A randomized double blind placebo control trial study will be conducted in critically ill patients with vitamin D deficiency. Investigator aimed to study the effect of oral vitamin D3 supplementation, compare to placebo, on skeletal muscle mass and body composition.

There is a high prevalence of vitamin D deficiency in critically ill patient which is associated with muscle wasting and physical disability. Recent study showed that treatment of vitamin D deficiency with high dose vitamin D improved muscle wasting and may prevent further muscle breakdown.

Investigator want to explore whether a high dose vitamin D3 supplementation, compare to placebo will be able to improve muscle wasting in critically ill patients.

The eligible participant will be asked to sign and date the informed consent document then they will be randomized to receive vitamin D3 supplement or placebo, using the computer generated code in conceal envelope.

Vitamin D3 or placebo will be given orally or feeding tube via feeding tube at a dose of 100,000 IU on day 1 and 3 then 50,000 IU on day 5,7,9,12 followed by 150,000 unit per week for 4 week.

Serum 25-Hydroxyvitamin D, 1,25-dihydroxyvitamin D will be measured at baseline (day 0) then day 10 and day 43 after vitamin D supplementation. Moreover, Investigator will assess the diameter of rectus femoris by using ultrasonography on day 0,10 and 43.

Inclusion Criteria:

• Age 18 - 70 years old
• Expected ICU stay ≥ 48 hrs

Exclusion Criteria:

• Participate in other clinical trial
• Contraindication to receive oral or enteral feeding
• Do not resuscitate /imminent death
• Vegetative state, generalize weakness, denervation of leg, both leg amputation
• Hypercalcemia or Hypercalcemia at risk
• Hyperphosphatemia,
• History of nephrolithiasis
• End stage renal disease on renal replacement therapy
• Pregnancy/lactation
• Consent refusal