Intestinal absorption of vitamin D: a systematic review

Abstract 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 fat-containing 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.