Vitamin D supplementation during illness
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Vitamin D Supplementation in Critically Ill Patients: Clinical Outcomes
Several studies have examined the effects of vitamin D supplementation in critically ill patients. Most randomized controlled trials and meta-analyses show that vitamin D supplementation does not significantly reduce overall mortality, length of stay in the ICU or hospital, or duration of mechanical ventilation in this population Langlois2017Šarić2024Lan2020+1 MORE. However, some recent analyses suggest that vitamin D may slightly reduce short-term mortality, ICU stay, and mechanical ventilation duration, especially in patients requiring mechanical ventilation, though these findings are not consistent across all studies and more data is needed to confirm them Zheng2025Shen2021. The benefits appear more pronounced in certain subgroups, such as surgical patients or those receiving parenteral vitamin D, but overall evidence remains inconclusive Zheng2025Shen2021.
Vitamin D Supplementation and Infection Outcomes
Children and Respiratory Infections
Research on vitamin D supplementation in children for infection prevention shows mixed results. High-dose vitamin D does not significantly reduce the incidence of upper respiratory tract infections, pneumonia, or diarrhoea in children under five years old Carboo2023Yakoob2016. However, daily supplementation with doses above 1,000 IU may reduce the incidence of influenza, cough, and fever, though the certainty of this evidence is low and based on a limited number of trials . There is no clear effect on hospitalizations or mortality in this age group Carboo2023Yakoob2016.
Older Adults and Hospitalization for Infection
In older adults, large-scale studies indicate that vitamin D supplementation does not significantly reduce the incidence of hospitalization for infections, including respiratory, skin, or gastrointestinal infections . There is some evidence that vitamin D may reduce the number of extended hospitalizations (lasting more than six days), but the overall impact on infection-related hospitalizations is minimal, especially in populations with low rates of vitamin D deficiency .
Vitamin D Supplementation in Specific Illnesses
Sickle Cell Disease
For individuals with sickle cell disease, limited evidence suggests that vitamin D supplementation increases serum vitamin D levels and may reduce pain days, but the quality of evidence is low and based on a single small study . There is no strong evidence to support improvements in other health outcomes, and more high-quality research is needed .
Dosing Strategies: Daily vs. Intermittent Bolus
The method of vitamin D administration may influence outcomes. Evidence suggests that daily maintenance dosing is more effective for preventing conditions like rickets and acute respiratory infections than high-dose intermittent bolus dosing . High-dose bolus regimens may induce biological mechanisms that inactivate vitamin D, reducing its effectiveness .
Limitations and Need for Further Research
Across all populations and illness types, studies highlight significant variability in trial designs, patient selection, dosing regimens, and outcome measures, making it difficult to draw firm conclusions . Standardized protocols and better-designed trials are needed to clarify the role of vitamin D supplementation during illness Šarić2024Lan2020Shen2021.
Conclusion
Vitamin D supplementation during illness, particularly in critically ill patients, generally does not improve major clinical outcomes such as mortality, length of stay, or infection rates. Some subgroups and specific dosing strategies may benefit, but the evidence is inconsistent. In children and older adults, the preventive effects on infections are limited. Daily maintenance dosing appears preferable to intermittent high-dose regimens. More rigorous and standardized research is needed to determine the true benefits of vitamin D supplementation during illness.
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