Intestinal absorption of vitamin D: a systematic review

Context: Vitamin D is frequently prescribed as a supplement, yet its absorption remains poorly understood. Objective: This systematic review was performed to evaluate data on mechanisms involved in the intestinal absorption of vitamin D. Data Sources: PubMed, Embase, and Cochrane Library databases were searched. Study Selection: The following studies were included: experimental laboratory studies of vitamin D absorption through the enterocyte brush-border membrane; absorption tests that used radiolabeled vitamin D; and clinical trials in adults that investigated a single dose of cholecalciferol or ergocalciferol and reported at least 2 measurements of serum cholecalciferol, ergocalciferol, or 25-hydroxyvitamin D. Data Extraction: From 2069 articles identified, 46 met the inclusion criteria. Results: Different methods were employed to evaluate vitamin D absorption. Recent research suggests that vitamin D absorption is not an exclusive simple diffusion process. Vitamin D was better absorbed when it was consumed with fatcontaining meals, but absorption also occurred without fat or oily vehicles. Factors that modified cholesterol absorption also altered vitamin D absorption. Conclusion: Vitamin D is probably absorbed through passive diffusion and a mechanism involving membrane carriers, especially cholesterol transporters, although data remain scarce. Some data suggest that fat, when consumed concomitantly with vitamin D, improves vitamin D absorption.