The effect of season and vitamin D supplementation on bone mineral densityin healthy women: A double-masked crossover study

Vitamin D status is known to be an important determinant of bone mineral de nsity (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 B MD could be detected in healthy normal subjects, along with associated vari ations in serum parathyroid hormone (PTH), intestinal calcium absorption an d biochemical markers of bone turnover. A second aim was to investigate whe ther, if such variations were identified, they could be suppressed by vitam in 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 receive d a daily oral supplement containing 800 IU (20 mug) cholecalciferol (group 1) and 35 subjects received a placebo preparation (group 2). During the se cond 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 (bo ne-specific ALP (BSAP) and urinary crosslinks (DYPD/creatinine)) and calciu m absorption were also measured at each visit. Cholecalciferol treatment in creased serum 25-OHD by 25.4 nmol/l (p < 0.001), while a reciprocal decreas e in serum PTH of 6.6 ng/l (p = 0.011) was seen in subjects in the lowest q uartile of baseline serum 25-OHD. The treatment had no significant effect o n 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 w as 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 h ealthy women studied there was no evidence of seasonal variation in spine, femur or total body BMD, serum PTH, calcium absorption or bone markers. Vit amin D supplementation was found to have no effect on BMD.