How does the gut's embryological development proceed?

During early development, the body forms a flat disc that undergoes head-to-toe and side-to-side folding, forming a tube that becomes the GI tract. This process, embryogenesis, occurs in the first trimester. The fetal period (second and third trimesters) then models these organs, making them functional enough to support a baby at birth. Outgrowths from this tube also form organs like the pancreas, gallbladder, lungs, and liver.

How is the liver connected to the gastrointestinal system?

The liver receives a unique venous blood supply from the gut (portal system), which breaks into capillaries within the liver (sinusoids). This allows the liver the first opportunity to metabolize absorbed nutrients from the gut. It also receives a separate arterial supply (hepatic artery) for its own oxygen needs.

Why is the enteric nervous system called 'the second brain'?

The enteric nervous system (ENS) is capable of functioning and controlling reflexes and behavior independently of the brain or spinal cord. No other nervous system outside the central nervous system (CNS) can do this. While it communicates bidirectionally with the CNS, it handles the detailed, local control of gut functions.

What are the common causes of the pain experienced from bowel distension?

The gut primarily feels pain in response to dilatation or pressure, not cutting. Most of these pain signals are transmitted via fibers in the dorsal root ganglia to the brain through the spinal cord. The vagus nerve carries some pain signals but is mostly involved in internal body balance (homeostasis).

How does the brain communicate stress and emotion to the gut?

The central nervous system acts like a CEO, transmitting general commands to the gut, such as those related to stress. For instance, anxiety can lead to tension-induced diarrhea, while fright can cause constipation. These CNS signals modulate the enteric nervous system, leading to changes in gut motility like more rapid peristalsis or slowed movement.

What role do hormones play in appetite regulation?

Appetite is regulated by several factors, including stomach distension and the rise of blood glucose (not just its absolute level). Enteroendocrine cells in the gut produce hormones like cholecystokinin (CCK) and leptin, which signal satiety to the brain. Conversely, ghrelin, another gut hormone, promotes food absorption and eating.

How do protein and fat affect gastric emptying and satiety compared to carbohydrates?

The stomach empties at different rates depending on food content. Protein and especially fat slow gastric emptying significantly more than saline, salt, or sugar. This slower emptying prolongs stomach distension, leading to earlier and more sustained satiety, reducing the desire to eat.

What is the connection between Varicella zoster virus, shingles, and gut health?

After chickenpox, the Varicella zoster virus remains latent in neurons throughout the body, including the enteric nervous system. When it reactivates as shingles, it can damage these nerve cells, leading to severe gastrointestinal issues like pseudo-obstruction (gut paralysis) or achalasia (esophagus inability to open), and even gut perforation. The Shingrix vaccine is highly effective at preventing reactivation.

What is the link between autism spectrum disorder (ASD) and gastrointestinal problems?

Children with ASD often experience a higher prevalence of GI issues, such as constipation, diarrhea, and food sensitivities. Research suggests a disorder of the nervous system within the gut contributes to these problems, rather than leaky gut. Deficiencies or excesses in serotonin signaling due to genetic mutations or prenatal SSRI exposure in mouse models show autism-like features and gut defects.

What is the actual role of serotonin in the gut?

About 95% of the body's serotonin is produced in the gut by enteroendocrine cells. It acts as both a neurotransmitter in the enteric nervous system and an endocrine substance. Serotonin is critical for gut pain, triggering nausea, acting as a growth factor for developing nerve cells in the gut, and interacting with the immune system to trigger inflammation.

What is 'leaky gut' and what causes it?

Leaky gut refers to increased permeability of the intestinal lining, allowing unwanted materials from the lumen (like toxins or microorganisms) to enter the body, and body proteins to leak into the lumen. Tight junctions between epithelial cells normally prevent this. The most common cause of leaky gut is allergy, where immune reactions can open these tight junctions.

How does the gut epithelium replace itself without compromising integrity?

The gut lining is highly folded with villi (projections) and crypts (clefts). Stem cells at the base of the crypts continuously generate new epithelial cells. These new cells migrate up the crypt and villus, pushing older cells towards the tip, where they undergo programmed cell death (apoptosis) and are sloughed off. This ensures constant turnover while maintaining tight junction integrity.

Are commercial gut microbiome tests reliable or actionable?

Commercial gut microbiome tests are generally not very useful or actionable. While they can identify classes and proportions of organisms, they lack the fine species-by-species resolution needed for meaningful clinical application. Many such tests are also paired with unproven supplements or probiotics, generating significant revenue without documented impact, with some companies even being fraudulent.

What is the best nutritional advice for maintaining optimal gut flora?

A balanced diet, including adequate fiber and nutrients like vitamin C (without excess), is generally recommended. Fiber, such as that found in celery or lettuce, is not digested by humans but is metabolized by gut bacteria, producing short-chain fatty acids that provide energy for colonic epithelial cells and absorb toxins. Avoiding "cleanses" is also recommended as they disrupt the natural symbiotic relationship with the microbiome.

What can be done to restore gut flora after antibiotics?

Taking appropriate probiotics during or after a course of antibiotics can be helpful. 'Appropriate' implies organisms (like specific lactobacilli, bifidobacteria, or yeast-derived Saccharomyces) that can survive stomach acid, transit the GI tract, and effectively colonize. It's crucial to rely on products with claims backed by evidence, as regulated by bodies like the FTC and FDA, rather than unregulated supplements.

Key Moments

215 - The gut-brain connection | Michael Gershon, M.D.

Peter Attia MD

Science & Technology4 min read133 min video

Jul 25, 2022|56,335 views|1,096|92

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Key Moments

TL;DR

The gut-brain axis is complex, with the gut having its own 'second brain' and influencing the central nervous system.

Key Insights

The gastrointestinal tract has an intrinsic nervous system, often called the 'second brain,' capable of independent function.

The gut and brain communicate bidirectionally, with the brain acting as a CEO and the gut handling detailed operations.

Gut hormones and mechanical factors (like distension) signal satiety and hunger to the brain.

The gut microbiome plays a crucial role in nutrient digestion, vitamin synthesis, and potentially influences mood and metabolism.

Serotonin, largely produced in the gut, is vital for nervous system development and function in both the gut and brain.

Leaky gut, characterized by compromised tight junctions between intestinal cells, can lead to protein loss and systemic issues.

EMBRYOLOGY AND ANATOMY OF THE GASTROINTESTINAL TRACT

The gastrointestinal tract originates from a simple tube formed during embryonic development. This tube, extending from mouth to anus, represents an internalized external space. Its lining, the mucosa, must maintain a barrier against the external environment while allowing for nutrient absorption. This involves complex molecular digestion and absorption, a challenge unique to the gut compared to systems like the respiratory tract. The gut's development involves folding and differentiation, leading to specialized regions like the foregut, midgut, and hindgut, each supplied by distinct arterial branches and playing roles in digestion and absorption.

THE INTRINSIC NERVOUS SYSTEM: THE GUT'S SECOND BRAIN

The gut possesses a complex intrinsic nervous system, the enteric nervous system (ENS), composed of the myenteric and submucosal plexuses. This 'second brain' can control reflexes and behaviors independently of the central nervous system (CNS), as demonstrated by studies where the gut continued to function even when severed from the brain and spinal cord. This intrinsic innervation is a vertebrate invention, increasing in complexity with vertebrate evolution. While it can function autonomously, it also engages in constant bidirectional communication with the CNS, which provides broad instructions while the ENS manages the detailed operations.

NEUROTRANSMITTERS AND SIGNALING IN THE GUT

Serotonin, with about 95% of the body's supply produced in the gut's enteroendocrine cells, plays a critical role. It acts as both a neurotransmitter and an endocrine substance, influencing gut pain, nausea, and crucially, nervous system development. Serotonin is essential for nerve cell growth in the gut, and its absence leads to fewer neurons and impaired function. Selective serotonin reuptake inhibitors (SSRIs), while targeting CNS serotonin, can impact the gut, often causing nausea due to enhanced serotonin signaling and potentially leading to motility changes or constipation with long-term use.

APPETITE REGULATION AND THE GUT-BRAIN FEEDBACK LOOP

The gut plays a significant role in appetite regulation through a sophisticated communication system with the brain. Factors like stomach distension and the rate of blood glucose rise are sensed by gut receptors and transmitted to the brain. Gut hormones, such as cholecystokinin and ghrelin, are released from enteroendocrine cells and signal satiety or hunger, respectively. The speed of gastric emptying, influenced by nutrient content and food form (liquids vs. solids), further modulates these signals, impacting how quickly and for how long an individual feels full, demonstrating a complex interplay between mechanical and chemical cues.

THE GUT MICROBIOME AND ITS BROAD IMPACT

The colon harbors an immense population of bacteria, outnumbering human cells and possessing far more genes. These anaerobic organisms play vital roles in digesting indigestible fibers, producing essential short-chain fatty acids for colonocyte energy, and synthesizing vitamins like K. The microbiome's composition is sensitive to diet and antibiotics. While specific gut bacteria can influence host metabolism and potentially mood, the complexity and difficulty in culturing most of these organisms mean our understanding, particularly regarding precise therapeutic interventions via supplements or fecal transplants, is still developing.

LEAKY GUT AND ITS IMPLICATIONS

Leaky gut refers to compromised intestinal barrier function, often due to disrupted tight junctions between epithelial cells. This permeability allows substances from the gut lumen to enter the body, potentially leading to inflammation and conditions like exudative enteropathy, where protein is lost from the bloodstream. While the causes are debated, allergies and toxins can contribute. Diagnostic tools exist to assess gut leakiness, such as measuring serum albumin or specific peptides and sugars that shouldn't be absorbed systemically. Leaky gut has been erroneously linked to autism, but current evidence doesn't support a causal relationship; rather, autism may involve nervous system dysregulation with concurrent gut issues.

THE ROLE OF THE GUT IN NEURODEGENERATIVE PROCESSES AND IMMUNITY

The gut's involvement extends to conditions like the varicella-zoster virus (VZV), which causes chickenpox and shingles. VZV can establish latency within the enteric nervous system, potentially leading to gut dysfunction such as pseudo-obstruction or achalasia if reactivated. This highlights how viral latency in neural tissue, even outside the CNS, can have profound effects. Furthermore, the gut's interaction with the immune system and its potential role in conditions like autism spectrum disorder, where gut issues are common, are areas of ongoing research, suggesting a deep and multifaceted connection between gut health and overall neurological well-being.

Mentioned in This Episode

●Products

●Companies

●Organizations

●Books

●Concepts

●People Referenced

Common Questions

The GI tract is effectively an internalized external space, beginning at the mouth and ending at the anus. Its interior hosts a vast community of microorganisms, requiring a strong protective barrier while simultaneously allowing for nutrient digestion and absorption. This creates a challenging balance between defense and transmission, unlike other systems like the skin or respiratory tract.

Topics

Gut-Brain Axis Health & Longevity Neuroscience & the Brain Digestive Health Immune Function Medicine & Clinical Enteric Nervous System Gastrointestinal Physiology Microbiome Health

Mentioned in this video

Organizations

Columbia University

The academic institution where Dr. Michael Gershon is a professor of Pathology and Cell Biology.

Oxford University

The university where Dr. Gershon conducted his postdoctoral fellowship.

Cornell University

The university from which Dr. Gershon earned his medical degree and where he did his senior thesis on serotonin.

FDA

A government agency that regulates medical claims for food and drug products, requiring tests for efficacy and safety.

FTC

A government agency that regulates advertising for food products, including probiotics, ensuring that claims made about their efficacy and safety are substantiated by evidence.

Books

The Second Brain

A book authored by Dr. Michael Gershon, which he named due to the gut's nervous system being able to function and control reflexes independently of the brain or spinal cord.

Concepts

Leptin

A hormone that can signal to the brain, affecting the urge to eat. It is transported across the blood-brain barrier to affect the central nervous system directly.

Gastric Bypass

A highly successful surgical procedure for treating obesity and type 2 diabetes by rerouting the upper GI tract, triggering strong satiety signals due to rapid food transit to the small intestine, thus reducing food intake.

Autism Spectrum Disorder

A neurodevelopmental disorder in which affected children often present with increased prevalence of gastrointestinal illnesses like constipation or diarrhea, and sometimes food sensitivities.

Apoptosis

The process of programmed cell death, through which epithelial cells at the tips of the gut villi are shed, maintaining continuous turnover while preserving tight junctions.

Serotonin

A molecule that functions as a neurotransmitter and endocrine substance, with about 95% of the body's serotonin produced in the gut by enteroendocrine cells, and 2-3% in the brain. It's crucial for gut pain, nausea, development, and immune system interaction.

People

Michael Gershon

Professor of Pathology and Cell Biology at Columbia University, whose research focuses on the neural control of the gastrointestinal tract and the role of serotonin in the gut as a neurotransmitter. He is the guest of this episode and author of 'The Second Brain'.

Bayliss and Starling

English scientists who, around the turn of the 19th to 20th century, conducted experiments on dog intestines, demonstrating the gut's independent response to pressure which they termed 'the law of the intestine'. Starling is also known for Starling curves in cardiac physiology.

Mike Rosenbaum

A researcher known for experiments on how different meal types (mixed meal shakes versus solids) affect satiety.

Rudy Leibel

A colleague of Dr. Gershon's at Columbia University, known for experimental research into how different meal types impact satiety and absorption.

Andrew Wakefield

A British gastroenterologist who published a discredited study in 1991, falsely associating GI inflammation and autism with the MMR vaccine.

Brian Dear

Mentioned as a guest who previously discussed the Andrew Wakefield controversy on a podcast, indicating his expertise on the subject.

Drugs & Medications

Cholera

A severe diarrheal disease caused by cholera toxin, leading to rapid dehydration and death, primarily due to impaired sodium absorption in the small intestine. Oral rehydration solutions with glucose are life-saving.

Cholecystokinin

A hormone produced by enteroendocrine cells in the gut that can signal satiety to the brain by crossing the blood-brain barrier.

Shingrix vaccine

A vaccine effectively used to prevent shingles by priming the immune system against the varicella zoster virus, showing about 97% effectiveness.

SSRIs

Medications that prevent the reuptake of serotonin, used to treat depression and anxiety. They can cause nausea and initial gut motility changes, eventually leading to constipation due to receptor desensitization. Fetal exposure to SSRIs in mice leads to abnormal nervous system growth in the gut, but they are still used in human pregnancy for depression due to its lethal potential.

Measles, Mumps, and Rubella (MMR) vaccine

A vaccine falsely linked to autism by Andrew Wakefield, a claim that has been widely disproven.

Oral vancomycin

An antibiotic used as a first-line treatment for C. difficile infections; it is not absorbed into the bloodstream and acts locally in the gut.

SNRI

A class of medication similar to SSRIs but less selective for serotonin, also inhibiting norepinephrine reuptake.

Supplements

Ghrelin

A hormone from the gut that promotes food absorption and eating, working in opposition to satiety signals. Its secretion is suppressed by nutrients like amino acids and fatty acids.

Companies

Ubiquitous Biome

A now-defunct company out of UCSF involved in gut microbiome testing, which turned out to be fraudulent, resembling the Theranos scandal.

Dannon

A food company that produces Activía yogurt, for which Dr. Gershon consults, and whose claims were carefully evaluated by the FTC.

Theranos

A disgraced health technology company known for its fraudulent claims regarding blood testing technology, used here as a comparison for the uBiome fraud.

Products

Cologuard

A commercial stool-based DNA test for detecting genetic signatures of GI cancers, capable of picking up about 90% of GI cancers, but not a substitute for colonoscopy in preventing cancer by finding polyps.

Activia

A yogurt product containing Bifidobacter animalis, which has been shown to survive transit through the GI tract and contribute to gut motility, according to claims substantiated with the FTC.