Obésité et ostéoporose

Abstract

A low body mass index (BMI) is a usual risk factor for fracture. Any decrease of a unit of BMI is associated with a moderate (but significant) decreased risk of fracture. The association is higher for fragility fractures (particularly for hip fractures). However, after adjusting for bone mineral density (BMD), this association is no longer significant. It is now well established that the protective effect of increased weight on the risk of fracture occurs only up to a certain threshold of BMI (25 kg/m2). Beyond that threshold, the incidence of fractures no longer varies. After adjustment for BMD, it seems that a BMI over 30 kg/m2 may be associated with an increased risk of fracture. The publications indicating that obesity is not always protective regarding the occurrence of fractures are recent. The analysis of data (which are otherwise not all unanimous) is complex since the effect of obesity depends on the site of the fracture. In women, obesity seems to be associated with an increased risk of fracture for tibia (upper and lower extremity), femoral shaft and the upper extremity of the humerus. Conversely, obesity appears protective regarding wrist, hip and pelvis fractures. Gender should also be taken into account. For men, obesity (after adjustment for BMD) is a risk factor for hip fracture. For vertebral fractures, obesity would be a protective factor for men but favoring for women. In mechanistic terms, many unknowns remain. Beyond the mechanical stresses, which can be an obvious explanation to understand the increased incidence of certain fractures in obese; the complexity of adipose tissue should also be taken into account. This endocrine organ has in fact substantial metabolic properties. It is especially capable of secreting adipokines. The best known of these is leptin. Leptin (measured in the serum) has a protective effect on bone. Another adipokine, adiponectin, may also be involved in bone metabolism with an opposite action compared to leptin. In addition, obesity belongs to the category of disorders causing low level of inflammation. Thus, secretion by adipose tissue of pro-inflammatory cytokines could also be an interesting explanatory track.