The Second Brain: How the Nervous System Affects Digestion

Listen, something I don’t think we talk about enough on this platform is digestion - and it’s not for lack of desire to talk about it! Obviously all dietitians are trained in digestive function and medical nutrition therapy for concerns regarding digestion, but not all of us develop special focus in this area (although given how much work the gut does for us, it’s hard not to have some sort of daily discussion around it). So let’s expand on this, and talk about an area I have been loving working in lately - neurogastroenterology!

Neurogastroenterology is the study of the way that the nervous system and the digestive system work together and influence each other. Digestion is often thought of as a purely mechanical or chemical process—food goes in, enzymes break it down, nutrients are absorbed. But behind the scenes, the nervous system plays a powerful and sometimes misunderstood role.

You have probably heard the term “gut-brain axis”... what does this mean? This is a two-way communication network connecting the brain, spinal cord, and the enteric nervous system (ENS) - your digestive systems’ own nervous system! We call it the “second brain” because the ENS is equivalent in the number of neurons to the spinal cord! And a significant portion of the neurotransmitter serotonin is produced in the digestive system, which can then be sent back to the central nervous system (CNS) for use.

Digestion is primarily regulated by the autonomic nervous system (think “automatic” nervous system, these functions happen without thinking about them!), which has two main branches: the sympathetic and parasympathetic nervous systems. The parasympathetic nervous system, often called the “rest and digest” system, promotes sleep, sexual activity and reproduction, and digestive function. When this system is active, it increases saliva production, stimulates stomach acid and digestive enzymes, enhances intestinal movement (called peristalsis), and improves nutrient absorption. This state is most active when you feel calm, safe, and relaxed.

On the other hand, the sympathetic nervous system, the “fight or flight” response (which is weird because we think “sympathetic” would mean relaxed or calm, but NO), suppresses digestion and other processes. When the body perceives stress or danger, blood flow is diverted away from the digestive organs toward muscles and the brain - it wants to get us out of danger quickly by preparing the sensory organs and the musculoskeletal system. We often feel our heart racing, our limbs tense and shaky, and our vision and hearing slightly enhanced to the environment around us. As a result, digestive secretions decrease, gut motility slows or becomes irregular, and absorption can be impaired. While this response is useful for short-term survival, chronic activation can disrupt digestion.

Chronic stress keeps the nervous system locked in a “sympathetic-dominant” state, which can contribute to a range of digestive issues. Conditions such as irritable bowel syndrome (IBS), acid reflux, bloating, constipation, and diarrhea are often closely linked to nervous system dysregulation. Stress can also increase gut sensitivity, making normal digestive sensations feel painful or uncomfortable and often confuse people about food sensitivities. It’s important to know that being stressed or anxious is not necessarily the REASON you have digestive issues (affirming this for folks in sessions can be so helpful, especially if their medical team has been dismissive or even engages in gaslighting), but it can be an influence on how severe and persistent digestive symptoms are. It can also explain why some foods tolerate poorly on one day, and totally fine the next - I’ve often heard folks describe it as “my food intolerances are always changing and I don’t know why”. I do!

One of the most important nerves involved in digestion is the vagus nerve, a major component of the parasympathetic nervous system. It’s a rest and digest pathway, sending signals from the brain to the digestive organs, and telling them when to produce enzymes, contract muscles, and relax sphincters. A vagus nerve that is working well supports digestion with minimal discomfort (as an example, being too full will still make us feel unwell, even if we’re relaxed). A vagus nerve with poor “tone” (meaning it’s not being used well) impairs digestive signaling and can lead to sluggish digestion, nausea and feeling full too quickly with eating small amounts. Factors like trauma, chronic stress, or significant fatigue can be contributors to poor vagal tone.

Because digestion is so closely tied to the nervous system, improving digestive health often requires more than dietary changes alone (which can be frustrating for individuals, relieving or a little bit of both). We are taught that we are totally responsible for how our body feels, so of course we feel discouraged when food isn’t the only thing we need to modify to feel better. Practices that regulate the nervous system can have a profound effect, such as eating more slowly, chewing thoroughly, and avoiding distractions as appropriate during meals; for folks with concerns like ADHD and ASD, or who are in eating disorder recovery, distractions may actually be helpful to reduce the impact of stress or overstimulation on digestion. Deep breathing, meditation, gentle movement, and adequate sleep also support nervous system balance. Think about nervous system regulation as both an opportunity to calm the body and brain, and allow functions that don’t trigger the fight-or-flight response (which may not be calming but can feel really good, like running, dancing, singing and more!)

Ultimately, digestion works best when the body feels safe - living in the wild with no food or not enough food sends messages of danger to a “caveperson” brain, and having plenty of food sends messages of safety. By recognizing the importance of the nervous system, and the unique relationship it has with the digestive system (instead of only one or the other), we can create the most optimal internal conditions necessary for efficient digestion, better nutrient absorption, and a more comfortable relationship with food!