Mechanisms of absorption of vitamin D delivered in protein nanoparticles in the absence and presence of fat.

Novel protein-based nanovehicles offer alternatives to fat for delivery of lipophilic bioactives (nutraceuticals and drugs), yet they raise important questions regarding the bioavailability and absorption mechanism of the bioactive without fat. To provide answers, we chose vitamin D (VD) as a model lipophilic-nutraceutical, re-assembled casein-micelles (rCM) as model protein-based nanovehicles, and non-fat yoghurt as a model food. We prepared three yoghurt formulations: 3% fat with VD dissolved in milk-fat, non-fat and 3% fat, both latter enriched with VD within rCM. Following in vitro digestion, VD retention and bioaccessibility were high (∼90% and ∼70%, respectively) in all formulations. VD uptake by Caco-2 cells was three-fold higher (p < 0.005) in the non-fat yoghurt enriched with VD in rCM compared with enriched fat-containing yoghurts. SR-BI, CD36 and NPC1L1 transporters were involved in VD absorption irrespective of the composition. Thus, our findings demonstrate that protein nanovehicles may improve VD bioavailability, without altering its absorption mechanism compared to that from fat.