Hyaluronic acid is also known as hyaluronan. It is a carbohydrate, or more specifically a mucopolysaccharide found in abundance in almost every cell of the body. It is a very large molecule and can be several thousand sugar molecules long.
Hyaluronic acid is a major component of connective tissue in the body and is also found in between cells. It is found in all bones and cartilage, a part of tendons and ligaments, in lips, and in greatest concentrations, in the skin. Almost half of the body's hyaluronic acid is found in skin. When not bound to other molecules, hyaluronic acid is bound to water. This large molecule has the ability to bind to a great deal of water! It is this property of hyaluronic acid that helps keep the skin healthy. As we age, the skin's ability to produce hyaluronic acid decreases.
Since hyaluronic acid is such a large molecule, it is unable to penetrate the 20 most superficial layers of the outer skin (stratum corneum), even in the presence of vitamin C, and pass into the deeper layers of either the epidermis (the top layer of skin) or into the dermis (the middle layer where wrinkling and fine lines are formed). Despite marketing claims by certain manufacturers that the formulation contains "low molecular weight" hyaluronic acid, reliable, reproducible science proving deep penetration of hyaluronic acid doesn't exist to support those claims. However, the ability of this large molecule to bind water helps hydrate the outer layers of skin, thereby improving the skin's appearance.
In skincare products, hyaluronic acid is used as a humectant, a substance that helps the skin hold onto water. In addition, it is used to help thicken the aqueous (water) portion of cosmetics and personal care products.
As for the claims that hyaluronic acid is the "key to the fountain of youth"....well, I'm sorry to say, marketing hype. Science has yet to find a single ingredient, molecule or product that can reverse or slow the aging process.