Lecithin contains dietary precursors to choline, an essential nutrient, which was formerly classified as a B vitamin (vitamin B). Lecithin is a mixture of fats that contains phospholipids, including phosphatidylcholine, and the human body can convert phosphatidylcholine into choline. The choline content in lecithin can vary, but it's found that phosphatidylcholine makes up about 25 to 35 percent of lecithin. Furthermore, only about 12 percent of phosphatidylcholine is actually choline. This adds up to a choline content of approximately 4 percent for lecithin. For example, 10 grams of lecithin has 2,500mg of phosphatidylcholine. 
Phosphatidylcholine is approximately 13.7% choline; as such, about 342mg of choline is present per 10 grams of lecithin. Therefore, 10 grams of lecithin can be a source for the body to produce about the same amount of choline (342mg) as can be produced by the body from 2 egg yolks. The recommended intake of choline varies depending on age, sex, and physiological conditions, and is roughly 500 mg per day for adults.
Lecithin is generally recognized as safe (GRAS) by the FDA.
There is no robust, scientifically validated clinical research investigating the safety and effectiveness of high-dose lecithin supplementation in lactating women and their infants. A meta-analysis found no evidence that high doses of lecithin improved milk flow in breast-feeding mothers or infants, though concluded that that "higher maternal choline intake was likely to be associated with better child neurocognition and neurodevelopment."
Soy lecithin does not contain enough allergenic proteins for most people allergic to soy, although the US FDA only exempts a few soy lecithin products from its mandatory requirements for allergenic source labeling. An alternative source of lecithin, derived from sunflowers, is available as a dietary supplement for those with concerns about soy-based foods.
A 2003 review of randomized trials found no benefit of lecithin in people with dementia.