PEAK BONE MASS - FACTS AND UNCERTAINTIES

Peak bone mass, which can be defined as the amount of bony tissue pres ent at the end of the skeletal maturation, is an important determinant of osteoporotic fracture risk in adulthood. The techniques of single or dual energy absorptiometry measure the so-called ''areal'' or ''sur face'' bone mineral density (BMD), a variable which has been shown to be directly related to bone strength. During puberty the gender differ ence in bone mass becomes expressed. This difference appears to be ess entially due to a more prolonged bone maturation period in males than in females, with a larger increase in bone size and cortical thickness , as there is no significant sex difference in the volumetric trabecul ar density at the end of pubertal maturation. Ar the beginning of the 3rd decade, there is a large variability in the normal values of areal BMD in axial and appendicular skeleton. This large variance, which is observed at sites particularly susceptible to osteoporotic fractures in adulthood, such as lumbar spine and femoral neck, is barely reduced after correction for statural height, and does not appear to substant ially increase during adult life. It is generally accepted that peak b one mass at any skeletal site is attained in both sexes during the mid -thirties. However, recent studies indicate that in healthy caucasian females, bone mass accumulation can virtually be completed before the end of the second decade, for both lumbar spine and femoral neck. Seve ral variables are supposed to influence bone mass accumulation during growth: heredity, sex, diet components, endocrine factors, mechanical forces, and exposure to risk factors. Quantitatively, the most promine nt factor appears to be the genetic determinant, as estimated by studi es comparing monozygotic and dizygotic twins. That heredity is not the only determinant of peak bone mass is of practical interest, since en vironmental factors can be modified. With respect to nutrition, the qu antitative importance of calcium intake in bone mass accumulation duri ng growth, particularly at sites prone to osteoporotic fractures, stil l remains to be clearly determined The same can be said for the impact of physical activity. Finally, the crucial years when external factor s will be particularly effective on bone mass accumulation remain to b e determined by longitudinal prospective studies in order to produce c redible and well-targeted recommendations for the setting up of osteop orosis prevention programs aims at maximizing peak bone mass.