What causes bone loss during weight loss?

Bone mass is, for the most part, regulated by the activity of two specialized cell types. Osteoblasts are tasked with adding bone mass, while osteoclasts actively digest it and cause bone resorption. In the context of diseases like osteoporosis, increasing osteoclast number or activity shifts the balance toward bone resorption, causing a progressive loss of bone density over time.^[1] Bone is capable of sensing mechanical stress, which lessens in response to weight loss. This shifts the balance of bone remodeling cells. Calorie restriction also alters a number of hormones that play a role in the regulation of bone mass. Reductions in body fat have been linked to reduced levels of estrogen and other sex hormones,^[2] and increased sex hormone binding globulin (SHBG), a protein that binds to and sequesters hormones, blocking their function. Although shifts in overall hormone levels during calorie restriction and weight loss can be subtle, small changes in hormone levels (particularly estrogens, and IGF-1)^[3][4][5] can work through both direct and indirect mechanisms to alter the balance of osteoblast vs. osteoclasts activity to promote bone resorption and decreased bone density.