Revised
[version 2; peer review: 2 approved]
Tracy N. Phiri 1, James W. Weatherill 2, Elena Monford-Sanchez 3, Jose-Ivan Serrano-Contreras 4, Callum Melvin 5, Mirriam Kunaka 1, Ian Chisenga 1, Perpetual Ngalande 1, Monica N. Mweetwa 1, Ellen Besa 1, Tafhima Haider 6, Nilanjan Mandal 3, Alex J. Thompson 3, Christine A. Edwards 5, Claire D. Bourke 7,8, Ruairi C. Robertson 6, Joram M. Posma 3, Rosemary Banda 1, Mulima Mwiinga 1, Lydia Kazhila 1, Leolin Katsidzira 9, Mutsa Bwakura-Dangarembizi 7,10, Beatrice Amadi 1, Isabel Garcia-Perez 4, Kathryn Maitland 11, Julian R. Marchesi 3, Douglas J. Morrison 2, Gary Frost 4, Paul Kelly 1,6
PUBLISHED 04 Mar 2025
REVIEWER STATUS
The gastrointestinal tract is a critical organ, simultaneously orchestrating digestion, absorption, and excretion of gut contents whilst providing a physical barrier between the external environment in the gut lumen and body tissues, creating conditions to tolerate dietary antigens and the gut microbiome. However, measurement of these intestinal functions, which we refer to hereafter as gut functional capacity, is currently crude and imprecise. There is an urgent need to develop improved tools for the evaluation of a broader range of intestinal functions in studies of pathophysiology and clinical trials in both adults and children. Following a golden age of research on human intestinal digestion and absorption in the 1970s and 1980s, work on gut physiology has slowed. Current gold-standard approaches to assess gut functional capacity, such as the lactulose-mannitol (or lactulose-rhamnose) test or endoscopy, are time- and labour-intensive or measure only one aspect of intestinal dysfunction, such as its permeability to molecules which normally are excluded from uptake in the healthy state.
Environmental enteropathy (EE) is a highly prevalent subclinical inflammatory intestinal disorder that is associated with exposure to unsanitary conditions and poor nutrition. In children, it is associated with growth failure, impaired neurocognitive development, and poor response to oral vaccines. EE is frequently associated with undernutrition. In moderate undernutrition, limited nutrient bioavailability, pathogen pressure, and gut barrier dysfunction can lead to impaired linear growth (stunting), impaired vaccine response and cognitive deficits in children. In acute undernutrition, mortality remains unacceptably high, particularly those hospitalised with severe acute malnutrition (SAM). Both stunting and incomplete recovery from SAM in childhood embed life-long and intergenerational health consequences which detrimentally impact population and economic health in low- and middle-income countries (LMICs).
EE probably represents a chronic adaptation of the proximal small intestine to marginal diets and exposure to environmental enteropathogens. In LMICs, this adaptation develops in early life and is characterised by reduced absorptive surface area, goblet and Paneth cell depletion and intraepithelial lymphocyte infiltration. Our work indicates that SAM is associated with a more severe enteropathy; a global disturbance of intestinal architecture and function, including maldigestion, malabsorption and impaired gut barrier function. Impaired gut barrier function allows translocation of microbes and their products, which may explain systemic endotoxemia and, in some cases, sepsis and septic shock, which are major drivers of mortality. Several non-invasive biomarkers of inflammation and microbial translocation have been proposed as biomarkers of severity of EE. Endotoxin core antibody (EndoCab), lipopolysaccharide (LPS) and myeloperoxidase (MPO) may be predictive of malnutrition status in children. α-1-antitrypsin (AAT), MPO and neopterin, are elevated in malnourished children when compared to healthy children. However, other studies do not report associations with malnutrition or lactulose-rhamnose tests. These inconsistent associations hinder their broader application. Recent research in children with stunting has revealed significantly reduced HLA-DR expression on memory CD4+ and CD8+ T cells, along with a higher proportion of regulatory T cells and classical monocytes compared to non-stunted children. Similarly, children with SAM exhibited lower HLA-DR upregulation on monocytes and neutrophils but demonstrated higher binding capacity for Escherichia coli than children without SAM. These findings suggest an altered immune cell phenotype in children with SAM. Increased microbial metabolites, and acute phase proteins (CRP and calprotectin) and inflammatory markers were associated mortality in children with SAM. These findings are further supported by another study which showed that markers associated with higher gut and systemic inflammation may be associated with higher mortality or hospital re-admission. Additionally, biomarkers of environmental enteropathy (EE) may correlate with antigen-specific immune responses in SAM children at 18 months of age. In the same study, maternal CRP and stool neopterin concentrations during pregnancy were associated with stronger heat-killed Salmonella typhimurium-specific IL-8 responses in the children at 18 months. Furthermore, children who received water, sanitation, and hygiene (WASH) interventions demonstrated higher inducible LPS-specific myeloperoxidase (MPO) levels compared to those who did not receive such interventions. These findings underscore the complex interplay between the immune system, SAM, and EE. Further research is needed to better understand and differentiate the immune responses in SAM and EE, across both children and adults.
Both SAM and the antibiotic treatments recommended for children admitted to hospital with SAM also affect the gut microbiome, the composition of which is associated with lower bacterial diversity and maturity, which fail to persistently recover with standard nutritional therapies. Characterization of the small intestinal microbiome in children is challenging due to the invasiveness of endoscopy procedures. Some studies showed through culture methods that small intestinal bacterial overgrowth (SIBO) is associated with stunting in children from Bangladesh, Central African Republic and Madagascar. These studies also provided the first evidence, using 16s sequencing, of decompartmentalization of the microbiome along the GI tract in stunted growth and EE and demonstrated a loss of carbohydrate-metabolising microbes in the small intestine. These findings suggest that small intestinal microbes contribute to the pathophysiology of stunting however, these findings are relatively novel and need validation with different cohorts. There is a need to take a holistic approach to understanding the impact of undernutrition and EE on gut physiology across the whole gastrointestinal tract, which means developing tools for assessing multiple domains of gut function. This necessitates simultaneous assessment of the structure of the intestine (e.g. villus height, and therefore surface area), barrier function and microbial translocation, digestive and absorptive capacity (e.g., enzyme activity for protein, fat, and carbohydrate digestion, transporter expression for absorption), systemic, and intestinal immune responses to pathogens, expression of epithelial pattern recognition receptors (PRRs), pathogen-associated molecular patterns (PAMP; an indi
