Relation of fractional 47Ca retention to season and rates of bone loss in healthy postmenopausal women.

Fractional whole-body retention of 47Ca (retention fraction) in 58 healthy postmenopausal women participating in a calcium supplementation trial was examined for seasonal variation and for relationships to rates of bone loss and plasma vitamin D levels. Retention fraction was measured after 18 months in the trial. Bone mineral densities of the radius, femoral neck, and lumbar spine were measured at baseline, 12 months, and 24 months in the trial, and plasma 1,25-dihydroxyvitamin D [1,25-(OH)2D] and 25-hydroxyvitamin D (25-OHD) at 12, 18, and 24 months. The adjusted retention (retention fraction adjusted for total calcium intake, smoking status, and log years since menopause) was significantly higher in women evaluated in the months of August through October (mean +/- SD, 19.8 +/- 4.1%, n = 13) than in March through May (mean +/- SD, 17.0 +/- 4.7%, n = 18, p = 0.05). Plasma 25-OHD was associated with retention fraction only in women with low calcium intakes (partial r = 0.69 controlling for total calcium intake and log body mass index, n = 14, p = 0.01). Plasma 1,25-(OH)2D was not related to retention fraction. Adjusted retention, independent of total calcium intake, log years since menopause, smoking status, and season, explained 8% of the variability in the annual change in radius density (partial r = 0.29, p less than 0.05). No significant associations were seen at the spine and femur.(ABSTRACT TRUNCATED AT 250 WORDS)