Choline is an essential nutrient that is naturally present in some foods and available as a dietary supplement. Choline is a source of methyl groups needed for many steps in metabolism. The body needs choline to synthesize phosphatidylcholine and sphingomyelin, two major phospholipids vital for cell membranes. Therefore, all plant and animal cells need choline to preserve their structural integrity. In addition, choline is needed to produce acetylcholine, an important neurotransmitter for memory, mood, muscle control, and other brain and nervous system functions. Choline also plays important roles in modulating gene expression, cell membrane signaling, lipid transport and metabolism, and early brain development.
Humans can produce choline endogenously in the liver, mostly as phosphatidylcholine, but the amount that the body naturally synthesizes is not sufficient to meet human needs. As a result, humans must obtain some choline from the diet. Premenopausal women might need less choline from the diet than children or other adults because estrogen induces the gene that catalyzes the biosynthesis of choline. When a diet is deficient in folate, a B-vitamin that is also a methyl donor, the need for dietary choline rises because choline becomes the primary methyl donor.
The most common sources of choline in foods are the fat-soluble phospholipids phosphatidylcholine and sphingomyelin as well as the water-soluble compounds phosphocholine, glycerolphosphocholine, and free choline. When these choline-containing compounds are ingested, pancreatic and mucosal enzymes liberate free choline from about half of the fat-soluble forms and some water-soluble forms. Free choline, phosphocholine, and glycerophosphocholine are absorbed in the small intestine, enter the portal circulation, and are stored in the liver, where they are subsequently phosphorylated and distributed throughout the body to make cell membranes. The remaining fat-soluble phospholipids (phosphatidylcholine and sphingomyelin) are absorbed intact, incorporated into chylomicrons, and secreted into the lymphatic circulation, where they are distributed to tissues and other organs, including the brain and placenta.
Choline status is not routinely measured in healthy people. In healthy adults, the concentration of choline in plasma ranges from 7 to 20 mcmol/L. According to one study, the range is 7–9.3 mcmol/L in fasting adults. Plasma choline levels do not decline below 50% of normal, even in individuals who have not eaten for more than a week. This may be due to the hydrolysis of membrane phospholipids, a source of choline, to maintain plasma choline concentrations above this minimal level, or to endogenous synthesis.