What is it?
The sodium salt of isostearic acid, a mixture of branched fatty acids with 18 carbons. Fatty acid salts like this one are soaps. Sodium isostearate cleanses and helps oil and water mix, and it is used in small amounts in bath soaps, face and neck products, moisturizers and night creams. It is a minor ingredient in cosmetics, with only a handful of reported uses.
Key information
- The CIR Expert Panel concluded in 2019 that sodium isostearate and 101 related fatty acids and fatty acid salts are safe as used when formulated to be non-irritating and non-sensitizing.
- It is a soap: the sodium salt of isostearic acid, a branched 18-carbon fatty acid.
- In 2019 it was reported in only 11 cosmetic products, at up to 3 percent.
More information
Sodium isostearate combines sodium with isostearic acid, a mixture of branched-chain fatty acids with 18 carbons, the same size as stearic acid. Like sodium cocoate and sodium palmate, it is a soap. The Cosmetic Ingredient Review (CIR) reported 11 uses in 2019, including bath soaps, face and neck products and moisturizers, at up to 3 percent in rinse-off products.
The CIR Expert Panel reviewed sodium isostearate with 101 other fatty acids and fatty acid salts. The report contains little data on sodium isostearate itself, but its parent acid has been well studied. Undiluted isostearic acid caused no irritation in 100 people in a 24-hour patch test, and at 10 to 35 percent it caused neither irritation nor allergy in 271 people. In one rabbit-ear test, a formula became comedogenic (pore-clogging) when 2.5 percent isostearic acid was added. The Panel noted that fatty acids and their salts can irritate skin and that some unsaturated fatty acids can oxidize into compounds that trigger allergies, and concluded that the group is safe when formulated to be non-irritating and non-sensitizing. Australia's industrial chemicals regulator (NICNAS) also found that sodium isostearate does not pose an unreasonable risk to workers or public health.
If you are prone to breakouts and already avoid isostearic acid, note that sodium isostearate is its salt. The pore-clogging finding comes from a single animal test of a formula, not from studies in people.
Sources: Cosmetic Ingredient Review: Safety assessment of fatty acids and fatty acid salts, final report (2019), copy hosted by FDA; Cosmetic Ingredient Review: Quick reference table, safe-with-qualifications findings (2020); PubChem: Sodium isostearate
