High dietary B12 is associated with reduced gut microbial B12 biosynthesis capacity and lower fecal short-chain fatty acids in healthy United States adults.
Hilliard M, Oliver A, et al. • Gut microbes • 2026
Dietary vitamin B12 greater than 8.51 µg/day is associated with reduced microbial synthesis of B12 and lower fecal SCFA production in healthy U.S. adults.
Key Findings
Results
High dietary B12 intake (>8.51 µg/day) was associated with a reduction in bacteria capable of anaerobic B12 biosynthesis.
Participants were stratified into adequate (2.4–8.51 µg/day) and high B12 intake (>8.51 µg/day) groups.
The study used fecal shotgun metagenomes integrated with habitual diet data in a cohort of n=277 healthy U.S. adults.
Nearly all participants were B12 replete regardless of intake group.
The association with reduced anaerobic B12 biosynthesis capacity was identified through metagenomic pathway analysis.
Results
High dietary B12 intake was associated with lower fecal short-chain fatty acid (SCFA) concentrations.
The association between high B12 intake and lower fecal SCFA concentrations remained significant after controlling for fiber and methionine intake.
Vitamin B12 intake in the cohort ranged widely from 2.4 to 1062 µg/day.
Both fecal and plasma SCFAs were measured alongside plasma B12 levels.
The finding extends prior animal model evidence linking B12 and/or methionine interventions to fecal SCFA concentration to a human context.
Results
Differences in microbial taxa between adequate and high dietary B12 intake groups were limited.
Despite changes in functional capacity (B12 biosynthesis pathways and SCFA associations), taxonomic composition differences between groups were limited.
The study used fecal shotgun metagenomics, which captures both taxonomic and functional (pathway-level) information.
This suggests that functional changes in the microbiome may occur without large shifts in community composition.
Results
Machine learning models could predict fecal propionate and butyrate from microbial pathways in adequate or no-supplement groups, but not in high intake or supplement groups.
Predictive models were built using microbial pathway data to estimate fecal propionate and butyrate concentrations.
Model performance differed by dietary B12 group: predictions were successful in adequate intake and no-supplement groups.
In high B12 intake and supplement groups, microbial pathways lost predictive power for these SCFAs.
This suggests that high dietary B12 disrupts the normal relationship between microbial metabolic pathways and SCFA output.
Methods
The study cohort consisted of 277 healthy U.S. adults with a wide range of vitamin B12 intake and nearly universal B12 repletion.
Sample size was n=277.
Vitamin B12 intake ranged from 2.4 to 1062 µg/day.
Nearly all participants were described as B12 replete, meaning plasma B12 status was generally adequate across groups.
The cohort was described as 'deeply phenotyped,' with measurements of habitual diet, fecal shotgun metagenomes, fecal SCFAs, plasma SCFAs, and plasma B12.
What This Means
This research suggests that consuming high amounts of vitamin B12 — more than about 8.5 micrograms per day, which is well above the standard recommended daily amount of 2.4 micrograms — may change how gut bacteria function in healthy adults. Specifically, people with high B12 intake appeared to have fewer gut bacteria capable of making their own B12, and they also had lower levels of short-chain fatty acids (SCFAs) in their stool. SCFAs are compounds produced by gut bacteria when they break down dietary fiber, and they are considered beneficial for gut and overall health. Importantly, the lower SCFA levels associated with high B12 intake could not be explained simply by differences in fiber or methionine consumption, suggesting B12 itself plays a role.
Interestingly, even though gut bacterial function appeared to shift with high B12 intake, the overall types and amounts of bacteria present were not dramatically different between people with adequate versus high B12 intake. The researchers also used machine learning models and found that in people with adequate B12 intake, microbial metabolic activity could reliably predict SCFA levels — but this relationship broke down in people with high B12 intake or those taking B12 supplements. This suggests that high B12 intake disrupts the normal connection between what gut microbes do and what they produce.
This research matters because vitamin B12 supplementation is common, and intakes far exceeding recommendations are widespread, particularly among supplement users. While this study does not establish that high B12 intake is harmful, it raises questions about whether very high B12 consumption may unintentionally alter gut microbial activity in ways that could affect the production of health-relevant compounds like SCFAs. Further research is needed to understand whether these associations reflect a causal relationship and what their long-term health implications might be.
Hilliard M, Oliver A, Wilson S, Shahab-Ferdows S, Hampel D, Bennett B, et al.. (2026). High dietary B12 is associated with reduced gut microbial B12 biosynthesis capacity and lower fecal short-chain fatty acids in healthy United States adults.. Gut microbes. https://doi.org/10.1080/19490976.2026.2718567