Seasonal variations in vitamin D status and calcium absorption do not influence bone turnover in young women

A study was done by the university of Bonn in Germany in 76 females to assess the potential benefit of a vitamin D supplementation during the summer for young women. The researchers concluded the following:

We could demonstrate seasonal differences in fractional
calcium absorption, in urinary calcium and in urinary
phosphorus excretion. This effect of season most likely
depends on variation in serum 25OHD levels. Decreased
vitamin D status and reduced fractional calcium absorption
in winter had no effect on bone turnover due to a diminished
vitamin D susceptibility of young women. Thus, with
a high calcium and phosphorus intake (>1200 mg=d) a
vitamin D supplementation of young females is not
required.

However, postmenopausal women and elderly subjects do benefit from vitamin D supplements.

http://www.nature.com/ejcn/journal/v52/n7/pdf/1600591a.pdf