The Effect of Calcifediol (Hy.D 25 SD/S) and Vitamin D3 on Muscle Strength in a Frail Elderly Population

Inclusion Criteria: * 25(OH)D levels 20-50 nmol/L. * age 65 or older. * physically frail (pre-frail or frail, based on the criteria designed by Fried et al.) (30). Frailty will be defined as a clinical syndrome in which one or more of the following criteria are present: unintentional weight loss (4.5 kg in past year), self-reported exhaustion (CES-D questionnaire), weakness (grip strength), slow walking speed and low physical activity (Minnesota questionnaire). More details on these criteria are described in chapter 6.3.2. * body mass index between 18.5 and 35 kg/m2. * willingness and ability to comply with the protocol, including performance of the knee extension strength test. Exclusion Criteria: * Medical Illness: * malabsorption syndrome: known intestinal malabsorption, celiac diseases, inflammatory bowel disease; * diseases that may enhance serum calcium concentration: sarcoidosis, lymphoma, kidney stone in the last 10 years, primary hyperparathyroidism; * abnormal indices of calcium metabolism, uncontrolled hypocalcaemia; * diagnosed renal insufficiency; * diagnosed cancer; currently diagnosed or undergoing treatment. * Hypercalcemia: serum calcium adjusted for albumin of \> 2.6 nmol/L. * Medication: interfering with vitamin D metabolism and vitamin D supplementation (bisphosphonate, PTH treatment, tuberculostatica, anti-epileptica). * Subject not able (when medically necessary/ advised) or not willing to stop the use of vitamin D containing supplements during the study. * (Expected) increase in exposure to sunlight (e.g. travelling to a sunny resort) during intervention period. * Patient heavily consumes alcohol containing products defined as greater than \> 21 drinks of alcoholic beverages per week. * Planned surgery which can affect study measures (taking into account duration of hospitalisation and recovery). * Participation in another clinical trial.