Metabolomic changes in the liver tissues of cows in early lactation supplemented with dietary rumen-protected glucose during the transition period

A fat-coated rumen-protected glucose (RPG) product was used to meet the high energy requirements of transition-period cows. Twelve multiparous Holstein cows (age, 4–5 years; body weight, 515 ± 42 kg; milk yield, 16.1 ± 3.7 kg/d) were blocked by body weight, previous milk production, and calving date and were assigned to two groups: the control (CON) group and the RPG group. For treatment, the CON group was fed a basal diet plus 90 g of coating fat, and the RPG group was fed the basal diet supplemented with RPG at 200 g/d. The treatments were continued from 7 d before to 14 d after calving. Five cows from each group were humanely euthanized on day 14 after calving, and liver tissues were obtained. The liver samples were analysed using untargeted liquid chromatography-tandem mass spectrometry (LC-MS/MS)-based global metabolomics to investigate potential hepatic metabolic mechanisms related to nutrient metabolism in cows in early lactation supplemented with RPG during the transition period. A total of 172 significantly differentially expressed metabolites between the RPG and CON groups were identified. Compared with the CON group, the RPG group exhibited a higher level of beta-D-glucose 6-phosphate (log₂[fold change, FC]= 1.49, P = 0.018) and lower levels of riboflavin (log₂FC = 0.64, P = 0.002), flavin mononucleotide (FMN; log₂FC = 0.71, P = 0.034), L-tryptophan (log₂FC = 0.83, P = 0.022) and L-serine (log₂FC = 0.68, P = 0.034). The differentially expressed metabolites were significantly enriched in 18 Kyoto Encyclopedia of Genes and Genomes (KEGG) pathways (P < 0.05), including the riboflavin metabolism pathway, glucose metabolism pathways (glycolysis/gluconeogenesis, the pentose phosphate pathway), and amino acid metabolism pathways (tryptophan metabolism; glycine, serine and threonine metabolism). The results of the present study help create a comprehensive and detailed understanding of the hepatic metabolic responses of transitioning dairy cows to exogenous glucose. These results may improve our understanding of the metabolic impact and mechanisms of action of RPG in dairy cattle.