Phosphatidylcholine is a phospholipid which is a polar ionic compound composed of an alcohol and a diacylglycerol, or sphingosine, linked by a phosphodiester bridge. It is a part of the membrane of cells and acts as an essential component of bile, aiding the solubilization of cholesterol. Supplementation with lecithin (which contains high levels of phosphatidylcholine) has been shown to decrease bile cholesterol levels.  All animal cells have a phospholipd bilayer that is principally composed of phosphatidylcholine. This phospholipid compound is also a main component of circulating lipoproteins.
Some sources confuse phosphatidylcholine with lecithin and use the two terms interchangeably. However, these are distinct compounds with lecithin being a mix of phospholipids and other fats. Lecithin is the primary dietary source of phosphatidylcholine. Supplements of lecithin that are formulated to contain more than 30% phosphatidylcholine are considered phosphatidylcholine concentrates. 
Phosphatidylcholine, the main component of lecithin, can be found in various food sources, most notably legumes (especially soybeans), egg yolks, whole grains, fish, and brewer’s yeast. [4, 5]
Applications of phosphatidylcholine are diverse. For many years, it was thought to be useful for dementia, but the bulk of evidence does not support this use. Phosphatidylcholine supplementation, often using the form of lecithin, is most commonly employed for treatments of bipolar disorder, gallstones, and conditions of the liver.
Phosphatidylcholine as soybean lecithin has demonstrated the ability to dissolve gallstones when used in combination with cholic acid. A study of 7 patients with radiolucent gallstones and 2 patients with radiolucent calculi in the biliary tree was conducted using 2250 milligrams (mg) of lecithin daily, combined with a 750mg dose of cholic acid over a period of 6 months.  The results showed that in 2 patients, gallstones completely disappeared. Another participant exhibited a decrease in gallstone size as well. This is a clinically significant study indicating the possible benefit of phosphatidylcholine for patients with gallstones. It is important to note that studies have not shown this effect when using phosphatidylcholine alone.
Bipolar disorder has been treated with phosphatidylcholine. This used is based on its ability to raise acetylcholine levels, which is a proposed mechanism of action for the effectiveness of lithium therapy. Two studies have documented clinically significant effects in numerous patients suffering from bipolar disorder. [7, 8]
Although a number of studies have been conducted to examine the effect of phosphatidylcholine in cases of dementia, the majority were poorly designed. Many trials showed only a modest effect or demonstrated no physiological effect at all. [9-11] A Cochrane Review was conducted in 2003 for lecithin and phosphatidylcholine in the treatment of dementia and cognitive impairment, to determine whether these studies warrant further investigation.  The inclusion criteria for studies was cited as: “all unconfounded, randomized trials comparing lecithin with placebo in a treatment period longer than one day, in patients with dementia of the Alzheimer type, vascular dementia, mixed vascular and Alzheimer’s disease, unclassified or other dementia or unclassified cognitive impairment not fulfilling the criteria for dementia”.
The conclusion of the study was that the evidence from randomized trials does not support the use of lecithin in the treatment of patients with dementia. The authors do note that a moderate effect cannot be ruled out, but suggest that a large randomized trial is not indicated at this time.
A novel application for phosphatidylcholine is in the treatment of infraorbital fat pad hernaition. This condition results when fat deposits contained in a specific location under the eye has herniated. In an open label study, an injectable form of phosphatidylcholine was examined in patients with this condition.  Every two weeks patients received a 0.4 ml injection of a 50mg/ml solution of phosphatidylcholine, administered directly into the infraorbital fat pad. After three to five treatments, patients rated their improvement at 70%, while physicians graded the improvement as 80%. Adverse effects noted in this trial included burning, erythema, and swelling at the injection site. In addition, at a 9 month follow up, 50% of patients assessed noted lasting effectiveness from treatment. This treatment appears to be preferable to the standard blepharoplasty, a surgical procedure.
Other conditions for which phosphatidylcholine may be beneficial include; peptic ulcers, sprains, strains, and contusions. [14, 15] Double-blind studies have also confirmed the utilization of phosphatidylchoine for hepatic/alcoholic steatosis and Hepatitis B and C. [16-18]
The dosage of phosphatidylcholine from supplementation may vary depending on the condition being treated. The dosage range is 1800 - 2400mg daily, with even higher levels required for liver damage.  The absence of noticeable effect in dementia cases after two weeks would indicate withdrawal of the supplement, as it will likely not be effective. 
Synthesis of phosphatidylcholine in the body requires choline, which can be obtained from the diet or from catabolism, or the breakdown of phospholipids in the membrane of all cells. If choline is not readily available from either of these sources it can be synthesized in the body from two different amino acids, either methionine or serine.  Biosynthesis of choline takes place in the liver.
Low levels of choline may be associated with fatty liver, gastric ulcers, kidney and liver impairment, cardiac symptoms, hypertension, and difficulty with fat digestion.  Lower levels of phospholipids in general, may also result in the formation of gallstones. 
Phosphatidylcholine is generally considered a safe therapeutic agent. Daily doses of up to 18 grams have been well tolerated in humans.  When adverse effects occur, it is a sign of intolerance and appears as gastrointestinal discomfort. Patients often experience symptoms of diarrhea, nausea, and fullness. 
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