Since the sun is the largest source of vitamin D for most people, individuals who live in areas where winter days are shortest synthesize little or no vitamin D on the skin during those months and have to rely on dietary sources of vitamin D.
Building up bone growth during puberty may be important in preventing osteoporosis and fractures later in life. It is well documented that vitamin D decreases the incidence of fractures in the elderly, but this is one of the first studies to investigate vitamin D�s performance in bone accumulation during adolescence.
As a principal researcher, Viljakainen and her colleagues studied 225 11- and 12-year old girls for one year after they were assigned to three study groups receiving daily doses of 5 micrograms or 10 micrograms of vitamin D3 as a single tablet vs. placebo. The background data on the subjects about pubertal development, height and weight, physical activity and dietary intake of vitamin D and calcium, as well as bone and urine samples were collected. The bone mineral density of the lumbar spine and left femur were measured at the beginning and end of the study with dual x-ray absorptiometry ( DXA ).
For study participants who received 10 micrograms/day, no wintertime decrease in vitamin D status was found and parathyroid hormone concentration, which increases during winter due to low vitamin D status and can negatively affect bone, remained stable throughout the year. Bone mineral accumulation in the femur increased in study participants receiving 5 micrograms/day by 14.3 percent, and by 17.2 percent in the 10 microgram/day group, as compared to the placebo group. Corresponding results were seen in the lumbar spine. Furthermore, the mechanism through which vitamin D affects bone mineral gain is by decreasing resorption, which supports the previous results.
The Food and Nutrition Board at the Institute of Medicine currently recommends 5 micrograms/day as adequate intake for vitamin D. In Finland and other Nordic countries, the current recommended rate is higher at 7.5 micrograms/day. However, these recommendations do not fluctuate throughout the year. �To maintain optimal vitamin D status gained during the summer,� says Dr. Viljakainen, �supplementation should begin in autumn to maintain sufficient vitamin D in winter months.� She adds, �We believe the current recommendation for adequate vitamin D intake for this age group currently is too low and adolescent girls must receive 10-15 micrograms/day, at least in the northern latitudes.�