A remarkable 400% increase in uptake of vitamin D when supplemented in milk

Recently, vitamin D has become a popular supplement in milk and various fruit juices due to its important immuno-regulatory functions in humans. Because milk is enriched with calcium and vitamin D acts as a hormone in enhancing the calcium uptake, it is plausible to spike vitamin D into milk as a daily requirement. However, the rationale and physiologic advantage beyond this purpose have not been thoroughly studied in the last two decades. We have recently discovered that β-lactoglobulin (LG), a major component comprising about 15% of milk proteins, is a vitamin D carrier forming a LGvitamin D complex at a ratio of 2:1. Based on the x-ray crystallographic structure, LG contains two binding sites for the hydrophobic vitamin D molecules. One is located in the central calyx surrounded by beta-configured barrels; the other is at a surface hydrophobic pocket containing an alpha-helix and a beta-sheet. Using mouse as an animal model, we showed that LG is the only milk fraction able to transport vitamin D in vivo. When vitamin D is supplemented to LG, the uptake is markedly increased by 400% compared to no LG being present. There is a 200% increase in uptake if the central calyx-binding site of LG is blocked. Since there are limited sources of vitamin D in the human diet, milk supplemented with vitamin D should be considered as an excellent vehicle in enhancing its bioavailability.