Does systemic inflammation lower measured 25-hydroxyvitamin D?

Extensive clinical and mechanistic evidence confirms that systemic inflammation significantly lowers measured levels of 25-hydroxyvitamin D [25(OH)D]. During acute inflammatory states, 25(OH)D behaves as a negative acute-phase reactant, meaning its concentration drops rapidly in response to physiological stress rather than reflecting a true change in total body vitamin D stores.

Clinical evidence

The behavior of 25(OH)D as a negative acute-phase reactant is well-documented in surgical, trauma, and critical illness cohorts.

• Perioperative Trends: In patients undergoing major surgery, such as hip arthroplasty or cardiac procedures, serum 25(OH)D levels typically fall to a nadir within 48–72 hours post-surgery. This drop correlates strongly with the peak of inflammatory markers like C-reactive protein (CRP) and Interleukin-6 (IL-6).
• Observational Data: Large-scale studies (including NHANES and MESA) demonstrate a robust inverse relationship between systemic inflammatory indices (e.g., neutrophil-to-lymphocyte ratio) and serum 25(OH)D.
• Recovery and Rebound: Levels generally return toward baseline as inflammation resolves, suggesting that measured "deficiencies" during acute illness are often transient and represent a snapshot of the inflammatory state.

Mechanistic explanations

The decline in circulating 25(OH)D during inflammation is driven by three primary physiological mechanisms:

• Reduced Carrier Proteins: Over 99% of 25(OH)D is bound to Vitamin D-binding protein (DBP) and albumin. Pro-inflammatory cytokines (IL-6, TNF-α) suppress the hepatic synthesis of these proteins. A reduction in DBP directly results in a lower capacity to hold 25(OH)D in the blood, leading to lower total measured levels.
• Increased Capillary Permeability: Systemic inflammation increases vascular leakage, allowing the DBP-25(OH)D complex to shift from the intravascular space into extravascular tissues, further diluting serum concentrations.
• Immune Consumption ("The Sink"): Activated immune cells (macrophages and monocytes) upregulate the enzyme CYP27B1 and the Vitamin D Receptor (VDR). This creates an "extra-renal sink" that pulls 25(OH)D from the circulation to convert it locally into active calcitriol [1,25(OH)₂D]. This localized metabolism fuels the production of antimicrobial peptides (like cathelicidin) and supports immunoregulatory functions.

Bottom line

Systemic inflammation consistently lowers measured 25-hydroxyvitamin D by suppressing its carrier proteins and increasing its utilization by the immune system. Consequently, a low vitamin D level found during acute illness may not reflect a nutritional deficiency, and status should ideally be assessed when the patient is in a non-inflammatory state.