This 2019 narrative review by the Walk laboratory at Montana State University synthesizes evidence on how the human gut microbiome modulates arsenic metabolism, bioavailability, body burden, and toxicity. The review covers arsenic speciation chemistry in the environment and food supply, human arsenic metabolism (methylation, thiolation, glutathione conjugation), bacterial arsenic biochemistry (reduction, efflux, methylation, demethylation), and the documented and proposed pathways through which gut microbial activity alters host arsenic fate. The companion primary study from the same laboratory, Coryell et al. 2018, demonstrated that antibiotic depletion of the gut microbiome in mice reduced fecal arsenic excretion and increased organ accumulation; this 2019 review situates that finding within the broader literature.
The review identifies three overlapping mechanisms through which the gut microbiome influences arsenic exposure in the host. First, bacteria in the intestinal lumen biochemically transform arsenic-containing compounds via reduction, methylation, demethylation, thiolation, and oxidation, altering which chemical species are available for absorption. Second, bacterial biomass can sorb dissolved inorganic arsenic and sequester it for fecal excretion, reducing intestinal uptake. Third, arsenic exposure perturbs microbiome taxonomic and functional composition, with downstream effects on host nitrogen homeostasis, energy metabolism, gut immune signaling, and epithelial barrier integrity.
Study design
This is a narrative review, not a primary study. The article reviews in vitro bacterial culture experiments, rodent experiments (germ-free, antibiotic-treated, and humanized gnotobiotic mouse models), ex vivo incubation studies of rodent and human intestinal contents with arsenic-spiked solutions, and observational human cohort studies that collected microbiome composition data alongside arsenic biomarkers or exposure estimates. No new primary data are reported.
Key numbers
The paper does not report primary measured concentration values. It synthesizes findings from cited studies, the most quantitatively specific of which are as follows.
Rodent in vivo exposure reference range: studies cited in Table 1 (pp. 6-7) used inorganic arsenate or arsenite in drinking water at 0.01 to 100 ppm (mg/L) in mice. The authors calculate human equivalent doses (HED) for three representative exposures using the FDA allometric scaling formula (HED equals animal dose multiplied by mouse Km divided by human Km, where mouse Km = 3 and human Km = 37): 10, 25, and 100 ppm mouse exposures correspond to HEDs of approximately 811, 2,027, and 8,108 ppb in humans, respectively (p. 7). The lowest HED (811 ppb) is noted as lower than drinking water arsenic concentrations recently reported in arsenic-rich regions of Chile.
Microbiome perturbation thresholds: significant changes in gut microbial community composition in mice were reported after exposures as low as 10 ppb of inorganic arsenite in drinking water (the current WHO and US EPA maximum contaminant level), based on a cited study (Dheer et al., reference 64 in the paper).
Human cohort findings (cited): in a cohort of US infants with low to moderate arsenic exposure, microbes from 8 genera in the Firmicutes phylum were positively associated with urinary arsenic concentrations at 6 weeks of age, while 15 genera including Bacteroides and Bifidobacterium were negatively associated; effects were stratified by sex and feeding status, with male and formula-fed infants more susceptible to arsenic-related effects on the microbiome (Hoen et al., reference 128). In Bangladeshi children aged 4 to 6 years, high arsenic in home drinking water was associated with greater abundance of Gammaproteobacteria, specifically members of the Enterobacteriaceae family (Dong et al., reference 129).
Protective microbiome findings (cited): in the authors’ own prior mouse study (Coryell et al. 2018, reference 121), antibiotic treatment significantly reduced fecal arsenic excretion and increased host arsenic accumulation in liver and lung tissue; the Gram-positive bacterium Faecalibacterium prausnitzii, but not Gram-negative Escherichia coli, provided protection to gnotobiotic As3mt-knockout mice during exposure.
Methods (brief)
No analytical methods are reported in this review; it describes methods from the primary studies it cites. Arsenic speciation in cited studies is performed by HPLC-ICP-MS in several instances; the review notes the importance of speciation given the large differences in toxicity among inorganic and organic arsenical species. The review discusses dosing considerations for translating animal study results to human relevance, including allometric scaling and the ATSDR lowest-observed-adverse-effect level (LOAEL) of approximately 0.05 mg inorganic arsenic per kg body weight per day for acute oral exposure. Limits of detection, quantification, and certified reference material recovery are not reported in the review itself.
Evidence fitness
This review supports mechanistic and contextual understanding of the gut microbiome as a modulator of arsenic metabolism, bioavailability, and toxicity. It is context-only evidence for occurrence or threshold-setting purposes, as it does not report measured arsenic concentrations in any food matrix or finished consumer product. The review provides strong rationale for including gut microbiome composition as a variable in arsenic exposure risk assessment and in epidemiological studies of arsenic-related health outcomes.
The review does not support brand-level contamination assessments or certification limit determinations. The experimental findings it cites are primarily from rodent models at exposure levels above those typical of dietary intake in non-endemic regions; the human cohort data are observational and cannot establish causation. The review itself acknowledges that few clinical or epidemiological studies have investigated relationships between the gut microbiome and arsenic-related health outcomes in humans, and calls for prospective cohort studies tracking oral or fecal microbial communities over time.
Limitations
The review is a narrative synthesis rather than a systematic review or meta-analysis; study selection criteria are not stated explicitly and the review does not report a search strategy or PRISMA flow. The majority of mechanism evidence cited comes from rodent experiments using inorganic arsenate or arsenite at doses above typical human dietary exposures in non-endemic regions, making direct translation to human dietary risk uncertain. Human cohort data cited are observational and cross-sectional, limiting causal inference. The review notes explicitly (p. 11) that in animal studies it is not always clear whether changes in the host are caused by changes in the microbiota, by arsenic toxicity, or by interactions between them. Inter-study reproducibility is flagged as a perennial challenge for microbiome research, partly attributable to vendor-dependent mouse microbiome variation. Bacterial organoarsenic metabolisms relevant to the human gut (ArsM methylation, ArsI demethylation, ArsH oxidation) have been identified in aerobic bacteria but their prevalence and activity in the predominantly anaerobic human gut microbiome remain unclear at time of publication.
Regulatory context
The review notes the WHO and US EPA maximum contaminant level for arsenic in drinking water is 10 ppb (10 micrograms per liter), and that this concentration is sufficient to perturb gut microbiome composition in mice according to a cited study. The ATSDR toxicological profile on arsenic is cited for the acute oral LOAEL of approximately 0.05 mg inorganic arsenic per kg body weight per day. The International Agency for Research on Cancer classification of arsenic and inorganic arsenicals as Group 1 carcinogens (sufficient evidence for cancers of the lung, skin, and bladder) is noted in the introduction. No regulatory limits on arsenic in specific food matrices are discussed in this review.
Related evidence
- Arsenic, Inorganic
- Arsenic, Total
- microbiome/arsenic-biotransformation-axis
- The gut microbiome is required for full protection against acute arsenic toxicity in mouse models
- Assessing the Role of the Gut Microbiome in Methylmercury Demethylation and Elimination in Humans and Gnotobiotic Mice
- Intestinal Microbiome and Metal Toxicity
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