We have previously introduced the potential of casein micelles (CM) as natural nanovehicles for hydrophobic nutraceuticals, e.g. vitamin D (VD) (E. Semo, E. Kesselman, D. Danino and Y. D. Livney, Food Hydrocolloids, 2007, 21, 936-942). The aims of the current study were to improve performance by adding an ultra-high-pressure homogenization step, and to evaluate the protection conferred by re-assembled CM (rCM) against VD thermal and oxidative degradation, and the bioavailability of VD(3) in rCM, by a clinical trial. Dynamic-light-scattering showed that VD(3)-rCM had a diameter of 91 ± 8 nm (average ± standard error). When VD(3) was encapsulated in rCM and subjected to thermal treatment (80 °C, 1 min), no significant loss was observed (P > 0.05), compared to 13 and 14% loss of VD(3) emulsified with Tween-80 (a synthetic emulsifier typically used for VD solubilization) and of unencapsulated VD(3) respectively (P < 0.05). VD(3) in rCM was also more stable during 28 d cold storage (∼40% loss) compared to Tween-80 emulsified (∼50% loss) or to un-encapsulated (∼70% loss) VD(3). Ultra-high-pressure homogenization of VD(3)-rCM (∼155 MPa) significantly enhanced vitamin stability, resulting in only ∼10% loss after 28 d of storage. Bioavailability of a single-dose of 50,000 international-units (IU) VD(3) encapsulated in rCM, in 1% fat milk, investigated in a randomized double blinded placebo controlled clinical study with 87 human volunteers, was at least as high as that using an aqueous Tween-80-emulsified VD(3) supplement. We conclude that ultra-high-pressure homogenization treated rCM protect VD(3) against heat- and storage-induced degradation, and VD(3) encapsulated in rCM is highly bioavailable.