Gastrointestinal permeability in calves: Validation of indigestible sugar markers and effects of dietary starch inclusion

Abstract

Gastrointestinal (GI) permeability refers to an animal’s ability to regulate the passage and absorption of nutrients, toxins, and even pathogens from the lumen of the gut into the bloodstream. During episodes of increased GI permeability, animals can experience increased systemic inflammation, decreased performance, and it has been hypothesized that increased GI permeability can lead to severe conditions such as liver abscesses (LA). Liver abscesses are topics of concern in feedlot animals, especially beef-dairy (BD) cross animals. Overall, increased GI permeability can negatively affect animal performance, health, and welfare, as well as farm productivity, making it an increasingly interesting issue in production animals. Currently, there are several gaps in our knowledge of the causes and consequences of increased GI permeability in cattle. Therefore, the objective of this thesis was to identify current knowledge and knowledge gaps and to validate methods of diagnosis of GI permeability, as well as evaluate the effect of starch inclusion level in starter feed. For the first chapter, a literature search was performed to identify published studies that provided valuable information on current knowledge of etiological risk factors, consequences, diagnostic methods, and mitigation strategies of increased GI permeability. The literature search resulted in 69 publications that were scanned for relevant information, with 37 being retained for the literature review. The primary etiological risk factors identified throughout the literature were nutrition, physiological stressors (e.g., weaning and heat stress), environmental, illness, and inflammation. Each of these etiological risk factors includes multiple mechanisms that can impair GI intestinal barrier function and contribute to increased GI permeability in cattle. Consequences of increased GI permeability were also identified with the main consequences negatively effecting animal performance, exacerbating inflammation, and contribution to the development of other disease conditions. These consequences are being studied through diagnostic methods for research settings that help study the GI permeability including quantifying inflammatory markers, using exogenous tracers, and evaluating tight junction gene expression. These diagnostic methods are only for research settings, therefore, identifying increased GI permeability in production settings is difficult and mitigation strategies are employed to reduce the effects of altered epithelial integrity. Throughout the literature, mitigation strategies re-volved around animal management and nutrition. Primary mitigation strategies include nutritional management and stress reduction for animals at higher risk for exhibiting hyperpermeability in the GIT. Overall, GI permeability in cattle remains a topic requiring further study to better understand the impacts and possible mitigation strategies in improving animal health, performance, and welfare for animals in production systems. Objectives of the first study were to assess the suitability of lactulose and sucralose, a novel sugar, as tracers of GI permeability and to determine appropriate blood sampling time for peak concentrations in functioning ruminants. Three yearling Holstein steers (290 ± 7.86 kg) were individually fed standard grass hay, ad libitum, before being restricted to 25% of their ad libitum intake to induce gastrointestinal permeability. An oral dose of lactulose (0.4 mg/ kg of body weight) and sucralose (0.4 mg/ kg of body weight) was administered and blood sampled at time 0 and every 3 hours until 36 hours post dosing. Serum was extracted and analyzed for sugar concentration by gas chromatography-tandem mass spectrometry. Arithmetic mean concentrations were 4.05 µg/mL for sucralose and 2.78 µg/mL for lactulose from 3-36 hours post dosing. The predicted probabilities revealed that at three hours both sugars had high probabilities of exceeding LOQ (Lactulose = 0.805, Sucralose = 0.905). In addition, the probability of sucralose being detected above LOQ was higher over the entire time period from 3-36 hours compared to lactulose (Lactulose = 0.857, Sucralose = 0.901). To conclude this study, the probability of sucralose concentration exceeding LOQ was consistently higher than lactulose indicating sucralose could be used as a more quantifiable and effective biomarker than lactulose with time points between 3 and 33 hours being equally acceptable time points for blood sampling. Objectives for the second study were to assess the effect of starch inclusion level in calf starter, fed during the hutch phase, on growth performance, rumen pH, GI morphology, and GI permeability during weaning and the finishing transition periods. Twenty beef-dairy (BD) cross calves, one week of age, were randomized to either a high (HS) or low starch (LS) calf starter diet, fed only during the hutch phase. During weaning rumen pH boluses were placed in four animals (n=4; HS=2, LS=2) and continuously measured rumen pH for a period of time. Dry matter intake and BW were collected and used to calculate ADG and G:F in each phase of the study. After the weaning period 10 animals (n=10, HS=5, LS=5) continued on to the grower phase, where they were fed a common grower diet for 131 days until moving on to the finishing transition. At weaning and finishing transition, GI permeability was assessed in all animals by orally dosing two indigestible sugars (lactulose and sucralose) and evaluating serum-sugar con-centration at 3 separate timepoints in each phase. At the end of both hutch and finishing phases 10 animals (n=10, Hs=5, LS=5) were harvested and GI contents and tissues were collected from the ventral sac of the rumen, duodenum, jejunum, ileum, and spiral colon. During the hutch phase, pre-weaning BW increased over time from day 0 to day 54 and the LS calves, on average weighed more than the HS calves (P=0.034), whereas, post-weaning BW showed no differences between treatments. Similarly, pre-and post-weaning ADG, DMI, and G:F revealed no treatment differences or interactions (P>0.05). Hutch phase GI morphology and GI permeability was also similar between treatments. Finishing phase performance was minimally affected by hutch phase treatment diets. Rumen morphology in the finishing phase was different between treatments, as papilla width (0.049) was greater in the HS group than in the LS group and there were tendencies for other rumen histologic measurements to be greater in the HS group (P ≤0.1). In the finishing phase, there were no other significant effects for GI permeability and GI morphology. Overall, starch inclusion level in calf starter diets had minimal effects on animal performance, GI permeability, and GI morphology in the hutch phase and minimal carryover effects in the finishing transition. These projects emphasize the difficulties and unknowns of researching, diagnosing, and mitigating GI permeability in cattle. Utilizing exogenous tracers and proven methods of inducing permeability as useful research approaches can provide insights that help bridge current knowledge gaps. This work establishes a foundation for future studies aimed at improving the GI health, performance, and welfare of animals at higher risk of exhibiting increased GI permeability.