The Gut Microbiome Goes Far Beyond Digestion: What Transfer Experiments Reveal

What if the microorganisms living inside us are not simply passengers—but active participants in our health?

The human gut contains a vast ecosystem of bacteria, viruses, fungi and other microorganisms collectively known as the gut microbiome. These organisms interact with food, medications, the immune system, metabolism and the nervous system. They help produce vitamins and biologically active compounds, transform bile acids, ferment nutrients, communicate with immune cells and generate signals that can travel far beyond the intestines.

For years, much of the research was observational: people with one condition had a different microbial pattern than people without it. That was interesting, but it could not answer the most important question. Did the microbiome contribute to the condition, or did the condition merely alter the microbiome?

Transfer experiments changed the conversation.

When scientists moved microbial communities, or selected organisms from those communities, into new hosts, they sometimes transferred part of the original biological phenotype with them. Susceptibility to infection, metabolic dysfunction, depression-related behavior, seizure resistance and even aspects of physical performance changed in the recipient.

The results do not suggest that microbes act alone. They show something more useful: the microbiome can be one active component of a larger biological system involving diet, genes, metabolism, immunity and the environment.

The clearest human example: recurrent C. difficile

The most dramatic clinical demonstration of microbiome restoration comes from recurrent Clostridioides difficile, commonly called C. diff.

Antibiotics can be necessary and lifesaving, but they can also disrupt the intestinal ecosystem. When the microbial community that normally provides resistance against pathogens has been severely altered, C. difficile can expand, produce toxins and cause severe diarrhea and inflammation. Antibiotics directed at C. difficile may suppress the infection, yet recurrence can occur when the surrounding ecosystem does not recover.

In a landmark 2013 randomized trial, researchers compared vancomycin with a protocol that included vancomycin, bowel lavage and infusion of screened donor microbiota.[1] The trial was stopped early because the difference was so large:

• 13 of 16 participants—81%—had resolution after the first donor infusion.
• Two of the remaining three improved after a second infusion, producing an overall response of 94%.
• Resolution occurred in 31% of participants receiving vancomycin alone and 23% receiving vancomycin plus bowel lavage.

After the transplant, the recipients’ microbial diversity increased and became more similar to that of the healthy donors.

A later double-blind randomized trial compared donor microbiota with a person’s own stool in people with multiply recurrent infection. Clinical cure occurred in 91% of the donor group and 63% of the autologous group. People who relapsed after receiving their own material were subsequently treated with donor microbiota, and all nine were then free of further infection during follow-up.[2]

This is the clearest proof of principle in humans: restoring an ecosystem can produce a major clinical effect that suppressing a single organism does not always accomplish.

Can features of depression travel with the microbiome?

The gut and brain communicate through immune signals, hormones, microbial metabolites, tryptophan pathways and the nervous system. Researchers often call this the microbiota–gut–brain axis.

In 2016, two research groups asked whether the microbial differences associated with major depressive disorder could do more than simply accompany depression.

One group transferred microbiota from people with major depression or healthy controls into microbiota-depleted rats. Animals that received depression-associated microbiota developed more anhedonia and anxiety-like behavior, together with altered tryptophan metabolism.[3]

Another group colonized germ-free mice with microbiota from people with major depressive disorder or healthy controls. Mice that received depression-associated microbiota developed depression-like behavioral and metabolic changes, including disturbances in microbial genes and host metabolites involved in carbohydrate and amino-acid metabolism.[4]

The recipients did not “catch depression” in the ordinary sense. Rather, the experiments showed that depression-associated microbial communities could transfer measurable behavioral and metabolic features into animal models. That moved the microbiome from correlation toward biological causality.

Human intervention research is now beginning to extend that story.

A 2023 randomized pilot established that enema-delivered fecal microbiota transplantation was feasible, acceptable and well tolerated in 15 adults with moderate-to-severe major depressive disorder.[5]

In a 2026 randomized study of 46 people with depressive episodes, participants received medication alone or medication plus fecal microbiota transplantation. After two weeks, the transplant group had a larger reduction in depression scores and a higher reported treatment-response rate. The intervention was also associated with changes in several measured intestinal organisms.[6]

An additional 2026 double-blind, placebo-controlled trial evaluated two weeks of oral microbiota capsules as an adjunct to escitalopram. Remission rates at week eight were not significantly different, but the microbiota group experienced greater reductions in depression scores at weeks two and eight. Donor strains engrafted in recipients, changes in bile-acid metabolism correlated with symptom improvement, and inflammatory signaling was reduced.[7]

These studies introduce a remarkable possibility: modifying the gut ecosystem may influence mood through metabolic and inflammatory pathways that connect the intestine and brain.

Ketogenic nutrition, the microbiome and seizure resistance

Ketogenic diet therapy has been used to treat epilepsy for more than a century, particularly when seizures do not respond adequately to medication. Ketones, changes in brain energy metabolism, neurotransmitter balance, mitochondrial function and inflammation may all contribute. Microbiome research suggests that intestinal organisms may be another part of the mechanism.

In a 2018 Cell study, ketogenic diets changed the gut microbiota in two mouse models of refractory epilepsy.[8] Antibiotic treatment or germ-free conditions reduced the diet’s seizure protection. Colonizing mice with selected ketogenic-diet-associated bacteria restored protection, while transferring ketogenic-diet microbiota to mice eating a control diet also increased seizure resistance.

The microbial changes affected amino-acid metabolism and increased the hippocampal ratio of GABA to glutamate—a shift consistent with reduced neuronal excitability.[8]

Researchers then asked whether microbiome changes seen in children receiving ketogenic treatment could produce a similar effect.

In a 2023 study, stool samples were collected from ten children with refractory epilepsy before ketogenic therapy and after one month of treatment.[9] Mice colonized with the children’s post-ketogenic-diet microbiota were more resistant to experimentally induced seizures than mice receiving the matched pretreatment microbiota. Microbial and metabolic changes found in the children were preserved in the recipient mice, along with changes in brain pathways involving energy production, glutathione metabolism and oxidative phosphorylation.[9]

The accompanying commentary discusses this antiseizure finding in mice.[10]

This work suggests that ketogenic therapy does not act only by producing ketones. It may remodel an intestinal ecosystem whose metabolites and signaling pathways become part of the brain’s response.

Kwashiorkor: when diet and microbiome interact

Kwashiorkor is a severe form of childhood malnutrition. In Malawi, investigators studied 317 twin pairs during their first three years of life. Some twin pairs were discordant: one child developed kwashiorkor while the other remained comparatively healthy despite sharing genetics and much of the same environment.[11]

Researchers transferred preserved fecal communities from affected and healthy co-twins into germ-free mice. When the mice ate a diet modeled on the children’s Malawian diet, recipients of the kwashiorkor-associated microbiota experienced pronounced weight loss and disruptions in amino-acid, carbohydrate and intermediary metabolism. A ready-to-use therapeutic food temporarily improved both the microbiome and metabolism, but many changes regressed when that food was withdrawn.[11]

This experiment did not show that a microbiome acts independently of nutrition. It showed the opposite: diet and microbiome can interact. The same food environment produced different metabolic consequences depending on which microbial ecosystem was present.

The findings helped expand the understanding of severe malnutrition from a simple deficiency model toward a systems model involving food, microbial development and host metabolism.

Body composition can also be transferred in mice

Another 2013 experiment examined adult female twin pairs in which one twin had obesity and the other did not. Researchers transferred each twin’s uncultured fecal microbiota into germ-free mice.[12]

Mice receiving obesity-associated microbiota gained more body mass and fat and developed obesity-associated metabolic changes. When mice carrying the two different microbial communities lived together, organisms from the lean-associated community moved into the obesity-associated community and prevented the increase in adiposity—but only in the appropriate dietary environment.[12]

Again, the lesson was not that the microbiome replaces diet. The protection was diet-dependent. The experiment demonstrated that microbial communities and food environment can work together to shape metabolism and body composition.

Can the microbiome influence strength and endurance?

Researchers isolated a strain of Lactiplantibacillus plantarum, named Tana, from a gold-medal-winning international weightlifter. Forty mice received different doses of the organism or a control for four weeks.[13]

Tana supplementation increased grip strength, endurance performance and glycogen storage in the liver and muscle. The effects increased with higher doses. It shows that a human-derived microbial strain can influence muscle performance and fatigue-related metabolism in mice.[13]

Newer transfer experiments support a broader gut–muscle connection. In a 2026 study combining a human cohort with a mouse model, researchers identified a distinct microbial pattern in physically active young adults. Mice receiving fecal microbiota from the active group developed better endurance capacity than recipients of microbiota from sedentary donors.[14]

In another 2026 experiment, microbiota from endurance-trained mice partially transferred improved exercise capacity, glycogen reserves and antioxidant activity to untrained recipient mice.[15]

These results suggest that exercise changes the microbiome—and that some exercise-associated metabolic effects may also travel in the opposite direction, from microbiome to muscle.

What makes the microbiome so influential?

Microbes can affect distant organs without leaving the intestine. They do this through the compounds they produce and the host pathways they influence.

Important connections under investigation include:

• Short-chain fatty acids, produced when microbes ferment certain nutrients
• Bile-acid transformation, which can affect metabolic and immune signaling
• Tryptophan and kynurenine metabolism, relevant to neurotransmission and immune activity
• GABA and glutamate balance, relevant to neuronal excitability
• Intestinal-barrier function, which affects which microbial products enter circulation
• Immune signaling and inflammation
• Glucose and amino-acid metabolism
• Mitochondrial and oxidative-stress pathways

This helps explain how changes in the gut can be associated with effects in the brain, liver, immune system and skeletal muscle.

Diet helps create the microbial environment

The microbiome is dynamic. Food provides both the nutrients used by the human host and the substrates used by intestinal organisms. Changing the diet can alter which organisms thrive, which genes are active and which metabolites are produced.

There is no single universally “perfect” microbiome. A microbial pattern that is helpful in one dietary or metabolic environment may behave differently in another. The kwashiorkor and obesity-transfer studies make this especially clear: the effect of the transplanted community depended partly on what the recipient ate.[11, 12]

Ketogenic nutrition is a powerful example. It changes carbohydrate availability, fat metabolism, ketone production and the intestinal environment simultaneously. In epilepsy models, the microbiome was not just altered by the diet; it contributed to the diet’s antiseizure effect.[8, 9]

This does not mean every health benefit of a ketogenic diet comes from the microbiome. It means the microbiome belongs in the larger explanation of how food changes human biology.

A living biological system

The most important lesson from microbiome research is not that one bacterial species determines health. It is that humans and their microbial communities function as an interconnected biological system.

In recurrent C. difficile, restoring microbial ecology can resolve a serious human disease. In experimental models, microbiota transfer can reproduce elements of depression-associated behavior, malnutrition, adiposity, seizure resistance and exercise capacity. New human trials are now exploring whether targeted microbial restoration can influence depression and other conditions.

The science is moving from cataloging which organisms are present to understanding what microbial communities do: which metabolites they produce, which host pathways they alter and how diet changes those interactions.

We are not simply feeding ourselves. We are also feeding an ecosystem and continuously reshaping it. That ecosystem communicates with the rest of the body.

What microbiome-transfer experiments have changed

• Infection: Restoring microbial diversity can resolve recurrent C. difficile.
• Mood: Depression-associated microbiota transferred behavioral and metabolic features to rodents.
• Seizures: Microbiota from children receiving ketogenic therapy increased seizure resistance in mice.
• Malnutrition: Kwashiorkor-associated microbiota produced weight loss and metabolic disruption when combined with a regional diet in mice.
• Body composition: Obesity-associated microbial communities transferred increased adiposity to mice in a diet-dependent manner.
• Performance: Selected athlete-associated organisms and exercise-associated microbiota improved strength or endurance in mice.

References

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