There are no clear international guidelines for dosing vitamin D based on deficiency severity. Therefore, a new clinical trial is needed to evaluate the benefits of early vitamin D supplementation in maintaining sufficient levels for critically ill patients. The investigators conducted a multicenter clinical trial in Taiwan focusing on vitamin D and critically ill patients. 240 patients with low calcidiol levels will be enrolled and be provided varying supplementation doses to maintain their serum calcidiol levels ≥ 30 ng/mL within 30 days of ICU admission. The serum levels of calcidiol and PTH will be measured on Day 0, Day 7, Day 14 and Day 30 before and after vitamin D supplementation. The results will serve as a valuable reference for intensivists when formulating appropriate vitamin D treatment strategy to maximize clinical benefits for critically ill patients.

Recent studies have highlighted a prevalent vitamin D deficiency in critically ill patients, ranging from 26% to 82%. These patients experience longer ICU stays, higher medical expenses, and increased sepsis-related mortality. The investigators conducted a multicenter clinical trial in Taiwan focusing on vitamin D and critically ill patients. In the first phase, the epidemiological investigation found significantly lower serum calcidiol levels of approximately 20.9 ng/mL compared to the normal range of 30-60 ng/mL in ICU patients. The second phase, a randomized control study, preliminarily demonstrated that supplementing 576,000 IU of vitamin D3 in critically ill patients with serum calcidiol levels below 20 ng/mL significantly reduced the risk of multidrug resistant bacterial infections within 30 days. An Austrian trial also showed that adequate vitamin D supplementation lowered in-hospital mortality in severely deficient patients. The importance of vitamin D supplementation for critically ill patients with vitamin D deficiency is evident, as their clinical prognosis is closely related to achieving adequate serum calcidiol levels. However, there are no clear international guidelines for dosing vitamin D based on deficiency severity. Therefore, a new clinical trial is needed to evaluate the benefits of early vitamin D supplementation in maintaining sufficient levels for critically ill patients. The investigators will enroll 240 patients with low calcidiol levels and provide varying supplementation doses to maintain their serum calcidiol levels ≥ 30 ng/mL within 30 days of ICU admission. The serum levels of calcidiol and PTH will be measured on Day 0, Day 7, Day 14 and Day 30 before and after vitamin D supplementation. The results will serve as a valuable reference for intensivists when formulating appropriate vitamin D treatment strategy to maximize clinical benefits for critically ill patients.

Inclusion Criteria:

• ≥18-year-old critically ill patient.
• ICU admission < 24 hours.
• Baseline 25(OH)D levels within 24 hours of ICU admission < 20 ng/mL.
• Expected ICU length of stay ≥ 72 hours.

Exclusion Criteria:

• Hypercalcemia (ie. total serum calcium levels > 2.6 mmol/L).
• Disorders affecting serum 25(OH)D levels, calcium metabolism, or bone metabolism (eg, parathyroid disease, rickets, or severe cirrhosis [Child C]).
• Having received high-dose vitamin D3 therapy (ie. > 2,000 IU daily or a single dose of ≥ 10,000 IU) within the past four weeks.
• Active COVID-19 at ICU admission.
• Organ transplant.
• Having had tuberculosis, sarcoidosis or kidney stones within the past year.
• Having renal dialysis, continuous kidney replacement therapy (CKRT), acute kidney injury (AKI).
• Having ICU admission within the past three months.
• Non-native-speaking patients and their families
• Pregnant women.