Supplementation regimens of the study groups
All participants (n = 100) were divided into three groups: Control (CON), Vitamin D daily (VDD) or Vitamin D monthly (VDM) groups. The CON group consumed placebo consisting of 400 mg calcium (Mamisch GmbH Prorenal, Ludwigshafen, Germany) daily. The VDD group received 400 mg calcium 800 IU of vitamin D3 (Vitactiv Natural Nutrition, FeelGood Shop BV, Venlo, The Netherlands) daily. Participants of the VDM group were provided with 400 mg calcium daily and 50.000 IU vitamin D3 once every four weeks. The supplements were placed into 28-day pill boxes and handed out to the study participants in order to be able to consume their daily intake supplementations at home. To ensure that all supplements have been consumed, the returned pill boxes were controlled for remaining pills by study staff. Additionally, all study participants were advised to consume the vitamin D and calcium supplements daily, always at the same time, together with a meal.