Isoleucine
What isoleucine does as a conditioning amino acid in skincare blends, why it isn't an exfoliating acid, and who it suits.
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Quick facts
- INCI name
- Isoleucine
- CAS number
- 73-32-5
- Origin
- Biotech (fermentation) or Synthetic
- Chemical class
- Amino acid
- What it does in a formula
- skin conditioning, humectant, hair conditioning
- Why it is in the formula
- Usually one of several amino acids in a blend that helps a formula hold water and leave a conditioned feel.
- Regulatory notes
- The Cosmetic Ingredient Review panel concluded that alpha-amino acids, including isoleucine, are safe in the present practices of use and concentration in cosmetics.
What is Isoleucine?
Isoleucine is one of the amino acids your body uses to build proteins. Cosmetic isoleucine is usually made by fermentation or chemical synthesis, and the label name doesn’t tell you which.
Isoleucine is one of the three branched-chain amino acids, along with valine and leucine. Its side chain is oil-like rather than water-loving, so in skincare it’s mostly part of a larger blend of amino acids.
On a label it usually sits in a group with other amino acids, such as glycine, alanine, serine, proline and aspartic acid. You’ll find it in serums, toners, moisturizers and some hair products.
What does Isoleucine do?
- Conditions skin. The safety review describes amino acids as working mainly as skin and hair conditioning agents.
- Helps hold water. Amino acids are also listed as humectants, so they help a product hold moisture at the skin’s surface.
- Rounds out a blend. Alongside other amino acids and humectants, it adds to a soft, hydrated finish.
It’s a supporting ingredient. The safety review reported it at no more than about 0.002% in cosmetics, so it isn’t a reason to buy a product on its own.
Who is Isoleucine for?
It suits most skin types, including dry and sensitive skin. The safety panel reviewed alpha-amino acids as a group and concluded they are safe as used in cosmetics.
The “acid” in “amino acid” sometimes worries people with sensitive skin. Isoleucine isn’t an exfoliating acid. If a product that lists it stings, other ingredients in the formula, such as fragrance or an exfoliating acid, are more likely to be responsible. Patch-test new products if your skin reacts easily.
What to pair Isoleucine with, and what to avoid
It already sits alongside other amino acids and humectants such as glycerin and hyaluronic acid in most formulas. There’s nothing you need to avoid with it, and you don’t need a separate isoleucine product.
Quick picks with Isoleucine
Best pick with no fragrance listed
No scent or sensitivity flags recorded in our glossary
Brickell Restoring Eye Serum Treatment for Men
Hyaluronic acid, peptides, vitamin C, reishi mushroom · oily, dry, combination, sensitive, normal skin · no scent flags recorded
Good for most skin types
Brand lists it for all skin types
Brickell 6 in 1 Gel Face Moisturizer
Aloe, centella, jojoba, sodium hyaluronate, sodium ascorbyl phosphate, amino acids · oily, dry, combination, sensitive, normal skin · no scent flags recorded
Most covered in our guides
Featured in our guides
Baxter of California Herbal Mint Toner
Witch hazel water, cucumber, aloe, chamomile, vitamin E, allantoin · scent ingredients flagged
All 17 products with Isoleucine
Daily Moisturizers & Water Gel Creams (2)
Hydrating Toners, Essence Waters & Face Mists (2)
Eye Creams & Under-Eye Patches (2)
Gel, Foam & Cream Facial Cleansers (1)
Hydrating Serums, Essences & Ampoules (1)
From stores: products with Isoleucine
Hydrating Serums, Essences & Ampoules (6)
Hydrating Toners, Essence Waters & Face Mists (1)
Mineral Pressed Powders & Skin-Nourishing Primers (2)
Common questions
What does isoleucine do in skincare?
It's an amino acid used as a skin-conditioning ingredient and humectant. It helps the formula hold water and leaves skin feeling softer, usually as part of a blend with other amino acids.
Is isoleucine an exfoliating acid?
No. The word acid refers to its chemistry as an amino acid. In skincare it's used for conditioning and water-binding, not to exfoliate like glycolic or salicylic acid.
Is isoleucine safe for skin?
A cosmetic safety panel concluded alpha-amino acids, including isoleucine, are safe as used in cosmetics. As with any product, stop using one that stings or leaves you red.
What are branched-chain amino acids in skincare?
Isoleucine, leucine and valine are called branched-chain amino acids because of the shape of their side chains. In skincare they usually appear together in amino acid blends for conditioning, not as separate actives.