Dose Finding Study in Physically Frail Elderly to Measure 25(OH) Vitamin D Levels After Supplementation With Hy.D Calcifediol 25 SD/S and Vitamin D3

Inclusion Criteria: * 25(OH)D levels between 25 - 50 nmol/L; * Age 65 and older; * Physically frail, based on the criteria designed by Fried et al. \[1\]. 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), selfreported exhaustion (CES-D questionnaire), weakness (grip strength), slow walking speed and low physical activity (Minnesota questionnaire). These criteria are easily assessable and have been commonly used to determine frailty; * Men and women; * Body mass index between 20 and 35 kg/m2 (used for stratification); * Willingness and ability to comply with the protocol. 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, diagnosed liver failure. * Hypercalcemia : serum calcium adjusted for albumin of \> 2.6 nmol/L. * Medication: bisphosphonate or Parathyroid hormone (PTH) treatment, tuberculostatics, epilepsy medication, interfering with vitamin D metabolism and vitamin D supplementation. * Use of vitamin D supplementation in the last three months (excluding multivitamin supplementation). * Not willing to stop the use of multivitamin supplementation during the study. * (Expected) increase in exposure to sunlight (e.g. travelling to a sunny resort). * Patient heavily consumes alcohol containing products defined as greater than (\>) 3 drinks (beer, wine, or distilled spirits) of alcoholic beverages per day. * Donating blood in the period of 2 months before, until 1 month after the end of the study. * Planned surgery. * Participation in another clinical trial.