Seasonal variation of lumbar spine bone mineral content in normal women.

The seasonal influence on lumbar spine bone mineral was evaluated in a prospective study of 26 normal women aged 19-66 years. Bone mineral content of the second, third, and fourth lumbar vertebrae (lumbar BMC) was determined every 3 months during 1 year by using dual-photon (153Gd) absorptiometry. Lumbar BMC was, on an average (mean +/- SE), 0.86 +/- 0.27 arbitrary units or 1.7 +/- 0.5% higher in July to September than in January to March (P less than 0.005), when other sources of variation were eliminated. It is hypothesized that the seasonal variation in lumbar spine bone mineral reflects differences of the mechanical loading on the vertebrae. The interpretation of longitudinal studies of lumbar BMC may be erroneous if the seasonal variations in bone mineral are not considered.