<unk>  is essential for all animal life, with each cell capable of synthesizing it by way of a complex 37-step process. This begins with the mevalonate or HMG-CoA reductase pathway, the target of statin drugs, which encompasses the first 18 steps. This is followed by 19 additional steps to convert the resulting lanosterol into<unk> . A human male weighing 68 kg (150 lb) normally synthesizes about 1 gram (1,000 mg) of<unk>  per day, and his body contains about 35 g, mostly contained within the cell membranes. Typical daily<unk>  dietary intake for a man in the United States is 307 mg. Most ingested<unk>  is esterified, which causes it to be poorly absorbed by the gut. The body also compensates for absorption of ingested<unk>  by reducing its own<unk>  synthesis. For these reasons,<unk>  in food, seven to ten hours after ingestion, has little, if any effect on concentrations of<unk>  in the blood. However, during the first seven hours after ingestion of<unk> , as absorbed fats are being distributed around the body within extracellular water by the various lipoproteins (which transport all fats in the water outside cells), the concentrations increase. Plants make<unk>  in very small amounts. In larger quantities they produce phytosterols, chemically similar substances which can compete with<unk>  for reabsorption in the intestinal tract, thus potentially reducing<unk>  reabsorption. When intestinal lining cells absorb phytosterols, in place of<unk> , they usually excrete the phytosterol molecules back into the GI tract, an important protective mechanism. The intake of naturally occurring phytosterols, which encompass plant sterols and stanols, ranges between ≈200–300 mg/day depending on eating habits. Specially designed vegetarian experimental diets have been produced yielding upwards of 700 mg/day. Function in cells Membranes<unk>  composes about 30% of all animal cell membranes. It is required to build and maintain membranes and modulates membrane fluidity over the range of physiological temperatures. The hydroxyl group of each<unk>  molecule interacts with water molecules surrounding the membrane, as do the polar heads of the membrane phospholipids and sphingolipids, while the bulky steroid and the hydrocarbon chain are embedded in the membrane, alongside the nonpolar fatty-acid chain of the other lipids. Through the interaction with the phospholipid fatty-acid chains,<unk>  increases membrane packing, which both alters membrane fluidity and maintains membrane integrity so that animal cells do not need to build cell walls (like plants and most bacteria). The membrane remains stable and durable without being rigid, allowing animal cells to change shape and animals to move. The structure of the tetracyclic ring of<unk>  contributes to the fluidity of the cell membrane, as the molecule is in a trans conformation making all but the side chain of<unk>  rigid and planar. In this structural role,<unk>  also reduces the permeability of the plasma membrane to neutral solutes, hydrogen ions, and sodium ions. Substrate presentation<unk>  regulates the biological process of substrate presentation and the enzymes that use substrate presentation as a mechanism of their activation. (PLD2) is a well-defined example of an enzyme activated by substrate presentation. The enzyme is palmitoylated causing the enzyme to traffic to<unk>  dependent lipid domains sometimes called "lipid rafts". The substrate of phospholipase D is phosphatidylcholine <unk>  which is unsaturated and is of low abundance in lipid rafts.<unk>  localizes to the disordered region of the cell along with the polyunsaturated lipid phosphatidylinositol 4,5-bisphosphate (PIP2). PLD2 has a PIP2 binding domain. When PIP2 concentration in the membrane increases, PLD2 leaves the<unk>  dependent domains and binds to PIP2 where it then gains access to its substrate<unk>  and commences catalysis based on substrate presentation. Substrate presentation; PLD (blue oval) is sequestered into<unk> -dependent lipid domains (green lipids) by palmitoylation. PLD also binds PIP2(red hexagon) domains (grey shading) located in the disordered region of the cell with phosphatidylcholine <unk> . When<unk>  decreases or PIP2 increases in the cell, PLD translocates to PIP2 where it is exposed to and hydrolizes<unk>  to phosphatidic acid (red spherical lipid). Signaling<unk>  is also implicated in cell signaling processes, assisting in the formation of lipid rafts in the plasma membrane, which brings receptor proteins in<unk> ose proximity with high concentrations of second messenger molecules. In multiple layers,<unk>  and phospholipids, both electrical insulators, can facilitate speed of transmission of electrical impulses along nerve tissue. For many neuron fibers, a myelin sheath, rich in<unk>  since it is derived from compacted layers of Schwann cell membrane, provides insulation for more efficient conduction of impulses. Demyelination (loss of some of these Schwann cells) is believed to be part of the basis for multiple sclerosis.<unk>  binds to and affects the gating of a number of ion channels such as the nicotinic acetylcholine receptor, GABAA receptor, and the inward-rectifier potassium channel.<unk>  also activates the estrogen-related receptor alpha (ERRα), and may be the endogenous ligand for the receptor. The constitutively active nature of the receptor may be explained by the fact that<unk>  is ubiquitous in the body. Inhibition of ERRα signaling by reduction of<unk>  production has been identified as a key mediator of the effects of statins and bisphosphonates on bone, muscle, and macrophages. On the basis of these findings, it has been suggested that the ERRα should be de-orphanized and<unk> assified as a receptor for<unk> . Chemical precursor Within cells,<unk>  is also a precursor molecule for several biochemical pathways. For example, it is the precursor molecule for the synthesis of vitamin D in the calcium metabolism and all steroid hormones, including the adrenal gland hormones cortisol and aldosterone, as well as the sex hormones progesterone, estrogens, and testosterone, and their derivatives. Metabolism<unk>  is recycled in the body. The liver excretes<unk>  into biliary fluids, which are then stored in the gallbladder, which then excretes them in a non-esterified form (via bile) into the digestive tract. Typically, about 50% of the excreted<unk>  is reabsorbed by the small intestine back into the bloodstream.
Cholesterol
Lipoprotein
Oxycholesterol
Glycogen
Phytosterol
Saturated fat
Protein 
Medium-chain triglyceride