Betaine
In plain terms
A gentle humectant and amino acid derivative from sugar beets that draws water into skin and takes the edge off harsh detergents. It is well tolerated and often added to make cleansers milder.
What the evidence shows
Betaine, also called trimethylglycine, is a small molecule that plants like sugar beet use to hold water and manage stress, and it does the same work on skin as a humectant that draws and binds moisture. It has a second, quieter job: because its charge is nearly neutral, it blends into cleansers to soften them, and human testing shows it reduces the irritation that strong detergents like sodium lauryl sulfate cause. The safety record is clean. The US cosmetic-ingredient panel judged the betaines safe as used when formulated to be non-irritating, with betaine itself non-sensitizing in human and animal testing at high concentrations. It is the same compound the body and the diet already contain, and it carries no hormone, cancer, or contamination concern. One thing to keep separate: the surfactant cocamidopropyl betaine, which shares part of the name, is a different ingredient with its own impurity-related allergy question; plain betaine is a different, simpler molecule. For nearly everyone it is a gentle hydrator and a formula softener.
On the label
Listed as betaine or trimethylglycine, usually among the humectants. It is not the same as cocamidopropyl betaine, the coconut cleanser with the amidoamine allergy question; plain betaine is a gentle hydrator with no such issue.