Derms Say These 10 Hydrating Serums Are Dry Skin’s Worst Nightmare

Hyaluronic acid acts as a humectant, meaning it retains water. “HA is a chain of carbohydrates that has the ability to hold on to water up to 1000 times its weight,” says David Kim, MD, MS, a board-certified dermatologist. “It’s an ingredient that helps your skin appear hydrated and plump.”

There isn’t just one form of hyaluronic acid, either—it comes in different sizes that are able to penetrate different layers of the skin. “Each molecule is assigned a molecular weight, which inversely relates to how deep the molecule can penetrate the skin,” says Rina Allawh, MD, a board-certified dermatologist at Schweiger Dermatology Group in King of Prussia, PA. The smaller the molecule, the deeper into the skin it’s able to penetrate. 

It may seem like you’d always want to go for the hyaluronic acid with the lowest molecular weight possible, but that’s not always the case. “The advantage of higher-molecular-weight hyaluronic acid serums are the anti-inflammatory and soothing properties, as it forms a durable film with the top layer of the skin,” says Allawh. “A low-molecular-weight hyaluronic acid serum is more likely to have a lasting anti-wrinkle and regenerating effect and works in the deeper levels of the skin. As you can imagine, lower-molecular-weight hyaluronic acid serums are able to lock in the moisture in the connective tissue of the skin and, as a result, stimulate firming effects.”