Stearic acid, a waxlike fatty acid also known as octadecanoic acid, is an important component of stratum corneum lipids. Stearic acid is also found in cocoa butter, shea butter, and other vegetable fats, as well as animal tallow. As an FDA-approved ingredient in several cosmetic products, it is used as a surfactant and emulsifying agent for fragrance and as the base for other fatty acid ingredients that are synthesized into emollients and lubricants. Stearic acid is used most often to thicken and retain the shape of soaps (indirectly, through saponification of triglycerides composed of stearic acid esters), and it is also used in shampoos, shaving creams, and detergents.
There is limited evidence for the potential of exogenously produced stearic acid to play a significant role as a topical dermatologic therapeutic agent. Stearic acid is thought to be associated with behenyltrimethylammonium chloride through salt bridges, and the combination is believed to have the capacity to build bilayer vesicles with the aid of hinokitiol (beta-thujaplicin), a natural monoterpenoid found in the wood of trees in the Cupressaceae family that has been shown to exert topical inhibitory activity against Chlamydia trachomatis (Antimicrob. Agents Chemother. 2005;49:2519-21). These vesicles, used to enhance the skin permeation of hinokitiol, were tested in hairless mice and appear to have the potential to promote hair growth (Drug Dev. Ind. Pharm. 2010;36:556-62).
In 2000, Khalil et al. studied the effects of cream formulations on chemically induced burns in mice based on reports that the ingredients, docosanol or stearic acid, were associated with antiviral and anti-inflammatory activity. Burns were engendered by painting murine abdomens with a chloroform solution of phenol. Investigators then topically applied the test formulations 0.5, 3, and 6 hours after injury. They found that the docosanol- and stearic acid–containing creams significantly mitigated the severity and progression of skin lesions compared with untreated sites, yielding, respectively, 76% and 57% declines in mean lesion scores (Contact Dermatitis 2000;43:79-81).
In 2001, Fluhr et al. studied the effects of the free fatty acid pool on stratum corneum (SC) acidification and function by topically applying two phospholipase inhibitors – bromphenacylbromide and 1-hexadecyl-3-trifluoroethylglycero-sn-2-phosphomethanol – for 3 days to murine skin. This raised skin pH and yielded permeability barrier abnormality, altered SC integrity, and reduced SC cohesion. All malfunctions were normalized, including SC pH, with the coapplication of either palmitic, stearic, or linoleic acids along with the inhibiting agents (J. Invest. Dermatol. 2001;117:44-51).
In 2010, Mukherjee et al. evaluated a recently marketed mild, moisturizing body wash containing stearic acid and emollient soybean oil to ascertain the location and amount of stearic acid deposited in the SC after in vivo usage of the product. They conducted clinical cleansing studies for 1 and 5 consecutive days using the soybean product or petroleum jelly. The deuterated variant of stearic acid replaced the free stearic acid in the soybean formulation. The researchers detected deuterated stearic acid in all 10 consecutive layers of SC, with a total stearic acid level measured at 0.33 mcg/cm2 after five washes with the soybean oil product. They concluded that the estimated total fatty acid delivered to the skin from cleansing, probably incorporated into the SC lipid phase, is comparable to the fatty acid amount in an SC layer (J. Cosmet. Dermatol. 2010;9:202-10).
Stearic acid is incorporated into several over-the-counter products, including formulations by Aveda (Green Science Firming Face Cream), Yves Rocher (Les Plaisirs Nature), Kiss My Face (with alpha hydroxy acid), Valeant Pharmaceuticals’ Kinerase line (including Clear Skin Regulating Mask), Buster’s Skin Care for Men (peptide complex organic face moisturizer), and Dermalogica (Soothing Shaving Cream with Daily Defense Block), among others.
Conclusion
While stearic acid is an important component in stratum corneum lipids and a widely used ingredient in skin care products, there is a dearth of data on its significance, if any, in the topical dermatologic armamentarium beyond its primary activity as a surfactant and emulsifying agent. Specifically, it remains to be seen whether stearic acid can be replenished in the stratum corneum through topical treatment. Much more research is needed in this area to assess the potential of stearic acid as a therapeutic agent.
Dr. Baumann is chief executive officer of the Baumann Cosmetic & Research Institute in Miami Beach. She founded the cosmetic dermatology center at the University of Miami in 1997. Dr. Baumann wrote the textbook "Cosmetic Dermatology: Principles and Practice" (McGraw-Hill, April 2009), and a book for consumers, "The Skin Type Solution" (Bantam, 2006). She has contributed to the Cosmeceutical Critique column in Skin & Allergy News since January 2001 and joined the editorial advisory board in 2004. She has received funding for clinical grants from Allergan, Aveeno, Avon Products, Galderma, Mary Kay, Medicis Pharmaceuticals, Neutrogena, Philosophy, Stiefel, Topix Pharmaceuticals, and Unilever. E-mail sknews@frontlinemedcom.com to contact Dr. Baumann or to suggest topics for a future column.