Lactopontin (LPN) is an important milk protein with the potential to improve bone health; however, its specific effects have not been determined. This study aims to investigate the effects of LPN on early bone growth and development. 3 week-old SD rats (n = 32) were assigned to the control group, whey protein concentration (WPC) group, LPN-L (low-dose LPN) group, and LPN-H (high-dose LPN) group, with intragastric administration of deionized water, 65.8 mg kg⁻¹ WPC, and 5 and 45 mg kg⁻¹ LPN for 30 days, respectively. LPN-H supplementation increased body length and femur length, improved femoral bone volume and microarchitecture, and upregulated osteoblast activity-related mRNA (Bmp2, Smad8, and Runx-2) expression in the femur. The content of secondary bile acid glycolithocholic acid (GLCA) in stool and serum was significantly increased after LPN-H intervention and positively correlated with the increased abundance of Parabacteroides in feces. In addition, the bile acid receptor FXR in femur was also activated in the LPN-H group, suggesting that LPN may regulate bone health through the microbiota–metabolite–bone axis. The results of this study suggest that LPN has potential application value in regulating bone metabolism during growth.

Background: Lactopontin (L-OPN) is a pivotal bioactive protein present in breast milk that supports bone development, but its efficacy in a formula matrix is unknown. This study aimed to evaluate the effects of L-OPN-fortified formula on bone growth in a growing rat model and to explore the underlying mechanisms. Methods: Weanling rats (n = 8/group) received daily gavage for four weeks: (1) CON—deionized water; (2) PRO—750 mg/kg·BW mixed protein; or (3) L-OPN—750 mg/kg·BW of the PRO formula fortified with L-OPN. Results: The results showed that the formula fortified with L-OPN could significantly increase bone volume and trabecular bone number (p < 0.05). Furthermore, both femur length and thickness, as well as overall body length, were significantly increased (p < 0.001). In addition, the L-OPN-fortified formula specifically increased the relative abundance of Bacteroides and Parabacteroides in rat feces (p < 0.05). Metabolomic analysis revealed that L-OPN supplementation significantly altered bile acid metabolism, notably increasing serum levels of 12-ketolithocholic acid (12-KLCA), which correlated strongly with bone metrics. Conclusions: These preclinical findings provide a basis for future research in infant formula.