Mercury Metabolism by Bacteria in Humans

Study Overview
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Brief Summary
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The purpose of this study is to evaluate how the bacteria in your gut can improve the break-down and de-toxification of non-harmful levels of a naturally occurring form of mercury (methylmercury) that comes with eating fish. This research could help scientists and doctors understand whether or not mercury in fish that we are likely to eat poses any concern for the health of people.

Detailed Description
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The overall objective of this study is to investigate the role of gut microbes in mediating how humans metabolize and excrete the environmental neurotoxicant methylmercury (MeHg). Exposure to MeHg through consumption of fish continues to pose a health risk for many populations globally. There remains considerable uncertainty in advising the public on mercury risks associated with fish consumption. A great deal of uncertainty stems from the fact that the MeHg metabolism and elimination rate is known to vary widely from individual to individual. This translates into the possibility that two individuals consuming the same amount of fish with the same frequency could, unknowingly, experience as much as 4-fold difference in accumulation of MeHg in their bodies. Thus, there is a need for greater understanding of the mechanisms of MeHg metabolism and elimination, as well as for development of tools to assess these characteristics in people. Our scientific premise is that symbiotic microbes in the human gut are required for the efficient biotransformation (demethylation) and excretion of toxic MeHg. In this prospective intervention study we will examine the variation in the rate at which MeHg is excreted, both between human subjects and within subjects over time, and relate it to the MeHg demethylation activity that is harbored in their respective gut microbes. Furthermore, through intervention with a prebiotic dietary supplement, we will induce a change in the gut microbial composition within the same individual and evaluate if slower of faster MeHg metabolism ensues. With these approaches we will obtain gut microbiome samples that correlate with faster or slower MeHg elimination kinetics. We then aim to identify specific genera and species of bacteria in the human gut responsible for MeHg metabolism. We will do this by feeding volunteers fish meals with documented trace levels of MeHg that are below any harmful level of exposure. We will subsequently measure kinetic rates of MeHg elimination via mass-spectrometry analysis of hair strands. We will also sample feces from the subjects as a source of the gut microbiota and as a medium to analyze the extent of MeHg metabolism (demethylation) that parallels its elimination. Study team members at Montana State University will directly examine the ability of the human gut microbiota to induce MeHg metabolizing activity in germ-free mice at rates that correspond with that seen in the human subject it was derived from. In parallel, we will use metagenomic sequence-informed strategies to bring isolated strains of the human gut bacteria to culture and subsequently interrogate their MeHg demethylating activity. We anticipate our results will lead to a clearer understanding of the microbial basis of human MeHg metabolism.

Official Title
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Microbial Mechanisms of Methylmercury Metabolism in Humans

Conditions
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Mercury--Toxicology

Intervention / Treatment
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* Other: Tuna fish
* Dietary Supplement: prebiotic


Participation Criteria
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Eligibility Criteria
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Inclusion Criteria: Subjects must be 18-80 years in age. Subjects must be in good general health based on self-reported health status, with the exception of self-reported conditions listed in the exclusion criteria. Subject must be willing to comply with study procedures. Exclusion Criteria: Known allergy to fish. Pregnant or lactating women. Use of hair dyes or chemical treatments within the prior month and/or intent to use such treatments during the two trial periods. (Hair treatment during the washout period up to one month prior to the second trial is acceptable). Known gastrointestinal or renal disorders. Subjects with diminished mental capacity. Use of antibiotics within two months leading up to the study.

Ages Eligible for Study
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Minimum Age: 18 Years
Maximum Age: 80 Years

Sexes Eligible for Study
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All

Accepts Healthy Volunteers
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Accepts Healthy Volunteers

Study Plan
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How is the study designed?
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Design Details

Primary Purpose: Basic Science
Intervention Model: Single Group Assignment
Masking: None (Open Label)


Arms and Interventions

| Participant Group/Arm | Intervention/Treatment |
| --- | --- |
| Experimental: All Participants<br>All participant will eat 6 meals of tuna fish. All participants will take a prebiotic dietary supplement. | Other: Tuna fish<br>* Six meals of tunafish ( 200gms) will be consumed. Three meals will be consumed in a period of 14 days, and a subsequent 3 meals will be consumed within a 14 day period six month after the first 3 meals.<br>Dietary Supplement: prebiotic<br>* Prebiotin brand prebiotic will be consumed at 8grms/day for a period of 75 days. The prebiotic administration will begin with the second grouping of 3 fish meals.<br>|

What is the study measuring?
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Primary Outcome Measures

| Outcome Measure | Measure Description | Time Frame |
| --- | --- | --- |
| Mercury Elimination Rate Before Pre-biotic | Mercury elimination rate will be determined for each individual at the beginning of the study, prior to any prebiotic administration. This determination will take place over the first 75 days of the study, wherein the first three fish meals are consumed, a eliminaiton period follows and then hair is sampled and analyzed. Time-resolved levels of mercury will be determined by longitudinal analysis of single hair strands using mass spectrometry methods. The values will be used to calculate the mercury elimination rate for each individual. The average elimination rate across the cohort before prebiotic will be reported. This total analysis will require approximately 2.5 months. | 2.5 months | 
| Mercury Elimination Rate After Pre-biotic | Mercury elimination rate will be determined for each individual after commencing prebiotic self-administration for a period of 75 days, contemporaneously with the second set of fish meal consumption. This will begin approximately 6 months after the finish of the first round of fish meals and elimination (i.e. approximately 8.5 months after the participant starts the study). This second round will last approximately 2.5 months. Time-resolved levels of mercury will be determined by longitudinal analysis of single hair strands using mass spectrometry methods. The values will be used to calculate the mercury elimination rate for each individual. The average elimination rate across the cohort after prebiotic, from approximately 8.5 to 11 months will be reported. | 11 months | 

Secondary Outcome Measures

| Outcome Measure | Measure Description | Time Frame |
| --- | --- | --- |
| Change in Mercury Metabolism Ratio | Mercury metabolism ratio, i.e. conversion of methylmercury to inorganic mercury, will be determined for each individual before and after prebiotic administration. The difference in average metabolism ratio across the cohort before and after prebiotic will be reported. Mercury elimination ratios will be measured in stool samples using mass spectrometry methods. | Baseline to 10 months |