Active ingredient in lecithin. Every cell membrane in the body requires phosphatidylcholine (PC). It is also a major source of the neurotransmitter acetylcholine. Acetylcholine is used by the brain in areas that are involved in long-term planning, concentration, and focus, but all of that information is associated with ingesting PC, not putting it on the skin. PC is considered a very good water-binding agent and aids in the penetration of other ingredients into the skin. It absorbs well without feeling greasy or heavy (although other ingredients can perform similarly, including glycerin, ceramides, and hyaluronic acid) (Sources: Skin Pharmacology and Applied Skin Physiology, September–October 1999, pages 235–246; and Journal of Controlled Release, March 29, 1999, pages 207–214.) See lecithin