Gut–Brain Axis in GAPS: Toxins, Digestion and the Brain

The gut–brain connection is one of the reasons the GAPS books look beyond the digestive symptoms themselves. Dr. Natasha Campbell-McBride describes chemicals made by gut microbes, incomplete digestion, the intestinal barrier and the bloodstream as parts of a pathway that can affect the brain. Her explanation asks where a substance is made, how it is absorbed and what happens after it reaches circulation.^[1][2]

Our family began GAPS in July 2024. I have studied both books and use them in our daily life, but I still want the explanations to be understandable without a biochemistry background. Breaking this particular argument into its separate steps makes it much easier to follow.

The Pathway From Gut to Brain

The expanded Gut and Psychology Syndrome describes the digestive system as a continuing source of toxicity when abnormal gut flora and a damaged intestinal wall occur together. Its explanation has three parts:^[1]

1. Production: microbes make substances as they metabolize food. Digestion can also leave partially broken-down food proteins.
2. Absorption: the book describes a damaged gut wall allowing substances into circulation that would otherwise be handled differently.
3. Transport and effects: circulating substances can affect other tissues. Whether a substance reaches the brain also depends on its properties and the blood–brain barrier.

Campbell-McBride uses swallowed alcohol as a familiar example of a substance entering through the digestive system and affecting thought, coordination and behavior. She then applies that transport idea to substances made within the gut. Her account also says the mixture differs between people, which is why she does not describe one identical toxin exposure in every person.^[1]

The blood–brain barrier guide explains that second barrier and the book’s discussion of predisposition. A substance being present in blood and that substance reaching brain tissue are separate steps.

Ethanol and Acetaldehyde

The earlier book describes yeast fermentation of dietary carbohydrate producing ethanol and acetaldehyde. It recounts people appearing intoxicated without drinking alcohol and uses that example to explain internally generated exposure. The 2020 book also describes poorly digested carbohydrate feeding abnormal microbial fermentation and the production of alcohol, acetaldehyde and other substances.^[1][2]

In the older chapter, the author connects prolonged exposure with impaired stomach-acid and pancreatic function, gut-lining damage and poor absorption. Her list also includes nutritional deficiencies, effects on the liver and immune system, disturbed hormone and neurotransmitter handling, and effects on nerves, muscles, coordination, memory and behavior. The chapter uses these effects to explain why she considers continued microbial fermentation important.^[1]

She gives acetaldehyde particular attention because of its interaction with proteins. Her proposed sequence connects altered proteins with impaired function and immune reactions. She also describes a functional vitamin B6 deficiency: the vitamin may be supplied in food but unable to do its usual work when its protein-binding sites are occupied. This distinguishes intake from the way a nutrient is used.^[1]

The same chapter discusses maternal circulation, the placenta, breast milk and microbial inheritance, and includes observations about fathers’ gut health. It presents those as parts of the author’s developmental account. A family history does not establish a measured fetal exposure, an alcohol concentration or the cause of a child’s condition. Those individual conclusions need their own evidence.

Protein Digestion and Opiate-Like Peptides

The books describe another pathway through incomplete protein digestion. The 2020 protein section explains that healthy enterocytes, the cells lining the intestine, finish breaking peptide chains into amino acids. In the GAPS account, damaged enterocytes and a porous gut wall allow incompletely digested peptides to be absorbed.^[2]

Gluten-derived gluteomorphins or gliadorphins and casein-derived casomorphins are the examples Campbell-McBride discusses most often. She describes them as having opiate-like activity and links them with effects on the nervous system. “Opiate-like peptide” names that proposed biological action; it does not mean a slice of bread contains heroin or that a child has taken an illicit drug.^[1][2]

The earlier discussion follows digestion from stomach acid and pepsin through pancreatic enzymes to the intestinal surface. It cites researchers including Dohan, Reichelt, Shattock and Cade in its account of food peptides and neurological symptoms. The newer book retains the protein-digestion explanation while placing it within the wider gut-flora framework.^[1][2]

Use the guide to gluten, casein and protein digestion for the actual food decisions. GAPS’s dairy rules cannot be reduced to a permanent instruction to remove all casein-containing foods. The source includes preparation and individual dairy tolerance, and those details matter.

Where DPP-IV and Other Peptidases Fit

The expanded earlier text discusses DPP-IV, an enzyme involved in breaking down certain peptides, and describes low activity in several conditions. It notes its inclusion in some digestive-enzyme products. It also asks a wider question: how many other intestinal enzymes are affected when the cells that produce them are damaged?^[1]

A second part of that discussion concerns enzymes that clear the body’s own peptides, hormones and neurotransmitters after use. The author proposes that excessive incoming dietary peptides can interfere with this clearance. These are related ideas, but a digestive enzyme and a treatment for every brain symptom are not the same thing. The DPP-IV and peptidases guide follows the enzyme discussion and its limits.

Other Toxins in the Earlier Book

The “Other Toxins” section recounts William Shaw’s reports of improvement during anti-Clostridia treatment and recurrence after treatment stopped. Campbell-McBride uses those reports to explain her emphasis on a lasting microbial balance rather than repeated short-term suppression. This is her interpretation of the reported cases, not a direction to stop a prescribed medicine.^[1]

The same section describes Alan Friedman’s proposed connection between deltorphin, dermorphin and a fungus that might live in the gut. The book itself leaves that fungal explanation for future research. It belongs here as a historical hypothesis in the chapter, not as a confirmed finding that a particular child carries a frog-associated toxin.^[1]

Neurotransmitters Are Another Part of the Connection

Gut and Physiology Syndrome also discusses the gut’s contribution to neurotransmitter production and the activities of the gut’s own nervous system. That is another part of gut–brain communication, alongside the toxin and peptide pathways. The neurotransmitter production guide keeps the book’s account beside research on the actual cells and signaling routes involved.^[2]

For histamine and related amines, read the histadelia and histamine guide. Their normal roles, excess production, breakdown and immune effects need to be considered together.

What This Means for the GAPS Approach

The books’ proposed treatment target is the wider digestive environment: food that fits the protocol, microbial balance, digestion and the integrity of the intestinal wall. The author’s criticism of focusing on one peptide or organism is that several processes can be operating at once.^[1][2]

For a household, the useful distinction is between understanding a mechanism and proving what caused a symptom. A dated record of food, sleep, bowel symptoms, medicines and observed changes gives you something concrete to discuss. It cannot, by itself, identify a toxin, diagnose a neurotransmitter deficiency or establish a damaged blood–brain barrier.

Questions About the Gut–Brain Connection

Is GFCF the Same as GAPS?

No. The earlier book acknowledges improvement in some children following a gluten- and casein-free diet, while explaining that GAPS addresses a broader set of digestive and microbial factors. The gluten, casein and protein digestion guide keeps those differences and the dairy qualifications together.^[1]

Do Different Symptoms Prove Different Toxins?

No. The book proposes individual toxin mixtures as one explanation for differing patterns. A difference in behavior or mood is not a chemical measurement.^[1]

For the household side of this explanation, read GAPS behavior and the mind–gut connection, including the book’s discussion of tantrums, panic and food cravings.

Your Next Step

Choose the part you need to understand next: neurotransmitter production, histamine and other amines or the blood–brain barrier. My Understanding & Preparing for the GAPS Diet course and Care Team Log are listed in the WFLI Shop. A notebook can hold your dated questions and observations.

References

1. Campbell-McBride N. Gut and Psychology Syndrome, expanded text represented by the second German edition (2015), gut–brain connection, “Ethanol und Azetaldehyd,” food-derived opiates and “Andere Toxine.” English paraphrases checked against that text; no English page numbers are implied.
2. Campbell-McBride N. Gut and Physiology Syndrome (2020), gut-flora neurotransmitter discussion; carbohydrate and protein digestion; casein and casomorphins.