IGFBP1 and age: The mechanism behind this may be attributed to BMSCs preferring to form adipocytes over osteoblasts or osteoclasts enhancing its bone resorption via FGF21-IGFBP1 axis (Wei et al., 2012; Wang X. et al., 2015); however, Lee et al. (2020) rejected the second half of the hypothesis and demonstrated that age-related bone loss caused by FGF21 is irrelevant to the FGF21-IGFBP1 axis.