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Year : 2023  |  Volume : 3  |  Issue : 1  |  Page : 12-16

A perspective on gut health: The redox potential and pH

1 MYAS-NIN Department of Sports Science, Extension and Training Division, ICMR National Institute of Nutrition, Hyderabad, Telangana, India
2 Center of Excellence for Clinical Microbiome Research; Department of Biochemistry, All India Institute of Medical Sciences, Bhubaneswar, Odisha, India

Date of Submission02-Dec-2022
Date of Decision03-Dec-2022
Date of Acceptance03-Dec-2022
Date of Web Publication28-Dec-2022

Correspondence Address:
Balamurugan Ramadass
Department of Biochemistry, Head, Center of Excellence for Clinical Microbiome Research, All India Institute of Medical Sciences, Bhubaneswar - 751 019, Odisha
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Source of Support: None, Conflict of Interest: None

DOI: 10.4103/ghep.ghep_38_22

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The diet, microbiota, and gut epithelium primarily determine gut health. Two fundamental physiochemical factors that regulate and coordinate the interaction between these three determinants of gut health are the redox potential (Eh) and pH. The Eh-pH coordinates determine the solubility, bioavailability, and toxicity of macronutrients and micronutrients. There is spatial heterogeneity in the redox potential, pH, and microbial composition/density both along and across the lumen of the Gastrointestinal (GI) tract. The optimal functioning of any compartment within the GI tract depends on its Eh-pH coordinates. Any abnormal deviation will likely result in pathophysiology and a shift in its resident flora to suit the altered Eh-pH state. Diet and digested products have a significant influence in regulating the local Eh-pH coordinates along with the microbiome and the mucosal secretions. This review emphasizes the importance and the need for simultaneous Eh-pH assessment of the lumen and mucosa of various compartments of GI tract in diagnosis and treatment. Since pH is a well-studied variable in the context of the gut, this minireview will focus on the relation between redox potential and gut health/disease.

Keywords: Gut health, microbiome, redox

How to cite this article:
Venkatesh K, Ramadass B. A perspective on gut health: The redox potential and pH. Gastroenterol Hepatol Endosc Pract 2023;3:12-6

How to cite this URL:
Venkatesh K, Ramadass B. A perspective on gut health: The redox potential and pH. Gastroenterol Hepatol Endosc Pract [serial online] 2023 [cited 2023 Jan 27];3:12-6. Available from: http://www.ghepjournal.com/text.asp?2023/3/1/12/365727

  Introduction Top

The redox potential and pH are measures of electron and proton content (activity), respectively.[1] Any chemical reaction that involves an exchange of protons is influenced by pH. Likewise, any reaction that involves an exchange of electrons is influenced by the environmental redox potential. While pH is a well-established master regulator in physiology, its redox potential has not received the attention it deserves. The redox potential (Eh, measured in millivolts) is positive or negative depending on oxidizing or reducing nature of the environment, respectively. In the past, many studies on microbes, gut health, and diet have addressed Eh and pH independently. However, there is a need to assess these two variables simultaneously as Eh and pH can potentially influence each other either directly (protons neutralize electrons and vice versa) or indirectly by influencing the dissociation of oxidant or reductant.[2],[3] The pH and Eh are critical parameters for living organisms, and their constancy is ensured by utilizing pH buffers and redox buffers for optimal physiological functioning.

There is spatial heterogeneity in the redox potential, pH, and microbial composition/density both along and across the lumen of the GI tract.[4],[5],[6] In addition, microenvironments exhibit varied Eh-pH coordinates within a GI compartment. The niche pH and Eh are essential players contributing to the biogeography of the bacterial microbiota. The oral cavity, stomach, and duodenum are less densely populated (CFU of about 103–105/g of contents). There is a gradual increase in microbial density along the small intestine. The terminal ileum has about 108–109 CFU/g of its contents. A maximal density of 1010–1014 CFUs/g is observed in the colon. The gastric fundus has a low pH (~1–2.5) and high redox potential (+400 to +500 mV).[7],[8] The pH increases along the small intestine and reaches a maximal value in the terminal ileum (~7.4), with a luminal redox potential as low as -150 mV. The colon's lumen is anaerobic and has a redox potential of about −200 to −300 mV.

This perspective envisages redox potential and pH as a central hub of the triumvirate core network consisting of the gut epithelium, microbiome, and diet that reciprocally balances their interaction [Figure 1]. A change in any of these three significant determinants of gut health can be propagated to the other two determinants through Eh and pH perturbations.
Figure 1: Triumvirate core network

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  The Eh-pH and The Microbiome Top

On an Eh-pH plot, individual microorganisms have demarcated distributions based on their natural Eh-pH requirements.[9] In a newborn gut, there is a sequence of progression of microorganisms. Aerobes first dominate the scene, reducing their oxygen content and redox potential, paving the way for facultative anaerobes, followed by obligate anaerobes. This concept is known as metabiosis, where one organism creates a favorable niche for another unrelated organism. Two critical determinants that determine the microbial population present in an ecosystem are its Eh and pH. On an Eh-pH plot, aerobes, facultative anaerobes, and obligate anaerobes are distributed in different regions. Individual species and strains can also be differentiated based on the plot, their maximum oxidation-reduction potential (Eh), and the rate of Eh change upon culturing these organisms in a pH-buffered media.[10] Nutrients, electron acceptors, and a microbial strain's genetically coded enzyme repertoire determine its Eh-pH distribution. Thus, extracellular Eh and pH influence the growth of microorganisms and are reciprocally regulated by these microorganisms. One of the mechanisms by which commensal flora protects against invading pathogens is by regulating the niche Eh and pH to levels that do not allow the growth of pathogens. The importance of Eh and pH in defense against pathogens has been appreciated as early as the 1960s by Professor Meynell.[11] In a classic study, he demonstrated that antibiotic streptomycin induces an increase in redox potential and pH in the mice cecum. This was also associated with a decrease in the average lethal dose for Salmonella typhimurium. In a recent animal study, a cocktail of antibiotics increased fecal redox potential and was associated with a bloom in the family Enterobacteriaceae. Upon discontinuing the antibiotics, the mice cohoused with unaffected gut communities recovered the redox potential and pretreatment microbial composition earlier than isolated mice. Growth and toxin production of certain pathogenic obligate anaerobes such as clostridium depends on the absence of oxygen and the presence of optimal redox potential. Severe acute malnourishment is associated with increased fecal redox potential and depletion of anaerobic and methanogenic prokaryotes.

On the contrary, obesity is associated with increased methanogenic prokaryotes[12],[13] in the colon. Methane can be toxic at the colonic Eh-pH coordinates. Million et al. introduced a Metagenomic Aerotolerant Predominance Index (MAPI) that correlated with fecal redox potential.[14] Venkatesh et al. demonstrated the direct correlation between gastric tissue redox potential and the aerobe/anaerobe ratio (Microbial Redox Index).[15] MAPI and MRI redox potential surrogates need to be extensively used by the research and practicing clinician community to diagnose and assess intervention measures. Pop et al.'s network analysis of bacterial species in moderate-to-severe diarrhea revealed an increase in facultative anaerobes/microaerophiles and a decrease in obligate anaerobes. This observation is suggestive of an increase in redox potential in infectious diarrhea.[16] Thus, the relative abundance of aerobes to anaerobes may serve as an excellent surrogate measure of redox potential in various gut pathologies.[17] Prebiotics has been shown to improve the relative abundance of beneficial obligate anaerobes in the colon, suggesting a likely decrease in gut redox potential.[18],[19]

  Eh-pH and Diet Top

Friedman et al. recently discovered that diet, acting through redox potential perturbation, alters the ruminal microbiome.[20] The stomach is likely to bear maximum insults due to dietary redox perturbations. Its acidic nature limits its microbial content and diversity. Ascorbic acid/dehydroascorbic acid, actively secreted into the lumen, serves as a primary redox buffer at the low gastric pH. Studies have established that ascorbic acid protects against gastric cancer. Helicobacter pylori, implicated in chronic gastritis and gastric carcinoma, is inhibited by ascorbic acid and another reducing agent, bisulfite.[21],[22] These observations suggest that a more oxidized gastric redox state may be linked to the primary pathogenesis of chronic gastritis and gastric cancer.

The Eh-pH coordinates determine the solubility, bioavailability, and toxicity of macronutrients and micronutrients [Figure 2].[23] At low redox potentials, nitrate (NO3-), sulfate (SO42-), Mn4+, Fe3+, and CO2 exist in their reduced forms, namely ammonia (NH4+), hydrogen sulfide (H2S), Mn2+, Fe2+, and methane (CH4), respectively. Oxygen, nitrate, manganese, ferric iron, and sulfate are undetectable below a redox potential of 0.33, 0.22, 0.2, 0.12, and -0.15 V, respectively.[2] The upper GI (oral cavity and stomach) is poised at a higher redox potential in comparison with the lower GI (ileum and colon). The availability of iron and phosphate for aerobes in the upper GI may be compromised due to the Eh-pH coordinates that determine their solubility. Excess iron in the colon can be toxic due to its acidic and reducing environment. Biological phosphate removal and storage by bacteria occurs in aerobic oxidation-reduction potential (ORP) of the oral cavity, whereas fermentative bacteria in the colonic anaerobic ORP release stored phosphorus. Methane and hydrogen sulfide production occurs in the colon's anaerobic ORP. Protein digestion in the upper GI is optimal in an oxidizing environment at a low pH, while protein digestion in the colon is associated with producing toxic ammonia and hydrogen sulfide. Colon is the site for the fermentation of complex carbohydrates.[19] The presence of colonic organisms in the stomach (implies a lowering of redox potential) due to altered pH, as in achlorhydria, is associated with fermentation in the upper GI (as inferred by an increase in facultative and obligate anaerobes) and affects not only protein digestion, but also the generation of vasodilator nitric oxide production.[24],[25] Chronic use of proton-pump inhibitors is also likely to change the ecology of the gastric mucosal microbiome toward fermentative organisms that can grow at a higher pH.
Figure 2: The Eh pH coordinates[23]

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Impaired oral health and an increase in obligate anaerobes in the oral cavity, as in periodontitis, are associated with hypertension and systemic inflammatory response.[26],[27] The negative redox potential of periodontal pockets in periodontal disease supports the growth of anaerobes.[28] Najmanova et al. describe an R/G index in the context of periodontal disease as a measure of periodontal health. The abundance of obligate anaerobes such as Fusobacterium, Treponema sp, and Prevotella has been implicated in periodontal dysbiosis.[29] The green taxa of organisms associated with periodontal health were predominantly aerobes, microaerophiles, or aerotolerant anaerobes.[30] Salivary nitrate availability has been associated with the abundance of aerotolerant health-promoting nitrate-reducing bacteria of the genera Neisseria and Rothia and a decline in anaerobic Prevotella and Veillonella.[31] Nitrate, an oral prebiotic, ensures a slightly negative redox potential in the oral cavity that does not support the growth of obligate anaerobes that thrive at a much lower negative redox potential. Thus, along the gut axis, the redox potential must be maintained high in the upper GI and low in the colon. An imbalance in the redox potential in the upper or the lower GI results in pathology.

The ORP of drinking water can vary with the source. Public water supply systems where chlorination is undertaken can reach ORP values as high as +1020 mV at pH 7.[32] Disinfection by-products (DBPs) are implicated in the incidence of adverse reproductive outcomes and cancers. These DBPs are formed when organic humic substances in water react with oxidizing disinfectants. ORP measures are used as indicators of disinfection. The redox potential of natural water at pH 7–8 can vary between -400 mV and +800 mV.[33] The ORP of shallow groundwater sources and the mobility/toxicity of trace elements such as zinc, copper, cadmium, lead iron, and arsenic will depend on the Eh-pH state. It is also observed that agricultural practices such as the application of nitrate fertilizers and submerged rice cultivation affect the redox state of soil and thereby determine the nutrient status/toxicity of agricultural products consumed by humans. The effect of drinking water ORP on gut health and microbiome needs further investigation.

In general, the Eh of food ranges between -500 and +500 mV.[34] The pH of vegetables ranges from 4.5 to 6.5, while fruits are more acidic with a pH range of 2–6. Polyphenols and Vitamin C are the main reducing constituents found in fruits and vegetables. In meat and animal products, thiols influence the redox state. Cooking by virtue of oxidizing food constituents increases the Eh.

On the contrary, fermented foods can attain lower and negative Eh values. Fermented food is also a good source of Vitamin B and minerals. Fermented food may be acidic, alkaline, or alcoholic based on the dominant microorganisms involved—Lactobacilli, Bacillus, or yeasts, respectively. In a prospective British study, dietary and circulating redox-modulating vitamins and minerals could predict mortality and disease in the elderly. The Eh-pH plot may be applied to individual dietary constituents. This approach must be extended to popular food items and different modes of culinary preparations.

  Eh-pH and the Gut Epithelium Top

The thiol GSH/GSSG couple constitutes the primary intracellular redox buffer. In addition, thioredoxins also function as critical intracellular redox buffers.[35],[36] The cysteine/cystine couple primarily buffers the extracellular environment. Analogous to the pH buffering system, different proportions of oxidized to reduced thiols (GSH/GSSG, Cys/CySS etc.) bring about distinct redox potential states in various intracellular and extracellular compartments. The thiol-disulfide redox status controls mucous fluidity, nutrient absorption, and chemical-induced oxidant injuries. Intestinal cell proliferation, differentiation, and apoptosis are associated with intracellular redox potential transformations.[37] Experimental perturbation of the extracellular redox state also regulates intestinal cell fate.[38] The gut epithelium, with its access to oxygen and nutrients from both blood and the lumen, regulates the extracellular redox environment of the lumen. Colonocyte oxidative metabolism using short-chain fatty acids derived from the microbiota is considered to regulate the availability of oxygen and the growth of facultative anaerobes.[39] SCFA is derived from the metabolism of complex carbohydrates in the diet by the beneficial obligate anaerobes. Mucosal inflammation is associated with higher oxygen availability in the environment and tissue hypoxia which can perturb the microbiome.[40] A high-fat diet impairs mitochondrial oxygen uptake in the enterocytes and increases nitrate availability in the mucous. This effect promotes the growth of oxygen and nitrate utilizing facultative anaerobes.[41] In contrast, in the oral cavity, a low redox potential enhances the pathogenicity/antigenicity of oral anaerobes.[42] Enterocyte metabolism, diet, and the microbiome thus regulate dysbiosis.

  Conclusion Top

Dietary carbon, nitrogen, phosphorus, sulfur sources, their breakdown products, and other redox-active minerals and vitamins influence the Eh-pH state of the gut. Electrochemical ORP values are global indicators of underlying proportions of oxidants and reductants. All the redox couples in a given environment may not be in equilibrium with each other. There are instances where dissolved oxygen may coexist with ferrous iron, hydrogen sulfide, or methane. A measure of common redox active constituents can provide a better assessment of the Eh state. Microbial relative abundance can be a useful measure of redox potential considering the difficulties associated with reproducible Eh measurements. Metagenomic Aerotolerant Predominance Index (MAPI), Microbial Redox Index (MRI), and R/G index described in the literature aim to quantify this elusive but crucial physiochemical variable “Redox potential” in terms of easily measurable microbial surrogates.

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