Fecal Transplant and Personality Changes: The Most Direct Evidence That Gut Bacteria Shape Who You Are

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The Procedure That Transfers Identity

Of all the evidence linking the gut microbiome to consciousness, the most unsettling comes from a procedure that most people find viscerally repulsive: fecal microbiota transplantation (FMT) — the transfer of stool from a healthy donor into the gastrointestinal tract of a recipient.

FMT is not a fringe therapy. It is an FDA-recognized treatment for recurrent Clostridioides difficile infection, with cure rates exceeding 90% — vastly superior to antibiotic treatment. It is being investigated for dozens of other conditions, from inflammatory bowel disease to metabolic syndrome to autism.

But the most philosophically provocative aspect of FMT is not its efficacy against infection. It is the growing body of documented cases in which recipients experience changes in personality, mood, food preferences, body weight, anxiety levels, and cognitive function after receiving another person’s microbiome.

You read that correctly. Transferring bacteria from one person’s gut to another can change who the recipient is.

Not their memories. Not their knowledge. Not their identity in the narrative sense. But their temperament, their cravings, their emotional baseline, their body composition — the aspects of selfhood that we typically attribute to genetics, psychology, or “just who I am.” These change. Measurably. Reproducibly. In response to bacteria.

If personality is partly a product of the gut microbiome, then FMT is not just a medical procedure. It is a partial transfer of identity — a biological demonstration that who you “are” is not solely determined by your brain, your genes, or your life experience. It is co-determined by the microbial ecosystem in your gut.

The History: From Ancient Stool Medicine to Modern FMT

The Premodern Precedents

The therapeutic use of fecal material is not new. The earliest recorded use is in fourth-century China, where the physician Ge Hong described the oral administration of a fecal suspension — called “yellow soup” — for the treatment of severe diarrhea and food poisoning. The sixteenth-century Chinese pharmacopoeia, Ben Cao Gang Mu by Li Shizhen, described multiple fecal preparations for gastrointestinal and systemic conditions.

Bedouin tribes reportedly consumed fresh camel dung as a treatment for dysentery — a practice that, while unsanitary by modern standards, may have introduced healthy commensal bacteria to a dysbiotic gut.

In veterinary medicine, “transfaunation” — the transfer of rumen contents from a healthy ruminant to a sick one — has been practiced for centuries and is still used today.

The Modern Era

Modern FMT was reintroduced to Western medicine in 1958 by Ben Eiseman, a surgeon at the University of Colorado, who successfully treated four patients with pseudomembranous colitis (now known to be caused by C. difficile) using fecal enemas from healthy donors. All four patients recovered.

The procedure remained obscure for decades until the early 2000s, when the epidemic of C. difficile infection — driven by broad-spectrum antibiotic overuse — created an urgent need for effective treatments. Thomas Borody in Australia and Alexander Khoruts at the University of Minnesota led the revival of FMT, demonstrating cure rates of 90-95% for recurrent C. difficile in cases where antibiotics had repeatedly failed.

In 2013, a landmark randomized controlled trial by Els van Nood and colleagues in the New England Journal of Medicine provided definitive evidence: FMT via nasoduodenal infusion was significantly more effective than vancomycin (the standard antibiotic) for recurrent C. difficile infection. The trial was stopped early because the FMT group’s outcomes were so superior that it was considered unethical to continue randomizing patients to the antibiotic arm.

The Personality Transfer Evidence

Animal Studies: The Foundational Evidence

The animal evidence for microbiome-mediated personality transfer is robust and unambiguous:

The timid mouse and the bold mouse: In a landmark 2011 study published in Gastroenterology, Premysl Bercik, Stephen Collins, and colleagues at McMaster University exchanged the gut microbiota between two strains of mice with distinct behavioral phenotypes: BALB/c mice (naturally timid and anxious) and NIH Swiss mice (naturally bold and exploratory).

After microbiome transfer, the timid mice became bold. The bold mice became timid. Each strain adopted the behavioral phenotype of the donor — not the recipient. The personality followed the bacteria, not the genetics.

Furthermore, the behavioral changes were accompanied by changes in brain-derived neurotrophic factor (BDNF) levels in the hippocampus — demonstrating that the microbiome transfer was altering brain chemistry, not just gut function.

Obesity transfer: The most famous FMT study in the animal literature was conducted by Jeffrey Gordon’s laboratory at Washington University in St. Louis. Gordon’s team transplanted gut microbiota from obese and lean human twins into germ-free mice. Mice receiving the obese twin’s microbiome became obese. Mice receiving the lean twin’s microbiome remained lean. Same genetics. Same environment. Same food. Different microbiomes. Different bodies.

When the obese-microbiome mice were co-housed with the lean-microbiome mice (allowing microbial transfer through coprophagy), the obese mice became lean — but only if they were fed a high-fiber diet. The lean phenotype was transmissible through the microbiome, but it required dietary substrate to establish.

Depression transfer: In 2016, Zheng and colleagues published a study in Molecular Psychiatry demonstrating that transferring gut microbiota from patients with major depressive disorder into germ-free mice produced depression-like behaviors — reduced social interaction, increased immobility in the forced swim test, and altered tryptophan and kynurenine metabolism. The depression was, in a very real sense, contagious through the microbiome.

Schizophrenia transfer: A 2019 study in Science Advances by the same group showed that FMT from patients with schizophrenia into germ-free mice produced schizophrenia-like behaviors and altered glutamate and GABA metabolism in the brain.

Human Case Reports and Studies: The Emerging Evidence

Human evidence for personality and behavioral changes after FMT is accumulating, though it remains largely in the form of case reports and small studies rather than large randomized trials.

Weight gain after FMT from an overweight donor: A 2015 case report by Alang and Kelly in Open Forum Infectious Diseases documented a woman who received FMT from an overweight donor for C. difficile infection. The patient, who had been of normal weight her entire life, gained 34 pounds in the 16 months following the transplant and was unable to lose the weight despite diet and exercise. Her BMI increased from 26 to 33 (obese range). No other explanation for the weight gain was identified.

This case prompted a policy change: many FMT programs now screen donors for obesity and use only lean donors.

Mood and anxiety changes: Multiple FMT recipients have reported changes in mood and anxiety following the procedure. While these are primarily anecdotal and reported in clinical observations rather than controlled studies, the pattern is consistent: recipients who receive microbiomes from psychologically healthy donors often report improvements in mood and anxiety, while recipients of microbiomes from donors with psychiatric conditions may experience mood deterioration.

Food preference changes: Clinicians performing FMT have reported that recipients sometimes develop new food preferences or aversions that are concordant with the donor’s preferences rather than the recipient’s pre-transplant baseline. While this has not been formally studied in controlled trials, the observation is biologically plausible: the gut microbiome influences food cravings through production of neurotransmitters and metabolites that modulate reward circuitry and appetite regulation.

The Autism Microbiota Transfer Therapy Trial: The Arizona State University trial (Kang et al., 2017, 2019) is the most rigorous human study demonstrating behavioral changes following microbiome transfer. Children with ASD who received FMT followed by a maintenance protocol showed significant improvements in core ASD symptoms — including social communication, repetitive behaviors, and gastrointestinal symptoms — that persisted for at least two years.

The behavioral improvements correlated with changes in microbiome composition — specifically, increased diversity and increased abundance of Bifidobacterium and Prevotella. The bacteria changed, and the children’s behavior changed with them.

The Mechanisms: How Transferred Bacteria Change the Recipient

Neurochemical Reprogramming

The donor’s microbiome brings its own neurochemical production profile. A microbiome rich in GABA-producing Lactobacillus and serotonin-supporting clostridia produces a different neurochemical cocktail than a microbiome dominated by inflammatory species.

When this new microbial community colonizes the recipient’s gut, it begins producing its characteristic metabolites — neurotransmitters, short-chain fatty acids, tryptophan metabolites — that alter the recipient’s brain chemistry through vagal, immune, and endocrine pathways.

The recipient’s mood, anxiety level, and cognitive function shift in response to the new neurochemical input — just as they would shift in response to a new pharmaceutical drug, except that the “drug” is a self-replicating, self-sustaining, ecosystem-based pharmacy.

Immune Recalibration

The donor’s microbiome interacts with the recipient’s immune system, reshaping the balance of pro-inflammatory and anti-inflammatory signaling. A donor microbiome with strong anti-inflammatory capacity (high butyrate production, robust Akkermansia, balanced immune-modulating species) can shift the recipient toward reduced systemic inflammation — with direct consequences for neuroinflammation and brain function.

Metabolic Reprogramming

The donor’s microbiome brings its own metabolic program — its own patterns of energy extraction, nutrient absorption, bile acid metabolism, and fat storage. This is the mechanism behind the weight gain case: the obese donor’s microbiome was more efficient at extracting calories from food and signaling the body to store fat.

The microbiome does not just influence the brain. It influences the entire metabolic system — which in turn influences energy levels, body composition, hormonal balance, and the subjective experience of vitality or fatigue.

Vagal Signaling Reset

The new microbiome produces different patterns of vagal afferent stimulation. Changes in gut motility, gut permeability, and the chemical signals reaching vagal nerve endings alter the information stream flowing from gut to brain. The brain’s representation of the gut state shifts, changing the “gut feelings” that contribute to emotional tone and decision-making.

The Philosophical Implications: Who Are You?

The Challenge to Personal Identity

Western philosophy has long debated the nature of personal identity. Are you your memories (John Locke)? Your body (biological continuity)? Your narrative (Paul Ricoeur)? Your consciousness (Descartes)?

The FMT evidence introduces a new variable: you are, in part, your microbiome. And your microbiome can be changed. Dramatically. In a single procedure.

If personality traits — anxiety level, food preferences, body composition, social behavior — are partly determined by the gut microbiome, and the microbiome can be transferred from one person to another, then aspects of your personality are transplantable. They are not fixed properties of your brain or your genes. They are properties of an ecosystem that can be reseeded.

This challenges the essentialist view of self — the belief that there is a fixed, unchanging core of identity beneath the flux of experience. The microbiome evidence suggests that the “core” is more dynamic, more environmentally responsive, and more biologically distributed than we assumed. Part of who you “are” is determined by who lives in your gut — and that community changes in response to what you eat, where you live, what medications you take, and — in the case of FMT — whose bacteria you receive.

The Extended Self

The philosopher Andy Clark has argued for the “extended mind” thesis — the view that cognitive processes extend beyond the brain into the body and the environment. The pen and paper you use to do mathematics are, in Clark’s framework, part of your cognitive system. The smartphone you rely on for memory is part of your extended mind.

The gut microbiome takes this thesis further. It is not an external tool you use. It is an internal ecosystem that you host, that produces the neurochemicals underlying your emotional and cognitive experience, and that constitutes part of the biological substrate of your selfhood. The extended self is not just cognitive — it is microbial.

The Buddhist Perspective: No Fixed Self

The Buddhist concept of anatta (no-self) — the teaching that there is no fixed, unchanging self — finds unexpected support in the microbiome research. If the “self” is partly constituted by a microbial community that is continuously changing (turning over its population every few days, responding to dietary and environmental inputs, exchangeable through FMT), then the self is indeed not fixed. It is a process, not a thing — a dynamic, continuously reconstructed phenomenon rather than a stable entity.

The FMT data is, in a sense, a biological demonstration of anatta. Transfer the bacteria, and aspects of the person change. The self is not solid. It is fluid, distributed, and co-created by a community of living beings.

Ethical and Practical Implications

Donor Selection as Personality Selection

If FMT can transfer personality traits along with bacteria, then donor selection is not merely a medical decision. It is, at some level, a decision about the recipient’s future personality, mood, and metabolic state.

Current FMT screening focuses on safety — excluding donors with infectious diseases, antibiotic-resistant organisms, and gastrointestinal conditions. But the personality transfer evidence suggests that screening should also consider the donor’s metabolic phenotype (lean vs. obese), psychological profile (low anxiety vs. high anxiety, low depression vs. high depression), and microbiome diversity.

Some FMT programs have already begun screening for donor BMI after the weight gain case report. The next logical step would be screening for donor psychological health — selecting donors whose microbiomes produce the neurochemical profiles associated with emotional stability, cognitive clarity, and resilience.

The Future of Microbiome-Based Personality Medicine

The logical extension of the FMT personality evidence is the development of targeted microbiome interventions for psychological conditions:

• Curated microbiome transplants: FMT from carefully selected donors with specific psychological profiles — high stress resilience, low anxiety, high cognitive function — for recipients with corresponding deficits
• Synthetic microbial communities: Designer ecosystems of defined bacterial species, assembled to produce specific neurochemical profiles — a “mood microbiome” or a “cognitive enhancement microbiome”
• Precision psychobiotics: Specific bacterial strains, selected based on individual microbiome profiling, to address specific neurochemical deficits in specific patients

These possibilities raise profound ethical questions: If we can engineer the microbiome to alter personality, mood, and cognition, should we? Who decides what constitutes an optimal personality? How do we distinguish between treatment of pathology and enhancement of normal function?

Informed Consent

The personality transfer implications of FMT raise novel questions for informed consent. If a patient is receiving FMT for C. difficile infection, should they be informed that the procedure may alter their mood, anxiety level, food preferences, and body weight? Current consent processes typically do not include this information — because the evidence, while growing, is not yet definitive. But the ethical obligation to disclose known risks and potential effects may require updating consent processes as the evidence matures.

The FMT Evidence and the Microbiome-Consciousness Synthesis

The FMT personality transfer evidence is the single most direct demonstration that the gut microbiome shapes who you are. It is not correlational — it is interventional. Change the microbiome, and the person changes. Transfer the microbiome, and aspects of the donor’s phenotype transfer to the recipient.

This evidence anchors the entire microbiome-consciousness synthesis:

• If depression can be transferred via FMT (Zheng et al., 2016), then the microbiome is causally involved in depression — not merely correlated
• If obesity can be transferred via FMT (Gordon et al.), then the microbiome is causally involved in metabolic regulation — not merely associated
• If ASD symptoms improve with microbiome restoration (Kang et al., 2017, 2019), then the microbiome is causally involved in neurodevelopmental function — not merely a biomarker
• If personality traits shift following FMT, then the microbiome is causally involved in the substrate of personal identity — not merely a peripheral system

The FMT evidence moves the microbiome from correlation to causation, from association to mechanism, from interesting observation to foundational biology. It demonstrates that the gut microbiome is not a bystander in the generation of consciousness. It is a participant — a co-creator of the mental, emotional, and behavioral patterns that constitute who you are.

The Integration: You Are the Ecosystem

The FMT research, taken together with the broader microbiome-consciousness literature, points to a radical revision of the human self-model:

You are not a single organism with a single genome generating a single consciousness from a single brain. You are an ecosystem — a superorganism comprising 30 trillion human cells and 100 trillion microbial cells, running two genomes (human genome and microbial metagenome), processing information through at least two nervous systems (central and enteric), communicating through multiple channels (neural, endocrine, immune, metabolic), and generating a conscious experience that is the integrated output of the entire system.

When you change the ecosystem — through diet, through antibiotics, through stress, through FMT — you change the output. You change the mood, the cognition, the personality, the consciousness. Because these are not properties of the brain alone. They are properties of the superorganism.

The ancient wisdom traditions never made the mistake of locating the self exclusively in the head. They located selfhood in the body, in the breath, in the belly, in the heart, in the web of relationships between the human being and the living world. They understood that identity is not isolated — it is relational, ecological, distributed across a community of beings.

The FMT data is the scientific confirmation of this indigenous insight: who you are depends on who lives within you. Tend the community well, and the community tends to you.

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Based on the research of Jeffrey Gordon (Washington University in St. Louis), Premysl Bercik and Stephen Collins (McMaster University), Dae-Wook Kang and Rosa Krajmalnik-Brown (Arizona State University), Peng Zheng (Chongqing Medical University), Alexander Khoruts (University of Minnesota), Thomas Borody (Centre for Digestive Diseases, Sydney), and the emerging literature on FMT and behavioral/personality changes. Key references include Bercik et al. (2011) in Gastroenterology, Ridaura et al. (2013) in Science, Zheng et al. (2016) in Molecular Psychiatry, and Kang et al. (2019) in Scientific Reports.