Conjugated linoleic acid decreases milk fat globule size by regulating lipid droplet proteins and diameter in dairy goat mammary glands

The size of fat globules in ruminant milk to some extent affects the nutritional quality of dairy products and plays potential roles in infant and adult health. Lipid droplets (LD) in mammary epithelial cells are the precursors of milk fat globules (MFG). However, it is unclear what happens to proteins during the transformation process from LD to MFG, and little is known about the regulation of LD diameter in vivo. In this study, 12 mid-lactation Saanen dairy goats were randomly divided into 2 groups: a control group fed a basal diet and an experimental group fed a basal diet supplemented with CLA at 90 g/d. Goat milk was collected for analysis of composition and MFG size. Mammary gland tissue was collected for analysis of LD diameter and proteins. The size of MFG was found to depend on LD diameter in the mammary glands of dairy goats. The regression equations for MFG size (Y) and LD diameter (x) were Y_D[3,2] = 1.8776x - 1.1984 (R² = 0.7765) and Y_D[4,3] = 2.4898x - 0.4453 (R² = 0.7693), respectively. Proteomic analysis revealed increased expression of proteins associated with lipid droplet autophagy and lipolysis (such as ATG5, ATG7, LDHA, MGL), whereas expression decreased for proteins involved in lipid synthesis (such as PLIN1, FASN, LPL, SCD, APOH). These LD proteins regulated LD diameter through glucose and lipid pathways metabolism, thereby affecting MFG size. Overall, these findings provide the first evidence that LD diameter determines MFG size and highlight the regulatory functions of LD protein in milk fat production.