Lactopontin in a simulated infant formula protein matrix promotes bone development via the gut–bone axis in growing rats

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.