Obese patients are known to have low levels of vitamin D metabolites. There is some evidence that vitamin D and/or dietary calcium may influence energy metabolism and body weight. We therefore perform a prospective controlled trial with 200 overweight (Body mass index 27-29.9 kg/m2) and obese subjects (Body mass index >= 30 kg/m2) who are on a telemedically guided weight loss program. Subjects randomly receive a daily vitamin D supplement or a placebo for 1 year. Participants have to send their body weight data to the study office weekly. In addition, a nutritionist at the study office has to be contacted weekly to receive further support concerning the weight loss program. Dietary records have to be completed monthly. Clinical parameters and blood samples are collected at baseline, and after 6 and 12 months. It is the aim of the study to investigate the vitamin D effects on weight loss and body composition. In addition, possible vitamin D effects on clinical and selected biochemical parameters should be assessed. These parameters include heart rate, blood pressure, inflammation markers, and parameters of lipid and glucose metablolism.