Easily one of my favorite cosmetic ingredients out there!

Hyaluronic acid is a super powerful humectant, which  means it draws water from the air and brings it to our skin, effectively hydrating it. As a result, this helps to plump the skin and give it a more youthful appearance.

The science

Hyaluronic acid(HA) is really big sugar molecule (weights 4 million Dalton, or 4000 kDa) found naturally in the body. It is the most important molecule in the extracellular matrix (a complex network of proteins that surrounds and supports skin cells). There can be many different forms of hyaluronic acid because it is a polymer and its subunits can be repeated as many or as little times as needed.

HA with high molecular weight (HMW): 500k-4M Dalton

In the skin: HMW has good water-binding abilities, binding 1000 times its own weight in water. It helps our skin attract and retain moisture. It can also maintain the skin’s elasticity, regenerate the skin barrier, and plumping up wrinkles.

HMW HA probably does not penetrate the skin, but instead it stays on top to give surface hydration and to form a protective barrier. It can alos work as an osmotic pump, which means it can push water-soluble actives deeper into the skin because it forms a hydrating surface.

HA with low molecular weight (LMW): 500K Dalton below

LMW HA might absorb a bit better than HMW-HA, and can help increase the skin’s self-defense and also help with rosacea and dermatitis. However, HA with a molecular weight of 50kDa or less shows some pro-inflammatory activity, but the study was not done on real human skin. A study found on real people though with 50kDA HA showed that it can plump up wrinkles. TLDR; just to be safe, I would stay with a normal molecular weight HA. 

P.S. You might see "hydrolyzed hyaluronic acid" in the ingredient list. This is just a chemically chopped up version of hyaluronic acid (low molecular weight).

P.P.S You might also see "sodium hyaluronate". It's the sodium form of hyaluronic acid. This and hyaluronic acid are functionally the same thing. The only difference is this form is more stable and easier/cheaper to formulate. If you search up sodium hyaluronate vs hyaluronic acid it may say that the salt form penetrates more, but it really doesn’t and they function similarly.