Leucine

Leucine is a small amino acid and one of the building blocks your body uses to make proteins.

It's also naturally found in your skin as part of your Natural Moisturizing Factor (NMF). Your NMF is a mix of water-binding molecules that keeps the outer skin layer hydrated and flexible.

In skincare, it's mainly used as a skin conditioning ingredient that helps reinforce the same moisture-retention function.

You'll usually see it used at low percentages (generally under 5%) which is in line with how amino acids are used in cosmetics.