A Model-Based 13C-Sucrose Breath Test Diagnostic for Gut Function Disorders Characterized by a Loss of Sucrase-Isomaltase Enzymatic Activity.

BACKGROUND: Environmental enteric dysfunction (EED) causes malnutrition in children in low-resource settings. Stable-isotope breath tests have been proposed as noninvasive tests of altered nutrient metabolism and absorption in EED, but uncertainty over interpreting the breath curves has limited their use. The activity of sucrose-isomaltase, the glucosidase enzyme responsible for sucrose hydrolysis, may be reduced in EED. We previously developed a mechanistic model describing the dynamics of the 13C-sucrose breath test (13C-SBT) as a function of underlying metabolic processes. OBJECTIVES: This study aimed to determine which breath test curve dynamics are associated with sucrose hydrolysis and with the transport and metabolism of the fructose and glucose moieties and to propose and evaluate a model-based diagnostic for the loss of activity of sucrase-isomaltase. METHODS: We applied the mechanistic model to 2 sets of exploratory 13C-SBT experiments in healthy adult participants. First, 19 participants received differently labeled sucrose tracers (U-13C fructose, U-13C glucose, and U-13C sucrose) in a crossover study. Second, 16 participants received a sucrose tracer accompanied by 0, 100, and 750 mg of Reducose, a sucrase-isomaltase inhibitor. We evaluated a model-based diagnostic distinguishing between inhibitor concentrations using receiver operator curves, comparing with conventional statistics. RESULTS: Sucrose hydrolysis and the transport and metabolism of the fructose and glucose moieties were reflected in the same mechanistic process. The model distinguishes these processes from the fraction of tracer exhaled and an exponential metabolic process. The model-based diagnostic performed as well as the conventional summary statistics in distinguishing between no and low inhibition [area under the curve (AUC): 0.77 vs. 0.66-0.79] and for low vs. high inhibition (AUC 0.92 vs. 0.91-0.99). CONCLUSIONS: Current summary approaches to interpreting 13C breath test curves may be limited to identifying only gross gut dysfunction. A mechanistic model-based approach improved interpretation of breath test curves characterizing sucrose metabolism.

Mechanistic inference of the metabolic rates underlying [Formula: see text]C breath test curves.

Carbon stable isotope breath tests offer new opportunities to better understand gastrointestinal function in health and disease. However, it is often not clear how to isolate information about a gastrointestinal or metabolic process of interest from a breath test curve, and it is generally unknown how well summary statistics from empirical curve fitting correlate with underlying biological rates. We developed a framework that can be used to make mechanistic inference about the metabolic rates underlying a 13C breath test curve, and we applied it to a pilot study of 13C-sucrose breath test in 20 healthy adults. Starting from a standard conceptual model of sucrose metabolism, we determined the structural and practical identifiability of the model, using algebra and profile likelihoods, respectively, and we used these results to develop a reduced, identifiable model as a function of a gamma-distributed process; a slower, rate-limiting process; and a scaling term related to the fraction of the substrate that is exhaled as opposed to sequestered or excreted through urine. We demonstrated how the identifiable model parameters impacted curve dynamics and how these parameters correlated with commonly used breath test summary measures. Our work develops a better understanding of how the underlying biological processes impact different aspect of 13C breath test curves, enhancing the clinical and research potential of these 13C breath tests.

Performance of empirical and model-based classifiers for detecting sucrase-isomaltase inhibition using the13C-sucrose breath test.

The13C-sucrose breath test (13C-SBT) has been proposed to estimate sucrase-isomaltase (SIM) activity and is a promising test for SIM deficiency, which can cause gastrointestinal symptoms, and for intestinal mucosal damage caused by gut dysfunction or chemotherapy. We previously showed how various summary measures of the13C-SBT breath curve reflect SIM inhibition. However, it is uncertain how the performance of these classifiers is affected by test duration. We leveraged13C-SBT data from a cross-over study in 16 adults who received 0, 100, and 750 mg of Reducose, an SIM inhibitor. We evaluated the performance of a pharmacokinetic-model-based classifier,ρ, and three empirical classifiers (cumulative percent dose recovered at 90 min (cPDR90), time to 50% dose recovered, and time to peak dose recovery rate), as a function of test duration using receiver operating characteristic (ROC) curves. We also assessed the sensitivity, specificity, and accuracy of consensus classifiers. Test durations of less than 2 h generally failed to accurately predict later breath curve dynamics. The cPDR90 classifier had the highest ROC area-under-the-curve and, by design, was robust to shorter test durations. For detecting mild SIM inhibition,ρhad a higher sensitivity. We recommend13C-SBT tests run for at least a 2 h duration. Although cPDR90 was the classifier with highest accuracy and robustness to test duration in this application, concerns remain about its sensitivity to misspecification of the CO2production rate. More research is needed to assess these classifiers in target populations.

Performance of empirical and model-based classifiers for detecting sucrase-isomaltase inhibition using the 13 C-sucrose breath test.

Background: Environmental enteric dysfunction (EED) is a syndrome characterized by epithelial damage including blunting of the small intestinal villi and altered digestive and absorptive capacity which may negatively impact linear growth in children. The 13 C-sucrose breath test ( 13 C-SBT) has been proposed to estimate sucrase-isomaltase (SIM) activity, which is thought to be reduced in EED. We previously showed how various summary measures of the 13 C-SBT breath curve reflect SIM inhibition. However, it is uncertain how the performance of these classifiers is affected by test duration. Methods: We leveraged SBT data from a cross-over study in 16 adults who received 0, 100, and 750 mg of Reducose, a natural SIM inhibitor. We evaluated the performance of a pharmacokinetic-model-based classifier, ρ , and three empirical classifiers (cumulative percent dose recovered at 90 minutes (cPDR90), time to 50% dose recovered, and time to peak dose recovery rate), as a function of test duration using receiver operating characteristic curves. We also assessed the sensitivity, specificity, and accuracy of consensus classifiers. Results: Test durations of less than 2 hours generally failed to accurately predict later breath curve dynamics. The cPDR90 classifier had the highest area-under-the-curve and, by design, was robust to shorter test durations. For detecting mild SIM inhibition, ρ had a higher sensitivity. Conclusions: We recommend SBT tests run for at least a 2-hour duration. Although cPDR90 was the classifier with highest accuracy and robustness to test duration in this application, concerns remain about its sensitivity to misspecification of CO 2 production rate. More research is needed to assess these classifiers in target populations.

13C-sucrose breath test for the non-invasive assessment of environmental enteropathy in Zambian adults.

Objectives: Environmental enteropathy (EE) is a subclinical disorder highly prevalent in tropical and disadvantaged populations and is thought to play a role in growth faltering in children, poor responses to oral vaccines, and micronutrient deficiencies. This study aims to evaluate the potential of a non-invasive breath test based on stable isotopes for evaluation of impaired digestion and absorption of sucrose in EE. Methods: We optimized a 13C-sucrose breath test (13C-SBT) in 19 young adults in Glasgow, United Kingdom. In a further experiment (in 18 adults) we validated the 13C-SBT using Reducose, an intestinal glucosidase inhibitor. We then compared the 13C-SBT to intestinal mucosal morphometry, immunostaining for sucrose-isomaltase (SI) expression, and SI activity in 24 Zambian adults with EE. Results: Fully labeled sucrose (0.3 mg/kg) provided clear breath enrichment signals over 2-3 h in both British and Zambian adults, more than fivefold higher than naturally enriched sucrose. Reducose dramatically impaired 13C-sucrose digestion, reducing 4 h 13CO2 breath recovery by > 50%. Duodenal biopsies in Zambian adults confirmed the presence of EE, and SI immunostaining was present in 16/24 adults. The kinetics of 13CO2 evolution were consistently faster in participants with detectable SI immunostaining. Although sucrase activity was strongly correlated with villus height (r = 0.72; P < 0.05) after adjustment for age, sex and body mass index, there were no correlations between 13C-SBT and villus height or measured sucrase activity in pinch biopsies. Conclusion: A 13C-SBT was developed which was easy to perform, generated clear enrichment of 13CO2 in breath samples, and clearly reports sucrase activity. Further work is needed to validate it and understand its applications in evaluating EE.