This protein is the building block of your hair. It is also responsible for keeping your strands strong and shiny.

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What is it?

Keratin is a structural protein that your body naturally produces, and the foundation of every strand of hair. It’s also present in your nails and skin. In your tresses, it’s referred to as alpha-keratin, and it’s what makes your hair flexible yet resilient. It constitutes 90 percent of each strand, explains Adir Abergel, celebrity hairstylist and creative director of Virtue Labs. The protein is found in both the cortex (which contains the pigment that gives hair its color) and in the cuticle (the protective outer layer).

How do you use it?

When hair is damaged, keratin proteins are affected, and the hair shaft becomes more porous because of cracks in the cuticle. In turn, this can lead to frizz and breakage. Everything from using hot tools and even combing can scrape away tiny amounts of this protein from your cuticle layer, explains Kevin Mancuso, global creative director for Nexxus. Adding keratin back with products that contain this wonder protein can patch up those cracks and repair hair over time. One thing to note: Don’t go all-in with lots of different keratin products—at least not at first. In some cases, too much of the protein can actually make hair even more brittle and prone to breakage, explains Abergel. This is because the balance between moisture and protein in the hair is out of whack. It’s a good idea to start with a weekly mask. After a few treatments, you should notice healthier, shinier strands. From there, you can experiment with adding in other keratin-based products. Thick serums tend to be great for coarser hair types. If you notice your hair reacting poorly, just dial back your usage.

Wondering where the in-salon smoothing treatment (often called a keratin treatment) fits in? You should know that it’s different from these at-home products. The good-for-your-hair protein isn’t always the main ingredient in this frizz-fighting process. A lot of formulas rely on chemicals that, when mixed with water, release formaldehyde. Your hair is covered in this mixture, and then high heat is applied to relax strands—a combo that can do more harm than good. If you’re interested in trying a temporary salon treatment to help defrizz, do some research and find a stylist who uses a glyoxylic-acid version of the straightening treatment (rather than formulas that release formaldehyde). Also ask for a list of ingredients, and avoid these: methylene glycol, formalin, paraform, and methanal.

This Way to Silky Strands

TRESemmé Keratin Smooth Shine Serum ($5; at Target). Put this nongreasy formula on wet hair as a treatment before heat-styling or on dry hair to create a polished, healthy look.

Keratin Complex Keratin Obsessed Multi-Benefit Treatment Spray ($26; It’s formulated with keratin and coconut oil, so a few spritzes on damp hair will nourish and hydrate—ultimately eliminating frizz, and even enhancing color.

It’s a 10 Miracle Deep Conditioner Plus Keratin($25; Once a week, skip your conditioner and apply this for up to 10 minutes before rinsing. The result: stronger, more moisturized strands.

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