In an ageing population, the need for interventions to help older people remain healthy, active and independent for as long as possible, increases. Although several studies suggest a beneficial effect of vitamin D3 on maintaining or improving muscle strength and physical functioning, particularly in vulnerable populations, results are contradicting. Randomized, placebo-controlled trials are needed to further establish the effect of vitamin D on muscle strength in the frail elderly population.The primary aim of this study is to determine the effect of daily supplementation with two different forms of vitamin D on muscle strength in frail elderly people over a period of 24 weeks.

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.