Observational studies among critically ill patients have shown strong associations between vitamin D deficiency and adverse outcomes, including increased length of stay, infection, and mortality. It is unknown whether vitamin D deficiency contributes directly to adverse outcomes or whether it is simply a biomarker of severity of illness or overall health status. However, vitamin D plays a key role in host defense, largely by stimulating production of the anti-microbial peptide cathelicidin (LL-37). We will test the hypothesis that administration of activated vitamin D (calcitriol) will increase serum levels of cathelicidin.