Reduced Oral Nutrition Contributes To Gastrointestinal Toxicity Of Total Body Irradiation Via Changes To The Gut Microbiome

Reduced oral nutrition (RON) is common after cytotoxic therapy for hematologic malignancies. We examined the interplay between nutrition, intestinal bacteria and intestinal barrier function in mouse models of total body irradiation (TBI). By 3 days following 9 Gy of a single dose of TBI, C57BL/6 mice had an approximately 50% reduction in oral intake (2g/day, data not shown) and 3 days later displayed marked changes in the microbiome by 16S deep sequencing, including an expansion of mucolytic bacteria Akkermansia muciniphila and Bacteroides thetaiotaomicron (Fig 1). These mice lost roughly 20% of their body weight and showed compromise of intestinal barrier function assayed by oral challenge with FITC-dextran (data not shown). To ask if RON could be driving microbiome changes, we limited the access of unirradiated mice to 2g/day of chow for 7 days. The microbiome was perturbed in these mice with changes reminiscent of that following TBI, including expansion of Akkermansia muciniphila and Bacteroides thetaiotaomicron (Fig 1). The colonic mucus layer was thinned in these mice (Fig 2), though intestinal barrier function remained intact (data not shown). To explore how RON could favor mucolytic bacteria, we characterized metabolic activity of colonic bacteria during RON. Bomb calorimetry demonstrated reduced substrates in the cecum during RON (Fig 2), and colonic luminal pH showed reduced acidity (Fig 2). IC-MS quantified lower concentrations of acetate, propionate, butyrate, and lactate, and elevated succinate (Fig 2). RNA sequencing of stool samples revealed broad downregulation by several intestinal bacterial species of phosphoenolpyruvate carboxykinase, an enzyme known to be associated with production of propionate from succinate (data not shown). Finally, we evaluated in our RON model the impact of administration of oral sugars by supplementing a well-absorbed sugar (glucose), and a poorly-absorbed prebiotic sugar in the drinking water of mice. We found that sugar supplementation with the prebiotic acidified the colonic lumen (data not shown) and prevented thinning of the mucus layer in mice undergoing RON (Fig 3). Additionally, in mice following 9 Gy of TBI, daily dosing of the prebiotic started 24 hours later was highly effective in preventing weight loss (Fig 3), nearly completely abrogated compromise of intestinal barrier function (Fig 3), and improved overall survival (Fig 3). In summary, reduced oral intake following total body irradiation contributes to thinning of the colonic mucus layer, an effect that appears to be independent of the cytotoxic effects on the host and is rather mediated by changes in the intestinal microbiome, including reduced metabolism of organic acids. Strategies to restore colonic acidity with prebiotic sugars may successfully target this phenomenon with potential clinical benefits, particularly in preventing disruption to the intestinal barrier.