Moisturizers: A Comprehensive Dermatology Review

1. Definition and Purpose

• Fitzpatrick's Dermatology, 9th ed.

2. Pathophysiology of Dry Skin (the Target)

• Natural Moisturizing Factor (NMF) - hygroscopic compounds (amino acids, PCA, urea, lactate, sugars) derived from filaggrin breakdown
• Intercellular lamellar lipids - ceramides, cholesterol, and free fatty acids in an approximately 1:1:1 molar ratio
• Aquaporin-3 (AQP3) channels - facilitate water transport through the epidermis

• Dermatology 2-Volume Set, 5th ed.

3. Classification of Moisturizer Ingredients

A. Occlusives

• Mechanism: Form a hydrophobic film on the skin surface that physically reduces TEWL. Work only while on the skin - once removed, TEWL returns to baseline.
• Key agents (ranked by occlusive efficacy):
  • Petrolatum - most effective; vapor loss resistance 170x that of olive oil; greasy texture limits cosmetic acceptance
  • Lanolin, paraffin, beeswax, squalene
  • Dimethicone (silicone) - good occlusion, cosmetically elegant
  • Mineral oil, soybean oil, grape-seed oil, propylene glycol
• Caveat: Reducing TEWL by >40% can lead to maceration and increased bacterial colonization.

B. Humectants

• Mechanism: Hygroscopic molecules that attract water from the atmosphere (most effective at >80% relative humidity) into the SC. In low humidity, they may paradoxically draw water from deeper layers, worsening dryness - hence they are most effective when combined with an occlusive.
• Effects: Rapidly draw water into the SC, causing slight swelling that fills fine lines and smooths surface texture. Effects are temporary (<24 hours) except for glycerin.
• Key agents:
  • Glycerin (glycerol) - most widely used; traverses AQP3 channels giving longer-lasting effects than other humectants; also has barrier repair properties
  • Hyaluronic acid (sodium hyaluronate) - binds 1000x its weight in water; popular in serums and lighter moisturizers
  • Urea - humectant at low concentrations (2-10%); keratolytic at higher concentrations (>10-20%); used for ichthyosis and hyperkeratosis
  • Sorbitol, propylene glycol, lactic acid, glycolic acid (AHAs), sodium PCA, arginine

C. Emollients

• Mechanism: Fill the spaces between desquamating corneocytes to create a smooth, cohesive surface. They flatten the curled edges of corneocytes, improving light reflection and giving an immediate visible improvement. They do not primarily affect hydration.
• Key agents: Fatty acids (linoleic acid, oleic acid), isopropyl myristate, cetyl alcohol, stearyl alcohol, shea butter, squalene, many plant oils

D. Barrier Repair Agents (Physiologic Lipid Replenishers)

• Mechanism: Replace the deficient intercellular lamellar lipids of the SC, addressing the underlying structural defect rather than temporarily masking it.
• Key agents:
  • Ceramides - most critical; historically animal-derived but now available as pseudoceramides formulated in multilamellar emulsions (MLE) that mimic the natural 3D barrier structure
  • Cholesterol
  • Free fatty acids (palmitic, stearic, linoleic)
  • Optimal ratio: ceramide:cholesterol:fatty acid = 1:1:1
• These agents may take 4+ days to produce noticeable results (vs. humectants/occlusives which work within hours), but produce lasting improvement by correcting the underlying defect.
• Fitzpatrick's Dermatology, 9th ed.

4. Formulation Vehicles

5. Ingredients of Special Clinical Interest

Ceramides

Niacinamide (Vitamin B3)

• Increases synthesis of free fatty acids, cholesterol, and ceramides
• Reduces TEWL by improving barrier function
• Anti-inflammatory: decreases inflammatory cytokines
• Inhibits melanosome transfer from melanocytes to keratinocytes (useful for hyperpigmentation)
• Potent antioxidant
• Concentration: typically 2-5% in OTC products

Colloidal Oatmeal (Avena sativa)

• FDA-approved skin protectant (1-3% concentration)
• Contains beta-glucans and avenanthramides with anti-inflammatory and antipruritic properties
• Restores SC lipids and NMF
• Useful in AD, contact dermatitis, pruritus

Hyaluronic Acid (HA)

• Endogenous glycosaminoglycan in the dermis and epidermis
• Multiple molecular weights have different penetration depths; lower MW penetrates deeper
• Humectant; does not form an occlusive barrier
• "HA5" formulations combine 5 molecular weight forms

Hydroxy Acids

• Alpha-hydroxy acids (AHAs): Lactic acid, glycolic acid - humectant properties, promote desquamation by reducing corneocyte cohesion, increase ceramide levels; lactic acid used up to 12% for ichthyosis
• Beta-hydroxy acid (BHA): Salicylic acid - lipophilic, penetrates the pilosebaceous unit, comedolytic, anti-inflammatory (salicylate activity), mildly drying
• Newer generation hydroxy acids also stimulate collagen, inhibit MMP and elastase activity, and block glycation

Urea

• Dual-function: humectant at 2-10%, keratolytic/emollient at 10-40%
• Clinical uses: ichthyosis, palmoplantar keratoderma, xerosis, nail dystrophy
• 20% urea: strong keratolytic for hyperkeratotic conditions
• May sting on broken skin

Petrolatum

• Gold standard occlusive; 99% effective in reducing TEWL
• Safe in all ages including neonates; no known allergenicity
• Aquaphor = ~41% petrolatum + mineral oil, ceresin, lanolin alcohol, panthenol, glycerin

6. Moisturizer Selection by Skin Type and Condition

Dry Skin

• Barrier repair moisturizers with ceramide:cholesterol:fatty acid (1:1:1)
• Occlusives (petrolatum, lanolin) + humectants (glycerin)
• Thick creams or ointments preferred
• Avoid: lightweight gels, high-alcohol formulations

Oily / Acne-Prone Skin

• Oily skin produces sufficient sebum for surface occlusion; moisturizer may not be needed
• If used: oil-free, non-comedogenic, lightweight lotions or gels
• Humectant-based (HA, glycerin) without heavy oils
• Avoid comedogenic ingredients: isopropyl myristate, isopropyl palmitate, coconut oil (some forms)
• Note: at least one study shows moisturizer use alone can improve acne

Sensitive / Rosacea-Prone Skin

• Soothing, anti-inflammatory ingredients: argan oil, niacinamide, colloidal oatmeal, chamomile, feverfew, licorice extract, green tea
• Avoid: retinoids, low-pH acids, fragrance, dyes, exfoliants, extreme temperatures
• Combine with prescription rosacea therapy (metronidazole, azelaic acid, ivermectin)

Atopic Dermatitis (AD)

• Cornerstone of maintenance therapy
• Liberal application recommended: 150-200 g/week in young children; 250-500 g/week in older children/adults
• Ointments > creams > lotions for occlusion and tolerability (ointments least stinging)
• Prescription emollient devices (PEDs) containing specific lipid ratios ± palmitoylethanolamide, glycyrrhetinic acid have been studied, but no preparation has been shown superior to OTC preparations
• Apply immediately after bathing ("soak and seal") to maximize hydration and penetration
• Dermatology 2-Volume Set, 5th ed.

Post-Procedure Skin

• Non-scented, hypoallergenic options (Aquaphor) post-laser/peels
• Avoid fragrances, acids, retinoids until re-epithelialization
• Daily moisturizer on thinned/post-procedure skin is protective against desiccation

7. Timing and Application Principles

• Apply within 3 minutes of bathing ("3-minute rule" / soak-and-seal): bathing hydrates the SC, and immediate application locks in water before it evaporates
• Frequency: 2-3 times daily for dry/atopic skin; once daily for maintenance in normal skin
• Direction: apply in the direction of hair growth to minimize folliculitis
• Separate from topical corticosteroids by 15-30 minutes to avoid diluting medication and reducing efficacy
• Hands and feet may need occlusive dressings overnight (wet wraps in severe AD)

8. Limitations and Key Facts for Clinical Practice

• No ingredient has been reliably shown to increase NMF production in vivo
• Humectants, occlusives, and exfoliants offer only temporary improvement - they do not repair the underlying barrier defect
• Only barrier repair agents (physiologic lipid replacers) address the structural cause
• Barrier repair ingredients may take 4+ days for noticeable effect
• Contact allergens in moisturizers are clinically important: fragrances (fragrance mix I/II), preservatives (methylisothiazolinone, methylchloroisothiazolinone), lanolin, vitamin E, propylene glycol, and certain plant extracts are common sensitizers - patch testing is important in suspected allergic contact dermatitis to cosmetics
• No particular moisturizing preparation has been shown superior to another in AD by RCT evidence - vehicle choice, patient preference, and compliance matter most

9. Summary Table: Ingredient Classes

• Fitzpatrick's Dermatology, 9th ed., Chapter 207 (Skin Care)
• Dermatology 2-Volume Set, 5th ed., Chapters on Atopic Dermatitis and Cosmeceuticals
• StatPearls - Moisturizers (Harwood A, Nassereddin A, Krishnamurthy K, 2023)
• Journal of Clinical and Aesthetic Dermatology - Update on Moisturizers

• Title page with subtitle and source attribution
• Navy banner headings for each of the 10 sections
• Justified body text with 15pt leading for easy reading
• Bullet and sub-bullet hierarchy with proper indentation
• Two colour-coded tables (vehicles table + ingredient summary table) with alternating row shading
• Running header (pages 2+) and page-numbered footer with source credits
• Bold callouts for key clinical facts (1:1:1 ratio, comedogenic ingredients, soak-and-seal, etc.)