To Test the Effect of Calcifediol Hy.D Supplementation on Muscle Function and Bone Quality in Younger Postmenopausal Women With Osteopenia

Inclusion Criteria: * Women age 50 to 70 * Post-menopausal (defined as: at least 1 year after the last menstrual period) * community-dwelling and ambulatory without help * with documented osteopenia (BMD by DEXA t-score: \< -1.0 and \> -2.5 at the spine or hip) in the 6 months prior to enrolment or with documented osteoporosis (BMD by DEXA t-score: ≤ -2.5 at the spine or hip) and a FRAX score (online calculation tool of absolute 10 year fracture risk) below the Swiss age-dependent indication threshold for pharmacologic treatment for the 10-year risk of major osteoporotic fractures - at the screening visit and including DEXA (in the 6 months prior to enrolment) as part of the calculation * body mass index \> 18.5 and \< 30 kg/m2 * 25(OH)D level \< 24 µg/l (\< 60 nmol/l) * understands German in reading and writing plus able to read, understand, and complete questionnaires and tests * willingness to limit additional vitamin D3 intake to 800 IU per day * willingness to limit calcium supplement intake to 500 mg/day * willingness to stop active vitamin D metabolites * participant understands the study procedures, alternative treatments available and risks involved with the study and voluntarily agrees to participate by giving a written informed consent * participant meets the routine clinical laboratory safety screening tests performed at screening visit * participant is able and willing to perform all study tests, attend all required office visits, and provide blood and urine samples * participant is able to swallow the study medication Exclusion Criteria: * Consumption of more than 1'000 IU vitamin D on any day in the 4 weeks prior to enrollment. * Elevated serum calcium \> 2.60 mmol/l adjusted for albumin if albumin ≤ 35 or ≥ 45g/l * estimated creatinine clearance \< 30 ml/min (Cockcroft and Gault = 140 - age(yr)\*weight(kg)/ serum Cr(mmol/l))×(1.04 for women) * severe visual or hearing impairment * malabsorption syndrome (celiac diseases, inflammatory bowel disease). * Pathologic fracture (excl. fractures due to osteoporosis) in the last year * Fracture due to osteoporosis in the last 10 years * Chemo therapy / Radiation due to cancer in the last year * Treatment which has an effect on calcium metabolism (e.g. PTH, calcitonin, chronic cortisone intake \> 5mg/day for more than 4 weeks in the last 12 months (except for inhalation and sporadic infiltration)) * Current treatment with a bisphosphonate * For participants of the ancillary study "Muscle Biopsy" only: Treatment which has an effect on blood coagulation (e.g. factor X inhibitor, thrombin inhibitor, NSAR, low-molecular heparin, inhibitor for platelet aggregation, vitamin K antagonist) and/or abnormal blood coagulation status . * Unwilling or unable to take study medication * Diseases with a risk of recurrent falling (e.g. Parkinson's disease/syndrome, Hemiplegia after stroke, symptomatic stenosis of the spinal canal, polyneuropathy, epilepsy, recurring vertigo, recurring syncope) * History of or current diseases that may enhance serum calcium: sarcoidosis, lymphoma, primary hyperparathyroidism * Individual that heavily consumes alcohol containing products defined as greater than (\>) 3 drinks (beer, wine, or distilled spirits) of alcoholic beverages per day. * Individual is unlikely to adhere to the study procedures, to keep appointments, or is planning to relocate during the study. * Individuals who are planning a stay in a "sunny" location (e.g. winter sun resort) for more than one month during the course of the study * Medication which has an effect on 25-hydroxyvitamin D level (e.g. certain anticonvulsants (e.g. Phenobarbital, Carbamazepin, Phenytoin)) * M. Paget (Ostitis deformans) * Inflammatory arthritis (e.g. rheumatoid arthritis, Reiter syndrome, psoriasis arthritis) * Participation in a study in the last 6 months, except for studies without drug-application, or any influence of the study-medication can be excluded