Does serum 25-hydroxyvitamin D decrease during acute-phase response? A systematic review.

Low levels of 25-hydroxyvitamin D, or 25(OH)D, are commonly associated with inflammatory diseases. These associations could be due to an increased prevalence of inflammatory diseases in hypovitaminosis D, although reverse causality cannot be excluded. We aimed to systematically review the longitudinal studies that reported serum 25(OH)D during an acute inflammatory response in humans. Using Ovid MEDLINE, EMBASE, and the Cochrane Library, an electronic search of the literature was conducted from database inception until January 2014 by combining the MeSH terms: vitamin D and acute-phase reactants. Other sources for obtaining articles were used as cross-referencing texts. Based on 670 titles and abstracts, 40 articles were selected for full-text review, and 8 of these studies met the final inclusion criteria. In 6 of the reviewed studies, 25(OH)D dropped after the inflammatory insult; this decrease was abrupt in the studies that measured 25(OH)D early after the insult. In 2 studies, there was no change of 25(OH)D during the course of the disease, but baseline levels were measured in both after days of symptoms onset. One study suggested that hemodilution decreased 25(OH)D, with no effect on inflammation. Serum C-reactive protein concentrations were used as inflammatory markers in almost all studies. The metabolic meaning and the functional importance of these changes are unknown. In light of the current evidence, the 25(OH)D measured during acute-phase response should be interpreted with care. Future research, including other markers of vitamin D adequacy, could help to clarify if hypovitaminosis D might be the cause or the consequence of inflammatory diseases.