Re-assembled casein micelles improve in vitro bioavailability of vitamin D in a Caco-2 cell model

The pandemic of vitamin D (VD) deficiency, and the global rise in obesity stimulate a need for staple low-fat foods and beverages enriched with VD. In light of consumer demand for a clean label, the use of natural endogenous food ingredients as delivery vehicles is of great interest. To this end, re-assembled casein micelles (rCM) have been shown to help retain VD during processing and shelf life and provide high bioavailability in low-fat milk and non-fat yoghurt. This follow-up study focused on the performance of VD-loaded rCM after drying and reconstitution, considering VD retention during simulated digestion, and the subsequent in vitro bioavailability of the vitamin. rCM conferred great protection to VD₃ during simulated digestion with a significant increase in vitamin retention for 1 h under gastric conditions. This observation is believed to arise from the vitamin-casein binding and the system's natural gelation (curd formation) near the casein isoelectric point that seclude the vitamin from environmental stressors and couple its release with digestive proteolysis of the rCM matrix. Vitamin absorption by Caco-2 cells from digested rCM was not significantly different from the absorption of the digested free VD. However, thanks to the highly protective effect of the rCM, against VD gastric degradation, the overall effect of the rCM was a 4-fold higher bioavailability, compared to the free VD.