Rates of bone loss in postmenopausal women randomly assigned to one of two dosages of vitamin D.

We conducted a study to determine whether increasing vitamin D intake above the recommended dietary allowance (RDA) of 5.0 μg (200 IU)/d reduces bone loss in healthy postmenopausal women residing at latitude 42 o N. In this double-blind, randomized 2-y trial, we enrolled 247 healthy ambulatory postmenopausal women who consumed an average of 2.5 μg (100 IU) vitamin D/d in their usual diets. The women were given either 2.5 μg (100 IU) or 17.5 μg (700 IU) vitamin D/d. All women received 500 mg supplemental calcium per day as citrate malate. Duplicate hip and spine and single whole-body scans were performed by dual-energy x-ray absorptiometry at 6-mo intervals selected to flank the periods when 25-hydroxycholecalciferol (calcidiol) concentrations are highest (summer/fall) and lowest (winter/spring). Plasma calcidiol and serum osteocalcin were measured in these seasons in year 1. Both treatment groups lost bone mineral density from the femoral neck, but the 17.5-μg group lost less than (-1.06±0.34%; x±SE) the 2.5-μg group (-2.54±0.37%, P=0.003). Seventy percent of the benefit each year occurred in winter/spring and 30% in summer/fall. Changes in spinal and whole-body bone densities did not differ by treatment group and were minimal after 2 y. Serum osteocalcin and plasma calcidiol (2.5-μg group only) fluctuated with season. In conclusion, in healthy, calcium-supplemented, postmenopausal women residing at latitude 42 °N, an intake of 5.0 μg (200 IU) vitamin D/d is sufficient to limit bone loss from the spine and whole body but it is not adequate to minimize bone loss from the femoral neck. We recommend that postmenopausal women at this latitude increase their vitamin D intake above the current RDA of 5.0 μg/d to reduce bone loss from the hip. Although an intake as high as 20 μg (800 IU)/d may not be needed, this amount is safe and effective