ARTICLE: Colloidal Oatmeal Part I: History, Basic Science, Mechanism of Action, and Clinical Efficacy in the Treatment of Atopic Dermatitis

October 2020 | Volume 19 | Issue 10 | Supplement Individual Articles | s4 | Copyright © October 2020

Published online September 21, 2020

Blair Allais MD, Adam Friedman MD FAAD

Department of Dermatology, George Washington School of Medicine and Health Sciences, Washington, DC

Avenanthramides have also been found to suppress IL-1β stimulated secretion of pro-inflammatory cytokines such as IL-6, IL-8, and MCP-1.13 Taken together, these findings highlight the anti-inflammatory properties of oat and their role in alleviating inflammation in various dermatologic conditions.

Colloidal oatmeal also has anti-pruritic properties via the inhibition of neurogenic inflammation. In a murine itch model, mice were injected with compound 48/80, which leads to mast cell degranulation and histamine release, key mediators of itch. Those mice that were treated with avenanthramide itched 40.7% less than controls.14 These findings in murine models have been evaluated further in clinical studies of patients who suffer from itch.

In one clinical study, 139 patients with a variety of pruritic dermatoses were treated with colloidal oatmeal bath and cleanser for 3 months. More than 71% of patients achieved complete or near-complete relief of pruritus during the study period.15 In addition to anti-inflammatory and anti-pruritic properties, colloidal oatmeal also has anti-oxidant properties. Phenolic avenanthramides exhibit antioxidant activity in vitro. Phenols as a class of chemical compounds exert antioxidant activity through several mechanisms: some are hydrogen atom donors that inhibit the cascade of radical chain reactions, and others function as metal ion chelators.16,17 The antioxidant properties of avenanthramides have been studied in detail. In one such study, eight avenanthramides identified in oat extracts were synthesized and assessed for antioxidant activity by determining reactivity toward 1,1- diphenyl-2-pic-rylhydrazyl and linoleic acid via the efficiency of hydrogen atom transfer from phenol to radical. Avenanthramides demonstrated higher antioxidant activity than other oat phenolic compounds, and authors hypothesized that it may be due to the resonance structure of its amide bond.16

Colloidal oatmeal also has anti-fungal and prebiotic properties. The Pc-2 gene in A. sativa plants confers resistance to Puccinia coronate, the crown rust fungus. Inoculation of oat plants with spores of P. coronate-induced avenanthramide production and inhibited further fungal growth.18 Emerging evidence has also shown that colloidal oatmeal has prebiotic properties. Colloidal oatmeal is metabolized by and promotes the growth of bacteria that is commensal to the skin includ-ing Staphylococcus epidermidis, Staphylococcus aureus, and Propionibacterium acnes. In a study by Liu-Walsh et al, colloidal oatmeal increased the growth rate of Staphylococcus epidermidis significantly more than that of Staphylococcus aureus, suggesting a differential response of these organisms to oats.19 Lactic acid and short-chain fatty acids are produced by microbes by fermentation, and have been shown to play an important role in the maintenance of gut and skin health. Lactic acid is also a known natural moisturizing factor and humectant. According to in vitro studies by Liu-Walsh et al metabolism of colloidal oatmeal results in increased pro-duction of lactic acid by Staphylococcus epidermidis and Staphylococcus aureus. In the same study, six weeks of use of a moisturizing lotion containing 1% colloidal oatmeal significantly increased the level of lactate in vivo.19

Colloidal oatmeal also has barrier repair properties. Lipids play a crucial role in the stratum corneum, particularly in its barrier function and ability to prevent excess loss of water. Stratum corneum lipids consist of an equimolar mixture of ceramides, cholesterol, and free fatty acids. Whole oat oil is rich in essential lipids including triglycerides, diacylglycerol, phospholipids, and free fatty aids. Oats also contain linoleic acid, which has been shown to be effective in reducing tran-sepidermal water loss (TEWL) and restoring the permeability of the skin barrier.20-21 The lipophilic molecules in oat also pos-sess agonist properties towards various receptors and genes involved in epidermal differentiation, further lipid synthesis, and ceramide processing. Peroxisome proliferator-activated receptors (PPARs) are ligand-activated nuclear receptors that have been shown to induce both the expression of epidermal differentiation proteins and lipid synthesis in keratinocytes.22 In in vitro cell and tissue culture studies, Chon et al demonstrated that lipophilic oat molecules possess dual PPARα and PPARβ/δ agonist activities. Oat oil treatment also resulted in a significant up-regulation of differentiation genes (including involucrin and transglutaminase 1) and ceramide processing genes (β-glucocerebrosidase, sphingomyelinases 3).23

Finally, topical application of oat extract has a beneficial effect on skin pH. The skin is a slightly acidic microenvironment with a pH of approximately 5.5.15 This acidity enhances the barrier function of the skin, protecting against entry of pathogens, and assisting in maintenance of the integrity of the superficial keratin layer. Inflamed skin increases the pH of the skin from acidic to basic. Topical application of oatmeal extract has been shown to decrease the pH of the skin from a basic pH towards a normal physiologic pH, indicating that it can act as a restor-ative buffer.15

Use in Atopic Dermatitis
The clinical benefits of colloidal oatmeal have been demon-strated through extensive research across diverse patient populations, particularly in atopic dermatitis (AD). Atopic dermatitis is a common, relapsing inflammatory skin disorder with a complex pathogenesis. It is characterized by genetic abnormalities in the skin barrier via mutations in filaggrin, deficiencies in ceramides and cathelicidins, immunologic disturbances with a shift toward the Th-2 inflammatory path-way, and an elevation in serum immunoglobulin (IgE) levels. Topical agents are a mainstay of AD therapy regardless of dis-ease severity. According to current clinical guidelines for the treatment of atopic dermatitis, the application of moisturizers should be an integral part of the treatment of patients with AD