NPC1L1 protein mediates the uptake of dietary sterols from the gut lumen. These include cholesterol and plant sterols (phytosterols). Half to two-thirds of the 250-500 milligrams of cholesterol consumed daily from a typical western diet is taken up by this route and exported from enterocytes into the lymph in chylomicrons. Of the 200-400 milligrams of dietary phytosterol, only about 5% is absorbed.The drug ezetimibe interferes with NPC1L1-mediated sterol uptake (Oram and Vaughan 2006).
Both cholesterol and phytosterols are secreted from the liver into the bile (Salen et al. 1970). Mutations affecting the ABCG5/8 transporter complex inhibit this process (e.g., Berge et al. 2000), while the molecular details of sterol export in human cells have not been worked out, they can be inferred from the properties of the homologous mouse proteins (Graf et al. 2003; Wang et al. 2006).