110 Effects of different carbohydrates replacing dietary lactose on metabolism of milk-fed calves

Abstract The disaccharide lactose represents the major carbohydrate source in milk-fed calves. Considering the complex and energy demanding processes of lactose synthesis in the mammary gland, the question arises why lactose is highly conserved as milk carbohydrate in evolution. In newborn and growing calves, lactose is enzymatically cleaved in the small intestine by lactase into its absorbable monosaccharides glucose and galactose. We investigated which effects different carbohydrate sources in milk exerted on metabolism of pre-weaned calves. Male and female calves [n = 44, age: 19.0 ± 0.3 d, body weight (BW): 53.7 ± 1.0 kg] were assigned to 6 different experimental diets (4 kg per meal, 2 meals/d) fed for 3 consecutive days. Experimental diets were formulated to achieve a composition similar to whole milk (WM; 4% fat, 3.4% protein, 4.7% lactose) by adding different carbohydrate sources to a milk replacer (based on a reconstituted protein extract with added milk fat): lactose (LAC), glucose (GLU), galactose (GAL), glucose + galactose (G/G), and xylose (XYL). Blood samples were taken from a jugular vein on the last day of treatments, right before feeding in the morning (T1), 2 (T2) and 7 (T3) hours later. Concentrations of glucose, non-esterified fatty acids (NEFA), beta-hydroxybutyrate (BHB), triglycerides (TG), insulin, and glucagon were determined in plasma. Data were evaluated using a mixed model (SAS, version 9.4) with treatment, timepoint of sampling, and treatment x time interaction as fixed effects and the individual calf as repeated subject. Plasma glucose concentration increased after feeding from T1 to T2 in WM and GLU (P < 0.05), but not in calves fed LAC, GAL, G/G, and XYL. In WM, glucose concentrations reached basal values at T3, whereas no changes were observed in the other groups. NEFA concentrations were increased by trend in GAL at T2 (P = 0.06). Plasma BHB concentrations increased (P < 0.05) in all groups except WM from T1 to T2. Blood TG concentrations were increased in all groups except WM at T2 compared with T1 (GLU, GAL, XYL: P < 0.05; LAC: P = 0.08; G/G: P = 0.09). Whereas insulin concentrations increased after feeding from T1 to T2 (WM, G/G: P < 0.05; GLU: P = 0.09), glucagon concentrations were greater at T2 (XYL: P < 0.05; GLU: P = 0.08) and T3 (GLU, G/G, XYL, GAL: P < 0.05; LAC: P = 0.06) compared with T1. In conclusion, we observed different metabolic and endocrine responses to feeding of milk replacers differing in their carbohydrate composition. Although gross nutrient composition of the experimental diets was similar, digestion, absorption, and post-absorptive metabolization seem to differ between the carbohydrate supplements. In calves receiving XYL, a more catabolic status could be shown as XYL is absorbed, but cannot be used in intermediary metabolism.