Meier 2004.

Methods	Randomised controlled trial using parallel group design (two intervention groups).
Participants	Country: Germany. Number of participants randomised: 55 healthy volunteers (65% postmenopausal women), 33 to 78 (mean 55,8) years of age. Inclusion criteria: healthy volunteers. Exclusion criteria: history or clinical evidence of significant skeletal or nonskeletal disease, taking any medication known to affect bone metabolism, including vitamin D and mineral supplements.
Interventions	Participants were randomly assigned to receive: Intervention group 1: vitamin D3 500 IU daily plus calcium 500 mg daily (n = 30); Intervention group 2 (Control group): no intervention (n = 25); for a six‐month period. Participants were followed an additional six‐month period. The first year of the trial after randomisation was designed as an observation period only, during which the participants followed their usual daily routine with no intervention per protocol. During the winter of the second year, from October to March, the participants assigned to the intervention group received a daily supplement of oral vitamin D3 (500 IU) and calcium (500 mg), whereas the participants in the control group received no supplements and were asked to remain off such agents. The trial medication was open label.
Outcomes	The primary outcomes were circannual changes in bone turnover, and bone mineral density and rates of bone turnover and bone loss during the winter months.
Stated aim of study	"To evaluate the circannual changes in bone turnover, and bone mineral density and to determine the effect of oral calcium and vitamin D3 supplementation on rates of bone turnover and bone loss during the winter months."
Notes	"Adherence to intervention was checked in monthly intervals through personal interviews."
Risk of bias
Bias	Authors' judgement	Support for judgement
Random sequence generation (selection bias)	Unclear risk	The trial is described as randomised but the method of sequence generation was not specified.
Allocation concealment (selection bias)	Unclear risk	The trial was described as randomised but the method used to conceal the allocation was not described, so that intervention allocations may have been foreseen in advance of, or during, enrolment.
Blinding (performance bias and detection bias) All outcomes	High risk	Trial was not blinded, so that the allocation was known during the trial. Placebo was not used.
Incomplete outcome data (attrition bias) All outcomes	Low risk	The numbers and reasons for dropouts and withdrawals in all intervention groups were described.
Selective reporting (reporting bias)	Low risk	Pre‐defined, or clinically relevant and reasonably expected outcomes are reported on.
Industry bias	Low risk	The trial is not funded by a manufacturer of vitamin D.
Other bias	Low risk	The trial appears to be free of other components that could put it at risk of bias.