Inclusion of 95 % range in the model to account for inter-individual variability increased the predicted intake of vitamin D needed to maintain serum 25(OH)D ≥ 50 nmol/l to 930 IU/d .
The Effect of Season and Vitamin D Supplementation on Bone Mineral Density in Healthy Women: A Double-Masked Crossover Study

Abstract : Vitamin D status is known to be an important determinant of bone mineral density ( BMD ) . There is a significant seasonal variation in serum vitamin D , and some studies have reported an associated seasonal variation in BMD . The present study was devised to investigate whether a seasonal variation in BMD could be detected in healthy normal subjects , along with associated variations in serum parathyroid hormone ( PTH ) , intestinal calcium absorption and biochemical markers of bone turnover . A second aim was to investigate whether , if such variations were identified , they could be suppressed by vitamin D supplementation . The subjects were 70 healthy female volunteers ( mean age 47.2 years , range 24–70 years ) recruited into a double-masked crossover study and followed over 2 years . During the first year 35 subjects received a daily oral supplement containing 800 IU ( 20 mg ) cholecalciferol ( group 1 ) and 35 subjects received a placebo preparation ( group 2 ) . During the second year the treatment each group received was reversed . Lumbar spine ( L1–L4 ) , left proximal femur and total body BMD were measured by DXA at 3-month intervals . Serum 25-hydroxyvitamin D ( 25-OHD ) , serum PTH , bone markers ( bone-specific ALP ( BSAP ) and urinary crosslinks ( DYPD/creatinine ) ) and calcium absorption were also measured at each visit . Cholecalciferol treatment increased serum 25-OHD by 25.4 nmol/l ( p < 0.001 ) , while a reciprocal decrease in serum PTH of 6.6 ng/l ( p = 0.011 ) was seen in subjects in the lowest quartile of baseline serum 25-OHD . The treatment had no significant effect on spine , femur or total body BMD , calcium absorption or bone markers . When Fourier analysis was used to analyze the data for seasonal effect ( defined as twice the amplitude of the 1-year period variation ) a highly significant effect for 25-OHD of 18 nmol/l ( p < 0.001 ) was found . However , no effect was found for BMD , PTH , calcium absorption or bone markers . The analysis set a 95 % confidence limit to the seasonal effect of less than 0.6 % for spine , total hip and total body BMD . It was concluded that in the population of healthy women studied there was no evidence of seasonal variation in spine , femur or total body BMD , serum PTH , calcium absorption or bone markers .  Vitamin D supplementation was found to have no effect on BMD
Inclusion of 95 % range in the model to account for inter-individual variability increased the predicted intake of vitamin D needed to maintain serum 25(OH)D ≥ 50 nmol/l to 930 IU/d .