Optimal Sampling Time for Indigestible Sugar Markers of Gastrointestinal Permeability in Yearling Cattle

Non-metabolizable sugars, such as lactulose and mannitol, are used in veterinary research to assess gastrointestinal permeability; however, there is uncertainty surrounding the persistence in serum. The objectives of this study were to assess the suitability of lactulose and a novel sugar, sucralose, as biomarkers of gastrointestinal permeability and determine the appropriate blood sampling time for peak concentrations in functioning ruminants. Three yearling Holstein steers (290 ± 7.86 kg) were individually fed the same standard grass hay ad libitum for four days and on days 5-9 were restricted to 25% of their ad libitum intake to induce gastrointestinal permeability. On day 9, lactulose and sucralose were orally dosed at a rate of 0.4 g/kg of BW, and blood was obtained via jugular venipuncture at hours 0, 3, 6, 9, 12, 15, 18, 21, 24, 27, 30, 33, and 36 relative to dosing. Serum was analyzed for lactulose and sucralose concentrations using gas chromatography- tandem mass spectrometry by Kansas State University Veterinary Diagnostic Laboratory. Serum sugar concentration for both sugars was analyzed with a linear mixed effects model (R v. 4.4.2) including fixed effects of sugar, time and their interaction and random effect of calf to account for repeated measures. The predicted values for each sugar and timepoint and the residual standard deviation from the model were used to estimate the probability observed concentrations would exceed the limit of quantification (LOQ). The mixed model revealed that time and sugar were significant (P < 0.001), but no interaction was found (P = 0.86). Sugar concentration was at its highest from 3-30 hours following dosing compared to 0 and 33-36 hours after dosing. Additionally, sucralose was detected at significantly higher concentrations than lactulose overall (P < 0.001; 3.81 ± 0.394 ug/mL vs 2.64 ± 0.394 ug/mL). Both sugars had a high probability of exceeding the LOQ at 3 hours (Lactulose = 0.971, Sucralose = 0.963); however, from 6 to 27 hours, the probability of exceeding the LOQ was, on average, 0.891 for lactulose and 0.989 for sucralose. In conclusion, sucralose concentration was greater than lactulose with timepoints from 3 to 30 hours after dosing being equally acceptable sampling timepoints for future evaluations.