Young postmenopausal women with osteopenia / or women with osteoporosis and a FRAX score below pharmacologic treatment indication have limited treatment options in the prevention of osteoporosis/treatment of osteopenia. Further, there is a concern about long-term side effects of bisphosphonate treatment among young postmenopausal women, and hormone replacement therapy has been controversial. In a pilot study 20 microgram Calcifediol Hy.D improved several muscle related function in this target population within 4 months of treatment, which can help to prevent falls and associated bone fractures. Thus the main aim of this study is to test whether Calcifediol Hy.D at a daily dose of 20 μg / day improves muscle function (lower extremity test battery) compared with (1) placebo and compared with (2) 3200 Vitamin D3 IU per day, at 3 and 6 month follow-up. As a secondary and exploratory objective of this study, the investigators will compare the beforementioned doses on muscle strength and the quality of the bones, beside muscle mass, body composition and systolic and diastolic blood pressure measurements.

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