From EurekAlert!: "In osteoporosis, excessive bone resorption occurs. The bones lose their density and are therefore prone to breakage. Even minor falls can lead to serious bone fractures. The interplay between two cell types determines bone density: bone forming cells (osteoblasts) and bone resorbing cells (osteoclasts). The equilibrium between these two cell types is strictly regulated to prevent the formation of either too much or too little bone. ... LAP is the term researchers use to denote the full-length isoform of [the gene] C/EBPbeta, and LIP is the term for the short isoform. LAP activates another gene switch [which] suppresses the formation of bone resorbing osteoclasts. In contrast, LIP, suppresses this gene switch and thus enhances the proliferation and activity of the osteoclasts. As a result, the osteoclasts resorb more bone substance than is built by the osteoblasts. The researchers suspect that imbalance in the ratio between LAP and LIP plays a role in osteoporosis. The activity of a signaling molecule – mTOR – determines which of the two isoforms LAP and LIP is formed. ... it may be possible to develop new drugs that regulate the activity of mTOR and, thus, remedy the disturbance in osteoclast function." mTOR activity, you might recall, is altered by the practice of calorie restriction.