Effects of rumen-protected choline supplementation on metabolic responses of dairy cows: a dose-response meta-analysis.

Several studies suggest that dietary supplementation with rumen-protected choline (RPC) during the transition period may support lactation performance in dairy cows, but the metabolic responses underlying these effects remain incompletely understood. We conducted a systematic review and dose-response meta-analysis following PRISMA guidelines to evaluate the effects of RPC supplementation on selected blood metabolites and liver-function indicators in dairy cows. A comprehensive literature search in Web of Science, Scopus, PubMed, and Google Scholar identified 18 articles and one dissertation, comprising 29 studies eligible for meta-analysis. Study quality and risk of bias were assessed using SYRCLE's risk of bias tool. A one-stage random-effects dose-response meta-analysis with restricted cubic splines was used to evaluate the effect of nominal choline chloride dose on blood concentrations of free fatty acids, β-hydroxybutyrate, glucose, total cholesterol, triglycerides, and aspartate aminotransferase. RPC supplementation did not significantly affect blood fatty acid, β-hydroxybutyrate, or glucose concentrations before or after calving. Prepartum total cholesterol showed a non-linear response, with the greatest estimated reduction at approximately 7 g/d of choline chloride and a significant increase from approximately 27 g/d onward. Prepartum triglyceride concentrations were reduced within the 18-21 g/d dose range. Postpartum aspartate aminotransferase activity showed the greatest estimated reduction at approximately 10 g/d, with significant reductions maintained up to approximately 18 g/d. These findings suggest that RPC supplementation may influence selected lipid-related and liver-function indicators during the transition period. However, the dose-response curves are based on nominal choline chloride equivalents and should not be interpreted as estimates of absorbed or bioavailable choline supply. These findings should be interpreted cautiously because methionine supply, DM intake, and product-specific rumen-protection characteristics were inconsistently reported across studies.