Reference
The ingredients, molecules, and biology behind the regimen — defined the way I would explain them in consultation: what each is, how it works, and why it matters for your skin.
A
African Marula Oil
Once considered sacred, the Marula tree was linked to fertility and happy marriage in ancient times. It is a highly anti-inflammatory plant oil rich in Omega-9, essential fatty acids, amino acids, and Vitamins C and E. It is known for its antioxidant, regenerating, rehydrating and reparative properties.
AHA (Alpha Hydroxy Acids)
Alpha hydroxy acids are a group of natural and synthetic ingredients that, when properly formulated, exfoliate the uppermost layers of skin and stimulate cell turnover and renewal. Glycolic and lactic acid are the most familiar members of this family.
Antioxidants
These ingredients help skin resist the effects of free radicals that damage and prematurely age it. Some antioxidants are also believed to support fibroblast activity involved in repairing the dermis.
Arbutin
Arbutin is extracted from the bearberry plant. Applied topically, it inhibits tyrosinase and so limits the formation of melanin. It evens skin tone, diminishes the appearance of dark spots, and is well tolerated across skin types.
B
Bearberry Plant
Made from the leaves of the bearberry plant, this extract is used to soften the look of age spots, freckles, and discoloration and to even blotchy tone for a more uniform, brighter complexion. It is also rich in antioxidants.
C
Caprylic/Capric Triglycerides
Used in cosmetics for over fifty years and usually derived from coconut oil and glycerin, this is an excellent emollient and skin-replenishing ingredient that helps smooth skin and acts as an antioxidant. It also binds other ingredients together and helps active ingredients last longer.
Ceramides
Lipids that occupy the spaces between skin cells and form a central part of the skin barrier.
Mechanism. Together with cholesterol and fatty acids, ceramides create the “mortar” that seals the outer layer of skin. That sealed matrix is what holds water within the skin and keeps irritants out. Ceramide levels fall with age and with over-cleansing, leaving the barrier porous.
Why it matters clinically. When ceramides are depleted, skin loses water through the surface and becomes dry, reactive, and prone to irritation. Replenishing them restores the barrier rather than masking the symptoms.
Collagen
The primary structural protein of the dermis — the scaffolding that gives skin its firmness, density, and resilience.
Mechanism. Collagen is produced by fibroblasts and must fold into a stable, triple-helix structure to become functional — a step that depends on adequate Vitamin C and healthy fibroblast activity. Synthesis declines measurably with age, and UV exposure degrades existing collagen faster than it is replaced.
Why it matters clinically. Collagen loss is the structural reason skin becomes thinner, less firm, and more lined over time. Supporting synthesis at the fibroblast — rather than only treating the surface — addresses the cause rather than the appearance.
E
ECM (Extracellular Matrix)
Aging and photoaging of the skin are chronic processes that progress gradually. The extracellular matrix, which constitutes over 70% of the skin, is the central hub for its repair and regeneration — and the area where changes related to photodamage are most evident.
Encapsulation
Encapsulation allows active ingredients to be delivered to skin in potent concentrations without causing irritation, by enclosing them in a protective carrier that releases them in a controlled way.
F
Fibroblasts
The principal cells of the dermis — the cells responsible for producing collagen, elastin, and the other structural components of skin.
Mechanism. Fibroblasts synthesize and maintain the dermal matrix that gives skin its strength and elasticity. Their activity slows with age and is impaired by UV exposure and oxidative stress, so less new structural protein is produced each year.
Why it matters clinically. Because the fibroblast is where skin structure is actually built, it is the meaningful target of corrective skincare. Ingredients that reach and support the fibroblast influence skin quality at its source; ingredients that act only at the surface do not.
Filaggrin
Filaggrin is a major structural protein found in the stratum corneum of the epidermis. Variations in the filaggrin gene are a significant risk factor for atopic dermatitis — an allergic irritation of the skin.
G
GAGs (Glycosaminoglycans)
Glycosaminoglycans are powerful cell regenerators. These complex proteins help rebuild collagen and increase skin's elasticity.
Ginger Root Extract
Ginger has been used medicinally in Asian, Indian and Arabic herbal traditions since ancient times. It contains an enzyme called zingibain, known for its ability to calm flare-ups and decrease oxidative stress. Ginger is also a powerful anti-inflammatory and antioxidant, helping to neutralize the free radicals that drive skin damage and aging.
Glycolic Acid
The smallest of the alpha hydroxy acids (AHAs), derived from sugar cane.
Mechanism. Glycolic acid loosens the bonds holding dull, dead cells to the skin surface, allowing them to shed and prompting renewal of the outer layer. Its small molecular size lets it act efficiently — which also makes correct concentration and pH essential to its safe use.
Why it matters clinically. Used appropriately, glycolic acid improves tone, smoothness, and clarity. Used too strongly or too often, it compromises the barrier. The benefit lies entirely in the right strength at the right interval.
Grapeseed Oil
A by-product of winemaking, grapeseed oil contains high levels of linoleic acid, an Omega-6 fatty acid that can help control acne by reducing clogged pores. It has anti-inflammatory, antimicrobial and antioxidant properties, and helps raise skin's moisture levels by reducing transepidermal water loss.
H
Hexylresorcinol
A phenolic lipid found naturally in whole-grain wheat and rye, hexylresorcinol functions as a skin-brightening agent with antioxidant and astringent properties. By neutralizing free radicals, it also helps protect skin from premature signs of aging.
Homeostasis
From the Greek for “same” and “steady,” homeostasis is the self-regulating process by which biological systems maintain internal stability while adjusting to the conditions optimal for their health.
Hyaluronic Acid
A molecule naturally present in skin — also called hyaluronate or sodium hyaluronate — that binds and holds water.
Mechanism. Hyaluronic acid can hold many times its weight in water. It is abundant in the dermis, where it keeps tissue hydrated, cushioned, and supple. Natural levels decline with age, contributing to a loss of plumpness and a more drawn appearance.
Why it matters clinically. Replenishing hyaluronic acid restores hydration and visible suppleness. It works best within an intact barrier — water drawn into skin must also be sealed in to last.
J
Jojoba Esters
The moisturizing properties of this unique seed oil have been valued through the ages. It is an exceptionally efficient moisturizer that does not block the pores, and it is also excellent for conditioning and soothing skin.
K
Kojic Acid
Derived from mushrooms and other fungi, kojic acid brightens, evens skin tone, and softens the look of fine lines by supporting collagen and elastin production.
L
Lactic Acid
An alpha hydroxy acid (AHA), gentler than glycolic acid and naturally present in skin.
Mechanism. Lactic acid encourages the shedding of dull surface cells while also acting as a humectant, drawing water into the outer layer. Its larger molecular size makes it act more gradually than glycolic acid, which is why it is generally better tolerated by sensitive skin.
Why it matters clinically. Lactic acid improves texture and radiance with less risk of irritation, making it the more appropriate exfoliating acid for delicate or reactive skin — and a sound entry point before stronger acids.
M
Malachite
A blue-green semiprecious stone long prized for its ability to detoxify skin. It contains copper, which supports collagen production and improves elasticity, and provides nourishing trace elements — zinc, copper, selenium, and manganese — that act as antioxidants against free radicals.
Mango Butter
Rich in beta-carotene, essential fatty acids, and Vitamins A and E, mango butter is an emollient with healing and regenerative properties. It is ideal for treating and protecting dry skin — and the better hydrated skin is, the flatter the aging curve.
Matcha Green Tea
Matcha is a special green tea in which the whole leaf is consumed, delivering more of the healthful elements of green tea than other forms. Alongside trace minerals and vitamins, it is rich in catechin polyphenols — compounds with high antioxidant activity that help protect against the aging process.
Matrikines
Matrikines and matrikine-like peptides help promote skin rejuvenation and collagen synthesis. They are frequently used in skincare for their small molecular size and ease of penetration.
Melanocytes
The pigment-producing cells of the skin, located at the base of the epidermis.
Mechanism. Melanocytes produce melanin and distribute it to surrounding skin cells, where it absorbs UV radiation and protects against damage. When melanocytes are overstimulated — by sun, heat, inflammation, or hormonal change — they produce melanin unevenly, which appears as dark spots and patches.
Why it matters clinically. Pigmentation concerns are, at their root, a melanocyte-regulation problem. Effective treatment calms and normalizes melanocyte activity rather than only lightening pigment already at the surface.
Melanosomes
Melanosomes are the melanin-containing pigment granules, produced within melanocytes, that provide skin with color and photoprotection.
Minerals
Minerals such as biotin, niacin, selenium, magnesium, copper and zinc help protect skin from the oxidative stress that drives premature aging. Several, such as magnesium, also help maintain moisture levels.
N
NMF (Natural Moisturizing Factor)
Natural moisturizing factor is a collection of naturally occurring amino acids and humectants derived from the structural protein filaggrin. Located within cells of the stratum corneum, NMF regulates skin's moisture by drawing water in, and also helps form a barrier against harmful microorganisms.
O
Oat Kernel Extract
Extracted from oat kernels, this ingredient contains tocopherols, phytosterols, phospholipids, and beneficial fatty acids with anti-inflammatory and anti-irritant properties. It moisturizes and nourishes skin, improves elasticity and smoothness, calms itchy skin, and is non-comedogenic.
Oleic, Palmitic & Linoleic Acids
Known as essential fatty acids (EFAs), these cannot be manufactured by the body. They are essential for maintaining a healthy stratum corneum, the outermost protective layer of skin. They decrease transepidermal water loss and are antimicrobial and anti-inflammatory.
Opuntia Oil
Also known as prickly pear, this oil helps calm acne and soothe irritation. It supports collagen production and elasticity, and is valued for brightening dark under-eye circles, calming redness, and evening complexion.
P
Parabens
A family of preservatives that help prevent microbial contamination and maintain product freshness. Parabens have since been designated as questionable ingredients that may cause side effects, and for that reason have been eliminated from all of Dr. Koo's formulas.
Peptides
Short chains of amino acids that act as signaling molecules in the skin.
Mechanism. Peptides carry instructions to skin cells — for example, signaling fibroblasts to produce more collagen, or modulating other repair processes. Their small size allows them to be formulated for delivery into the skin, where this signaling occurs.
Why it matters clinically. Peptides support skin's own repair machinery rather than replacing it. They are most effective as part of a considered regimen, reinforcing the structural work done by other actives.
pH
pH describes how acidic or alkaline a solution is — below 7 is acidic, above 7 is alkaline. Normal skin pH is about 4.8, slightly acidic. Acidic products in the pH 2.5 to 3.5 range cause natural exfoliation of the outer layer; the lower the pH and the higher the free-acid concentration, the greater the exfoliation. It is a delicate balance, and every skin has its own optimum.
PHA (Polyhydroxy Acids)
Often called the next generation of AHAs, polyhydroxy acids — gluconolactone, galactose, lactobionic acid — exfoliate skin in a manner similar to AHAs while also offering antioxidant and humectant benefits. They are notably better tolerated by delicate, sensitive skin and skin of color.
Phthalates
Phthalates are a class of chemicals used in fragrance as solvents and carriers. They have been shown to carry negative health impacts, and have been eliminated from Dr. Koo's formulas.
R
Retinol
A derivative of Vitamin A and one of the most evidence-supported ingredients for renewing skin.
Mechanism. Retinol prompts faster turnover of surface cells and signals fibroblasts to produce more collagen — together refining texture, softening lines, and evening tone. Because it accelerates cell activity, skin must be introduced to it gradually and at the correct strength.
Why it matters clinically. Retinol delivers measurable improvement in fine lines and texture — but only when the form is stable, the concentration is meaningful, and it is introduced at a level the skin can tolerate. Strength without tolerance produces irritation, not results.
S
Skin Barrier
The outermost defensive layer of skin — chiefly the stratum corneum — that holds water in and keeps irritants, allergens, and microorganisms out.
Mechanism. The barrier behaves as a brick-and-mortar system: skin cells set within a lipid matrix of ceramides, cholesterol, and fatty acids. When that lipid matrix is depleted — by over-cleansing, over-exfoliation, or age-related decline — water escapes outward and irritants enter more readily.
Why it matters clinically. A compromised barrier presents as tightness, redness, stinging, and reactivity to products previously well tolerated. Most “sensitive skin” is in fact barrier disruption — restored by replenishing lipids and easing aggression, not by adding more actives.
Sodium Hyaluronate
See Hyaluronic Acid.
Sun Protection Factor (SPF)
SPF is a number, designated by the FDA, that identifies a product's ability to protect skin from sunburn. Because sunburn results from UVB exposure, SPF is primarily a measure of UVB protection. There is currently no comparable numbering system for UVA radiation, which reaches the deeper layers of skin.
T
Tachyphylaxis
In medical terms, tachyphylaxis is a diminishing response to successive doses of a drug, rendering it less effective over time. Because the regimen is built on leveling up — raising concentrations as skin tolerates them — and on periodic change-up routines, skin does not reach tachyphylaxis.
THD-Ascorbate (Tetrahexyldecyl Ascorbate)
A lipid-soluble derivative of Vitamin C, formulated at a therapeutic concentration.
Mechanism. Conventional water-soluble Vitamin C penetrates the lipid-rich stratum corneum poorly and oxidizes quickly on contact with air. THD-Ascorbate is lipid-soluble: it crosses that barrier and is converted intracellularly by esterases into active ascorbate, where it supports the enzyme activity required to stabilize collagen.
Why it matters clinically. The antioxidant and collagen-supporting activity reaches the fibroblast — the dermal cell that actually builds collagen — rather than dissipating at the surface. It is also far more stable than conventional water-soluble Vitamin C, which is why it does not require refrigeration.
Tocopherol
Tocopherol acetate is an abundant form of Vitamin E with potent antioxidant properties. It helps neutralize free radicals and protect skin against visible signs of aging.
Tolerance
In medical terms, tolerance is the need for progressively larger doses of a drug to produce the same effect. Over time skin adjusts to a product, the effect plateaus, and a more potent formulation may be needed.
Transepidermal Water Loss (TEWL)
The continuous, largely invisible evaporation of water outward through the skin.
Mechanism. A small amount of water loss through the skin is normal. When the barrier is intact, that loss is restrained. When the barrier is compromised — through depleted lipids, over-exfoliation, or age — water escapes faster than skin can replace it, and the rate of transepidermal water loss rises.
Why it matters clinically. Elevated water loss is the measurable signature of a weakened barrier and a direct cause of persistent dryness and reactivity. It is corrected by repairing the barrier, not by layering on more hydrating product.
Tyrosinase
Tyrosinase is a copper-containing enzyme that catalyzes the production of melanin from the amino acid tyrosine. It is found inside melanosomes, which are synthesized within the skin's melanocytes.
V
Vitamin C
Vitamin C is a water-soluble vitamin. Unlike most mammals, humans cannot produce their own, so it must be obtained internally or topically. It is required for the synthesis of collagen and is a highly effective antioxidant; it also helps minimize hyperpigmentation by reducing melanin production. Because conventional water-soluble Vitamin C is unstable, THD-Ascorbate is the stable, effective form used in skincare.
Vitamin D
Most often linked to bone health, Vitamin D is a fat-soluble vitamin shown to support skin protection, growth, repair and metabolism. Because sun exposure should be limited — especially when using retinoids — topical supplementation is helpful.
Vitamin E
A lipid-soluble vitamin and antioxidant. Applied topically, Vitamin E provides meaningful antioxidant protection against ultraviolet rays, combats free-radical damage and photo-aging, enhances moisturization, and minimizes cell damage.
Vitamin F
Vitamin F (linoleic acid) is an essential fatty acid with emollient, water-binding, anti-inflammatory and antioxidant properties — a precious ingredient rarely found in meaningful concentrations in off-the-shelf serums.
Vitamin K
One of the best ingredients for healing and repairing skin, especially when paired with anti-aging actives like retinol. It helps reduce inflammation, calm redness, and fade dark circles by supporting circulation, and it strengthens skin's resiliency.
Philosophy & Approach
A few terms that are not ingredients or biology, but principles — the way I think about treating skin.
Breakthrough Medicine
Breakthrough medicine treats people as individuals with their own particular needs. It brings together the personal, one-on-one commitment of medicine decades ago with the precision and technology of today.
Leveling Up
Every person begins a skincare practice from their own baseline. Private Practice Skincare is designed in step-wise concentrations — the right products, in the right concentrations, at the right time, in the right sequence. Each level builds on the one before, advancing results as skin transforms and can tolerate greater activity. As your skin progresses, you move up to the next level.
Medical Quality Skincare
Medical-quality skincare contains purely sourced ingredients with genuine scientific and clinical data behind their claims. These products meet a higher standard of purity and safety, overseen by a chemist, biochemist, pharmacist or physician — which is why they are not sold as cosmetics in mainstream stores, and why they meaningfully improve skin quality with consistent use.
Skin Kind
A personal guarantee that you receive the right ingredients, in the right concentrations, in the right sequence — uniquely prescribed for you, so your recommendation is safe, non-irritating and non-allergenic across all genders, skin types, and skin colors.
The Ripple Effect
A personal philosophy. As an aesthetic plastic surgeon, I make small but truly meaningful changes in people's lives — and those small changes inspire others, emotionally and physically. That is the ripple effect.
— Dr. Michele Koo, MD, FACS
Board-Certified Plastic Surgeon
Founder, Dr. Koo Private Practice Skincare