rate, and hunger sensations could help to explain the link between gut disorders and ANS based response to emotions.
Trusting our gut may not be as unexplainable as many of us believe. That loss of appetite or “funny” feeling in our stomach may be the result of the gut-brain connection. Our physical wellbeing remains a complex issue. It is no longer just a task that involves going to the gym and getting your steps in, but also can be influenced by our inner feelings or emotions. When it comes to treating gut-brain disorders, better understanding this connection can shape future treatments that take the mind into account. So, the next time you experience a loss of appetite when making decisions, try to listen and trust your gut.