*Geek Box: Phytosterols
Phytosterols comprise both plant stanols and plant sterols, but there is no clinical or statistically relevant difference between the two regarding their cholesterol-lowering effects. Phytosterols are cholesterol-like molecules found in all plant foods, that act by inhibiting intestinal absorption of cholesterol; they also inhibit recirculating endogenous biliary cholesterol, which is a key step in cholesterol elimination.
Phytosterols have very low systemic absorption, and act by displacing dietary cholesterol from intestinal micelles, in turn decreasing the absorbable pool of cholesterol. In addition, phytosterols are taken up by enterocytes (intestinal cells) and increase the expression of ATP-binding cassette A1 transporter (ABCA1). ABCA1 is a cell membrane transport protein, responsible for the transport of cholesterol and phospholipids across the cell membrane; it also has been shown to have a role in determining plasma HDL levels. By increasing the clearance of cholesterol from macrophages (immune cells implicated in atherosclerosis and oxidation of cholesterol), and generating HDL particles, ABCA1 has an important role in mediating the secretion of excess cholesterol from the cell into HDL pathways, to be cleared by HDL.
There is a final pathway by which phytosterols may decrease cholesterol levels: transintestinal cholesterol secretion, or TICE, which is an important pathway in cholesterol excretion. In animal models it has been shown that phytosterols stimulate cholesterol secretion via TICE. It should be noted that to date there are no human studies regarding the role of TICE; however, TICE has been detected in human jejunal explants, so future work may confirm that phytosterols influence this pathway in humans. Nonetheless, significant cholesterol lowering effects can be obtained from 2g per day of phytosterols, with no adverse effects noted at this dose in long-term human studies.