Phosphatidylserine is a phospholipid, a kind of fat found throughout the body but particularly concentrated in the brain, where it keeps cell membranes fluid. It is believed to help keep the memory-related pathways in the brain working properly. Because levels of phosphatidylserine decline with age, it has been proposed as a treatment for age-related dementia. Some studies indicate it is effective, but although it is also touted as a treatment for the mental fatigue of fibromyalgia [and CFS], no one knows whether it works in healthy people.
(Source: Natural Treatments for Fibromyalgia: An A to Z Guide, by Kenna Simmons. An official publication of the Arthritis Foundation. To purchase, visit www.arthritis.org.)