The purpose of this study is to assess the dose of Hy.D Calcifediol 25 SD/S (a metabolite of vitamin D3) which will achieve the range of mean serum 25(OH)D between 75-100nmol/L in physically frail elderly people after 24 weeks of daily supplementation with different doses of Hy.D Calcifediol 25 SD/S compared to 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.