Differences in Digestibility Impact Glucose Regulation, Thermic Effect of Food, and Gut Transit Time in Healthy Adults

Objectives: Intestinal energy absorption (i.e., digestibility) is incomplete, with ∼2-9% of energy consumed from the diet excreted in stool. This study aimed to determine how differences in digestibility impact markers of glucose regulation, thermic effect of food (TEF), and gastrointestinal function. Methods: Healthy adults [n=16 (8m/8f), 31 ± 5 y, 23.8 ± 3.4 kg/m2] followed a controlled diet for 9 d. The energy and macronutrient content of the diet and stool were measured by bomb calorimetry and chemical analysis, respectively. Participants were dichotomized by mean digestibility and classified as absorber (ABS) or excreter (EXC). Participants wore a continuous glucose monitor to measure 24 h glucose concentrations and variability. TEF was measured for 4 h after a mixed meal challenge by indirect calorimetry. Segmental gut transit time was measured with a gas-sensing capsule and stool microbial composition was analyzed by 16S rRNA amplicon sequencing. Results: Energy digestibility was higher in ABS (93.0 ± 0.9%) compared to EXC (90.4 ± 0.6%; P< 0.0001). Fat and protein digestibility were higher in ABS (P< 0.02); however, there were no differences in carbohydrate digestibility (P=0.43). Even so, glucose variability was higher in ABS (MAGE: 54.8 ± 12.1 mg/dL vs. 38.3 ± 11.9 mg/dL; P=0.02). Glucose concentrations were lower overnight in ABS (88.6 ± 6.9 mg/dL) compared to EXC (101 ± 13.0 mg/dL; P=0.04), but there were no differences in mean glucose during the daytime (P=0.18) or over the 24-h period (P=0.09). In ABS, colonic transit time tended to be slower (34.0 ± 15.8 h vs 17.6 ± 9.5 h; P=0.07) and small bowel transit was faster (3.0 ± 1.1 h vs. 4.4 ± 1.5 h; P=0.04). When TEF was expressed as a percent of gross energy intake, ABS expended more energy on digestive processes compared to EXC (6.6 ± 1.8% vs 4.5 ± 2.2%, P=0.04). Findings were similar after adjusting for digestibility between ABS (7.1 ± 1.9%) and EXC (4.9 ± 2.4%, P=0.06). There were no significant differences in stool microbial composition. Conclusions: These data suggest intestinal energy absorption is related to glucose regulation. Personalized nutrition recommendations based on individual differences in energy digestibility may be beneficial for improving glucose regulation and overall health. Funding Sources: 2021 Herbalife Scholarship; Florida State University. Authors' views do not reflect official DoD or Army policy.