A number of types of lecithin are found in human cell membranes, particularly those of the liver, nerve tissue and semen. Lecithins are a type of phospholipid. Like fats, they consist of a backbone of glycerol but instead of having three fatty acids attached to it, the glycerol has only two fatty acids plus a phosphate group and a choline molecule. Lecithin is synthesized by the liver.
The fatty acids in lecithin and other phospholipids make it soluble in fat, and the phosphate-containing group makes it also soluble in water. Lecithin in cell membranes therefore enables fat-soluble substances, including vitamins and hormones, to pass easily in and out of cells. Lecithin also acts as an emulsifier, helping to keep fats suspended in the blood and body fluids.
Lecithin is synthesized by the food industry, where it is widely used as an emulsifier in products such as mayonnaise. It is also available as a food supplement, usually as granules or in capsules. Although the gut enzyme lecithinase probably breaks down most dietary lecithin before it reaches the body fluids, lecithin supplements – also known as phosphatidylcholine – usually contain large amounts of the nutrients choline and inositol, and are generally a safe and economical way to supplement them.
Good food sources of lecithin include liver, meats, fish, eggs, wheat, peanuts and soya beans. The calorific value of lecithin is similar to that of fats, at about 9 kcal per gram.