Reconsidering early-life bile acid amidation defect in environmental enteric dysfunction

Abstract

Hasan et al presented compelling cross-sectional evidence that Bangladeshi infants aged 6-9 months with environmental enteric dysfunction (EED) exhibit an age-dependent defect in primary bile acid conjugation, concurrent elevations in unconjugated and sulfated primary bile acids, reduced primary conjugated bile acids, and associations between bile acid profiles, small intestinal bacterial overgrowth, enteric inflammation, and impaired linear growth. Their findings have raised important mechanistic and translational questions: Was delayed maturation of bile acid amidation a driver of malabsorption and growth faltering in EED, or a downstream marker of intestinal injury and microbiome immaturity? Did altered bile acid pools contribute to intestinal inflammation and barrier dysfunction via signaling pathways (farnesoid X receptor/Takeda G protein-coupled receptor 5) or via nutrient malabsorption, like lipid and fat-soluble vitamins? Could targeted bile acid replacement, like glycocholic acid or microbiome-directed interventions, offer therapeutic benefit in selected infants? This Letter highlights these key considerations, outlines testable hypotheses and practical next steps for the field, and calls for coordinated longitudinal, mechanistic, and interventional studies to clarify causality and translate these observations into interventions that might mitigate EED-associated stunting.

Key Words: Bile acid metabolism; Malnutrition; Environmental enteric dysfunction; Small intestinal bacterial overgrowth; Bangladesh

Core Tip: This study identifies a previously unrecognized defect in bile acid conjugation among Bangladeshi infants with environmental enteric dysfunction, linking altered bile acid metabolism to poor growth and intestinal inflammation. Elevated unconjugated primary bile acids were strongly associated with anthropometry. Further, the findings highlight a possible age-related delay in the maturation of bile acid conjugation pathways in impoverished children. This study may provide the initial insights for exploring novel therapeutic targets through bile acid pathways for treating malnourished children worldwide.

TO THE EDITOR

Hasan et al[1] performed a rigorous study that highlights bile acid metabolism as a potentially pivotal axis in the pathophysiology of environmental enteric dysfunction (EED) and early childhood growth impairment. Their careful bile acid profiling provides a rich dataset with confirmation of prior hints in the literature and opens several new avenues for inquiry in paired serum and stool samples from Bangladeshi infants combined with markers of small intestinal bacterial overgrowth (SIBO) and intestinal inflammation[2,3]. By outlining the most salient implications, further steps about methodological considerations, mechanistic hypotheses, and pragmatic approaches could accelerate the translation of these findings into clinical impact. In this letter, we refer to “bile acid amidation (conjugation) defect” (hereafter “amidation defect”). Amidation, sometimes referred to as bile acid conjugation, is the hepatic enzymatic addition of glycine or taurine to primary bile acids, a process catalyzed by bile acid-CoA:amino acid N-acyltransferase (BAAT) and related enzymes. The present letter uses “amidation defect” consistently below to denote reduced hepatic conjugation of primary bile acids.

Hasan et al[1] distinguish delay from defect in conjugation with biological and clinical implications, and interpret the pattern through reduced primary conjugated bile acids in young infants with normalization in older children as consistent with a delayed maturation of bile acid conjugation rather than an intrinsic genetic BAAT/bile acid-CoA ligase (SLC27A5) defect[4]. A maturational delay would imply reversibility and opportunity for intervention; this interpretation is both plausible and clinically optimistic[5]. However, the cross-sectional sampling of distinct age cohorts seems not to exclude survivorship, nutritional, or microbiome-selection biases. Longitudinal sampling of the same infants from birth through 24 months, combined with serial bile acid panels, nutritional assessments, and genotyping for BAAT/SLC27A5 variants, would more definitively distinguish delayed maturation from congenital or acquired defects.

As for mechanistic pathways linking conjugation deficits to growth and inflammation, there are two non-mutually exclusive mechanisms meriting prioritization: (1) Malabsorption pathway: Lack of conjugation impairs micelle formation as well as reducing lipid and fat-soluble vitamin absorption, with nutritional deficits directly impairing growth. The success of glycocholic acid in genetically confirmed amidation defects underscores the plausibility of this route[6]; and (2) Signaling and immunomodulation pathway: Changes in bile acid composition alter signaling through farnesoid X receptor (FXR)/Takeda G protein-coupled receptor 5 (TGR5) and other receptors with downstream effects on intestinal barrier integrity, mucosal immunity, and epithelial regeneration[7-9]. Elevated unconjugated and sulfated primary bile acids may differentially engage these receptors and microbiome interactions, potentially exacerbating enteropathy[10,11]. Therefore, teasing apart these pathways requires integrated functional readouts, including lipid absorption tests, fat-soluble vitamin kinetic studies, epithelial permeability assessments, and receptor activation assays in patient-derived samples or relevant ex vivo models.

In addition, the microbiome-bile acid-host triad raises doubts about causation, consequence, or feedback loop. Hasan et al[1] report associations between SIBO area under the receiver operating characteristic curve and decreased secondary conjugated bile acids, consistent with bacterial deconjugation or dehydroxylation activity altering bile pools[12]. Yet SIBO could be both a cause and consequence of altered bile acid signaling that reduced conjugates might be less bacteriostatic in the small intestine, permitting overgrowth, while SIBO itself modifies bile acid composition[13]. Longitudinal microbial metagenomics/metatranscriptomics paired with bile acid metabolomics would help resolve directionality. Gnotobiotic or antibiotic-manipulation models using microbiota from affected children can provide causal insights into how specific bacterial consortia modify bile acid conjugation, absorption, and host outcomes.

For translational potential and cautions for bile acid-targeted interventions, the reported therapeutic benefit of oral glycocholic acid in genetic amidation defects offers a rational translational hypothesis that targeted bile acid replacement, such as glyco-conjugated or tauro-conjugated cholic acid, might restore micellar function and vitamin absorption, improve growth, and reduce enteropathy in infants with acquired conjugation delay[5]. Before human trials, it’s warranted for preclinical safety and pharmacokinetic studies in malnourished or EED-model animals. Key questions should be addressed, including optimal dosing in infants, effects on microbial ecology of supplementation that select for pathogenic taxa, hepatic handling in the context of immature liver function, and potential unwanted signaling through FXR/TGR5 to perturb metabolic homeostasis. Parallel strategies should be considered either alone or in combination with bile acid replacement, considering microbiome modulation like selective antibiotics, prebiotics, or probiotics, nutritional rehabilitation with focused lipid and fat-soluble vitamin supplementation, and interventions for promotion of ileal epithelial recovery.

For diagnostic and prognostic implications, serum bile acid patterns with elevated unconjugated primaries, high sulfated primaries, and low conjugates could serve as a noninvasive biomarker panel for early EED phenotyping to stratify infants for targeted interventions after validation longitudinally. However, the feasibility of such assays in low-resource settings remains a challenge, which requires cost-effective assay development and field validation. The integration of bile acid profiling with established EED biomarkers of myeloperoxidase, neopterin, and lactulose-mannitol ratios may yield composite diagnostics with higher specificity for mechanistic subtypes, for example, conjugation-defect predominant vs inflammation-predominant EED.

The present letter to the editor notes several limitations of Hasan et al’s study[1]. First, the cross-sectional nature of the dataset using distinct age cohorts rather than longitudinal sampling limits causal inference and leaves open survivorship, sampling, and confounding biases. Second, while Hasan et al[1] have discussed potential FXR/TGR5-mediated signaling effects, the original dataset did not include measurements of host expression of FXR target genes or downstream transcriptional readouts. Thus, conclusions about receptor activation remain inferential. Third, the sample size of some comparator groups, for example, the high-income country control, was relatively small, limiting statistical power for subgroup analyses and generalizability. Finally, the generalizability of findings across different geographic and ethnic populations remains to be confirmed in larger prospective cohorts.

After fully evaluation of methodologic strengths and limitations, there are strengths including concurrent serum and stool sampling, expanded bile acid panel like sulfated forms and muricholates, and inclusion of high-income country controls to provide a multifaceted picture. Limitations consist of a cross-sectional design, a non-paired older cohort, potential unmeasured confounders, and a relatively small American comparator group. Future studies should prospectively control for these variables with larger and ethnically diverse controls. The next step initiates with longitudinal birth cohort studies in EED-endemic settings with serial bile acid metabolomics, microbiome sequencing, dietary and clinical phenotyping, and genotyping for BAAT/SLC27A5 variants. The development of preclinical models, such as murine or organoid, is warranted using microbiota and sera from affected infants to test causality and mechanistic pathways. In addition, the inclusion of pilot randomized and mechanistically informed interventions consists of short-term oral conjugated bile acid supplementation with rigorous safety monitoring and absorption endpoints, microbiome-targeted modulation, as well as combined nutritional and bile acid therapy. Moreover, the simplified and field-adapted bile acid assay panels should be validated with an assessment of predictive value for growth outcomes.

CONCLUSION

Hasan et al[1] have provided a crucial dataset that reframes bile acid dysmetabolism from a peripheral observation to a central candidate mechanism in EED-associated growth failure. The study elegantly demonstrates associations with biological plausibility and potential actionability. The research community should be urged to prioritize longitudinal and mechanistic studies and to design cautious and hypothesis-driven interventional trials. Since the delayed bile acid conjugation proves causal and reversible, this discovery could transform therapeutic approaches to a major global health problem and exemplify how metabolic, microbial, and developmental biology converge to shape childhood growth. Further steps with a multicenter prospective protocol should be proposed by harmonizing bile acid metabolomics, microbiome analyses, nutritional phenotyping, and interventional pilot trials to accelerate translation of these findings into meaningful clinical impact.