Dual Osteoprotective Actions of the Kefir Peptide KFP-1: Enhancement of Bone Formation and Suppression of Bone Resorption in Cells and Murine Models

KFP-1, a kefir-fermented peptide, promotes osteoblast differentiation and bone formation while inhibiting osteoclast differentiation and bone resorption. We also confirmed the bioaccessibility of KFP-1 in bone tissue and its overall benefits for bone health. Kefir is a dairy beverage rich in bioactive peptides with diverse health benefits. We identified a kefir-fermented peptide, KFP-1 (TEVPAINTIASAEPTVH), which enhances intestinal calcium absorption and may modulate bone remodeling. This study investigated the effects of KFP-1 on osteoblast and osteoclast gene expression and differentiation in BMMSCs, MC3T3-E1, BMMs, and Raw264.7 cells. Using iodine-125 labeling, we tracked KFP-1 distribution in mice following oral and intravenous administration, and evaluated its osteoprotective effects in AKR1A1 knockout (AKR1A1-KO) osteoporotic mice. KFP-1 upregulated osteogenic markers (ALP, Col1a1, OCN, OPG, RUNX2, OSX, BMP-2, β-catenin) and promoted osteoblast differentiation and mineralization, while downregulating osteoclastic markers (CTK, CTR, DC-STAMP, TRAP, c-Fos, c-Src, NFATc1) and inhibiting osteoclast differentiation and resorption via the inhibition of NFATc1, c-Fos, and c-Jun nuclear translocation and attenuation of RANKL-induced p38 MAPK, JNK, ERK, and NF-κB signaling. Biodistribution analysis showed KFP-1 reached femur and tibia at approximately 0.4% (oral) and 1% (intravenous) of the administered dose. In AKR1A1-KO mice, oral KFP-1 prevented bone loss, with additional calcium supplementation further improving cortical bone mechanical properties. These findings highlight KFP-1's dual action in promoting bone formation and inhibiting bone resorption, supporting its potential as a therapeutic adjuvant or functional food ingredient for osteoporosis prevention and bone health.