Using Bomb Calorimetry to Investigate Intestinal Energy Harvest in Anorexia Nervosa: Preliminary Findings on Stool Calorie Loss

Objective: Renourishment and weight restoration are critical first steps in anorexia nervosa (AN) treatment. The ability of the gastrointestinal tract to harvest and utilize energy from food is essential for successful weight restoration, but the functional capacity of the intestine after prolonged caloric restriction remains unknown. In an exploratory study, we quantified the stool energy content of individuals with AN before and after renourishment.

Method: We used archived stool samples from a multisite cohort (NCT03119272) of 103 individuals with AN at admission and discharge from inpatient renourishment and 122 sex-matched non-eating disorder controls (non-ED). To determine whether bomb calorimetry may have utility in AN, we measured stool calorie loss as the number of kilocalories per gram of stool. Analyses included ANOVA, paired/unpaired t-tests, and Spearman rank correlations for associations between stool energy and clinical variables.

Results: Following 25.10 ± 19.73 days of renourishment, patients gained an average of 5.67 ± 4.27 kg. Raw stool energy at discharge (AN-DIS: 5.72 ± 0.86 kcal/g) was significantly higher than admission (AN-AD: 5.41 ± 0.86 kcal/g; p < 0.01). Weight and BMI at admission and discharge were negatively correlated with raw stool energy at discharge.

Discussion: Energy content in stool increases after inpatient refeeding, likely due to increased calorie consumption. Findings should be interpreted cautiously due to methodological limitations in this post hoc study design. Future studies should use a priori, gold-standard protocols to investigate stool calorie loss in AN, as prolonged restriction in AN may disrupt the gut and impede energy harvest.

Trial registration: clinicaltrials.gov: NCT03119272.