During my last visit to the dermatologist, I was introduced to a new companion that my skin has invited without my consent. It is, in a nutshell, a dry skin condition that crops up every so often on my feet. I was prescribed Keralac ointment to spread over the hardened areas on the soles of my feet every night after showering. Besides vitamin E, lactic acid, and zinc, this preparation contains 50% urea. That's a whole heap of urea, which makes me wonder- How does something manufactured by the body and excreted in urine and sweat come to constitute half the active portion of a skin treatment?
It appears that dry skin conditions like mine, as well as eczema, psoriasis, and ichthyosis, result in drastically reduced amounts of urea. The concentration of urea in healthy skin averages 28 micrograms of urea per square centimeter, which is diminished by 50% in dry skin and by 85% in skin afflicted with atopic eczema. This is a problem because urea is one of the three natural moisturizing factors in the outer horny layer of skin, the other two being lactic acid and amino acids. Representing 7% of the natural moisturizing factors in the stratum corneum (skin-building layer), urea plays a vital role in maintaining the skin's moisture balance and suppleness. Reduced levels of urea lead to a lower water-binding capacity within the skin, which in turn, can lead to roughness, tightness, flaking, and irritation.
While internal urea transported by the kidneys plays a vital role in human metabolism, synthetic urea often finds its way into topical dermatological products to rehydrate the skin. Lab-manufactured urea can be helpful in treating scaly skin, corns, calluses, and ingrown nails. By increasing moisture levels within the skin, urea not only softens and reduces rough texture, but it also has a localized anesthetic effect, helping to curtail cycles of itching, irritation, and flare-ups. It may also be used in helping remove dead tissue to speed wound healing because it can improve the capacity of the epidermal barrier to regenerate.
According to a New Zealand study in 2007, urea works best for treating ichthyosis and hyperkeratoic conditions (Australian Journal of Dermatology, Vol 11, June 28 2007). In this experiment, 58 patients suffering from a range of dermatoses were treated with 10% urea in a non-greasy base. Those with atopic dermatitis reported only fair results, and no improvement was noted in those with Psoriasis, Solar Keratosis, and Peri-oral Dermatitis. However, the participants with hyperkeratotic conditions saw excellent results.
Urea is joined by lactic acid, salicylic acid, and allantoin as the most common keratolytics, which serve to exfoliate and soften keratin (a key component of skin). You might recognize salicylic acid from treatments for removing warts and lesions, reducing dandruff, and clearing up acne. By dissolving the horny substance holding the top layer of skin cells together, urea (and other keratolytic agents) help the dead skin cells fall off as the skin retains water. This action ultimately improves the skin's moisture-binding capacity.
In addition to their keratolytic properties, urea and lactic acid are also effective humectants, which means that they bind water below the surface of the epidermis, right down to the stratus corneum. Urea is highly hygroscopic, or water-loving, and modifies the structure of amino chains and polypeptides in skin. This is significant for skin moisturizing since there is a direct correlation between water content and amino acid content in skin (the drier skin is, the lower its share of dissolved amino acids).
Ironically, urea can be irritating to the skin and may cause dermatitis with repeated exposure. Urea's Material Safety Data Sheet indicates that this substance (in its pure form) can cause skin and eye irritation from direct contact, blood electrolyte imbalance from ingestion, and adverse reproductive effects from prolonged exposure. Though side effects are generally mild and rare, the Cosmetics Safety Database gives it a hazard ranking of 4.
Urea also gets a bad name from a couple of common preservative compounds that rely on urea's natural antibacterial, antiviral, and antimicrobial properties. Diazolidinyl urea and imidazolidinyl urea (trade name Germall 115) both release formaldehyde, which raises a red flag for altered DNA (aka cancer), as well as a number of physical and psychological problems. A statistically significant percentage of people exposed to this chemical experience allergic reactions. These two microbials are some of the most widely used preservatives in the world and are generally considered safe at low concentrations. Nonetheless, extremely sensitive, oily, or infected skin should steer clear of all topical products containing urea.
Urea preparations typically range in strength from 3-40% and can take many forms, including creams, gels, shampoos, deodorants, foundation, and even toothpaste. While doctors often prescribe urea in skincare preparations for dry skin conditions, it can also be found in OTC products under the name carbamide (the synthetic version of urea). Another name to watch for is hydroxyethyl urea, a superior lubricating ingredient comparable to glycerin. I have already noticed a difference in the texture of my feet since starting a regimen with the urea-loaded ointment. Seriously dry skin can't even put up a fight with urea around.