Additionally we found that the positive associations between fat mass and bone size and the negative associations between fat mass and volumetric density persisted after adjustment for lean mass, suggesting this website that the relationships were not mediated by muscle mass. The emerging evidence that fat is not an inert tissue, but a highly active endocrine organ, yields some additional possible explanations. Adipocytes produce leptin, a

peptide hormone involved in the regulation of fat metabolism and appetite through hypothalamic mechanisms [19]. Recent work in animals has suggested that the primary effect of leptin on bone formation is negative via hypothalamic action on the sympathetic nervous system [20]. How this relates to mechanisms in humans is as yet unclear. Conversely leptin may push mesenchymal stem cells towards differentiation to osteoblasts rather than adipocytes [21] and [22] and leptin receptors have been found on osteoblasts, chondrocytes and bone marrow stromal cells [23]. Thus it is possible that leptin may explain some of the relationship between fat mass and bone, both positive and negative. Adiponectin is another hormone released by adipocytes;

in contrast to leptin it is negatively related to overall fat mass. Adiponectin is associated with increased insulin sensitivity and improved glucose tolerance. A recent study from a large UK cohort related adiponectin, measured TSA HDAC in vitro at 9.9 years, cross-sectionally to bone indices measured by DXA, and longitudinally to those measured by pQCT at 15.5 years [24]. The direct relationships between fat mass and volumetric density were not reported but total fat mass was negatively related to adiponectin concentration, which in turn negatively predicted volumetric density at 15.5 years. It seems unlikely, therefore, that adiponectin could explain negative relationships between fat mass and volumetric bone density. Insulin has been shown to have positive effects on bone in animal studies [25], with insulin resistance (and higher levels of insulin, as might be found in obesity)

associated with increased BMD [26], [27] and [28] and reduced fracture risk in humans [29]. Finally, recent work has suggested a role for peroxisome proliferator-activated receptors (PPARs) in the regulation of bone mass; reduced osteoblast Ergoloid function [30] and [31], increased osteoclastogenesis [32] and altered adipocyte/osteoblast differentiation [33] have been demonstrated in animal studies; thiazolidinedione drugs, which activate PPAR-gamma, have been shown to increase fracture risk [34]. Subtypes of these nuclear receptors also have a role in regulating insulin sensitivity and lipid metabolism [35], and thus are likely to relate to obesity, but there are currently insufficient data to allow detailed conclusions regarding any bone-related role in humans to be made.