The evidence
Where this comes from.
Proof Positive Skin Care™ sells no products and no treatments — only understanding. That means every claim I make has to stand on published research rather than on something I need you to buy.
This page is that research. It is organized by claim rather than by paper, so you can find the evidence behind the specific thing you just read. Every entry carries a PubMed ID you can look up yourself.
Where the evidence is strong, I say so. Where it is mechanistic, preliminary, or drawn from outside human skin, I say that too.
How to read this page
Not all evidence is the same, and pretending otherwise is how people get misled.
A finding in a dish is real science and it is not the same thing as a result in a person. Every citation below is labeled with what kind of study it is, so you can weigh it yourself instead of taking my word for the weighing.
Measured in living people. Controlled trials, randomized trials, and laboratory studies on human volunteers. The strongest evidence here.
Measured in human skin, human skin cells, or human biopsy samples — but not in a whole living person.
Shows how something works at a cellular or molecular level, often in cultured cells or animal models. Explains a mechanism; does not predict an outcome on your face.
A survey of the existing literature by researchers in the field. Useful for establishing what is broadly accepted, not for proving a single effect.
Foundations
Repair is expensive, and the skin cannot do everything at once.
The idea underneath everything I do: skin repair is metabolically demanding work, the skin's processes compete for the same resources, and what the rest of the body is doing changes what the skin has left over.
Skin repair is an energy-demanding biological process, and the skin reprograms its metabolism to carry it out.
Manchanda M, Torres M, Inuossa F, et al. (2023). Metabolic Reprogramming and Reliance in Human Skin Wound Healing. Journal of Investigative Dermatology.
Human skin biopsy transcriptomics and metabolomics, plus a human skin explant model, showing that healing tissue shifts its metabolic programme to meet the demands of repair.
Eming SA, Wynn TA, Martin P. (2017). Inflammation and metabolism in tissue repair and regeneration. Science.
Establishes that inflammation and repair are metabolically distinct states, and that the metabolic environment shapes which one the tissue can pursue.
Wang Z, Zhao F, Xu C, et al. (2024). Metabolic reprogramming in skin wound healing. Burns & Trauma.
Glucose, lipid and amino acid metabolism shift measurably across the inflammatory, proliferative and remodelling phases of skin repair.
A note on language: I describe this as the skin having a limited budget. The literature calls it metabolic competition and resource prioritization. Those terms mean the same thing and they are the ones to search if you want to read further. What the research establishes is that repair is costly and metabolically distinct from defense — not a fixed hierarchy in which the skin ranks its jobs.
Resolving inflammation is itself an active biological process, not simply the absence of one.
Sears DD, Saha AK. (2021). Dietary Control of Inflammation and Resolution. Nutrients.
Resolution is an actively driven phase with its own biochemical requirements, rather than inflammation simply fading.
Cañedo-Dorantes L, Cañedo-Ayala M. (2019). Skin Acute Wound Healing: A Comprehensive Review. International Journal of Inflammation.
Skin repair proceeds through ordered inflammatory, proliferative and remodelling phases — the sequence that makes "unfinished work" a meaningful idea.
Domain one
Barrier function & inflammation
21% of the Skin State Decoder™
The outermost layer holds water in and irritants out. When its lipid structure is disordered, everything applied to it behaves differently — including the products that used to work.
The barrier depends on a specific ratio of lipids, and the ratio matters as much as the ingredients.
Man MQ, Feingold KR, Thornfeldt CR, Elias PM. (1996). Optimization of physiological lipid mixtures for barrier repair. Journal of Investigative Dermatology.
Incomplete lipid mixtures delay barrier recovery; complete, correctly proportioned mixtures accelerate it.
Man MQ, Feingold KR, Thornfeldt CR, Elias PM. (1997). Optimal ratios of topical stratum corneum lipids improve barrier recovery in chronologically aged skin.
The same ratio principle demonstrated in aged human skin, where baseline barrier recovery is slower.
Andrew PV, Williams SF, Brown K, Chittock J, Pinnock A, Poyner A, Cork MJ, Danby SG. (2025). Topical supplementation with physiological lipids rebalances the stratum corneum ceramide profile and strengthens skin barrier function in adults predisposed to atopic dermatitis. British Journal of Dermatology 193(4):729–740.
Physiological lipid supplementation rebalanced the stratum corneum ceramide profile and improved barrier integrity measured by transepidermal water loss.
Ceramide-containing formulations measurably restore barrier function.
Spada F, Barnes TM, Greive KA. (2021). A daily regimen of a ceramide-dominant moisturizing cream and cleanser restores the skin permeability barrier in adults with moderate eczema. Dermatologic Therapy.
Randomized trial showing restoration of permeability barrier function in adults with moderate eczema.
Draelos ZD, et al. (2023). Ceramide-Containing Adjunctive Skin Care for Skin Barrier Restoration During Acne Vulgaris Treatment. Journal of Drugs in Dermatology.
Barrier restoration during a treatment course known to compromise the barrier — the clearest demonstration of barrier support running alongside an active.
Dimmers F, Reichert D, Nguyen T, et al. (2026). Monocentric, Vehicle-Controlled, Double-Blind Study to Assess the Short- and Long-Term Effects of a Ceramide NP C15-Containing Emollient on the Skin Microbiome and the Skin Barrier Function in Sensitive Skin. Skin Pharmacology and Physiology.
Fifty participants over six weeks. Transepidermal water loss decreased significantly in participants who began with impaired barrier function.
Industry-funded, and the effect was found in the impaired-barrier subgroup rather than across the whole cohort. Both facts are worth knowing when weighing it.
Niacinamide increases the skin's own production of barrier lipids.
Tanno O, Ota Y, Kitamura N, Katsube T, Inoue S. (2000). Nicotinamide increases biosynthesis of ceramides as well as other stratum corneum lipids to improve the epidermal permeability barrier. British Journal of Dermatology 143:524–531.
Increased ceramide biosynthesis in cultured human keratinocytes; topical application raised stratum corneum ceramides and free fatty acids and lowered transepidermal water loss in dry skin.
Transepidermal water loss is a real, objective measure of barrier integrity — not a marketing metric.
Green M, Kashetsky N, Feschuk A, Maibach HI. (2022). Transepidermal water loss (TEWL): Environment and pollution — A systematic review. Skin Health and Disease.
Systematic review establishing TEWL as a validated measure and documenting its response to environmental exposure.
Domain two
Immune reactivity & stress signaling
21% of the Skin State Decoder™
Your skin is wired into your nervous and immune systems. This is the domain with the strongest human evidence on this page — and it is the one people find hardest to believe.
The skin's nervous and immune systems are directly integrated, and talk to each other continuously.
Tamari M, Ver Heul AM, Kim BS. (2021). Immunosensation: Neuroimmune Cross Talk in the Skin. Annual Review of Immunology 39:369–393.
Sensory neurons and immune cells in skin form a bidirectional signaling network, with each capable of altering the other's behavior.
Barbon PV, Hoeffel G, Fohrer T, et al. (2026). Neural Regulation of Skin Immunity. Annual Review of Immunology.
Neural control of cutaneous immune function, including neuroendocrine pathways.
Psychological stress measurably damages barrier function in healthy people.
Altemus M, Rao B, Dhabhar FS, Ding W, Granstein RD. (2001). Stress-induced changes in skin barrier function in healthy women. Journal of Investigative Dermatology 117:309–317.
Twenty-five healthy women. Experimentally induced psychological stress delayed barrier recovery and raised inflammatory signaling. A sleep-deprivation arm produced the same direction of effect.
Choe SJ, Kim D, Kim EJ, et al. (2018). Psychological Stress Deteriorates Skin Barrier Function by Activating 11β-Hydroxysteroid Dehydrogenase 1 and the Cortisol Response. Scientific Reports.
Identifies the mechanism: stress raises the enzyme that generates active cortisol within the skin itself, and barrier function deteriorates accordingly.
Higher stress predicts slower wound healing — measured, not inferred.
Ebrecht M, Hextall J, Kirtley LG, Taylor A, Dyson M, Weinman J. (2004). Perceived stress and cortisol levels predict speed of wound healing in healthy male adults. Psychoneuroendocrinology 29:798–809.
Twenty-four healthy men, punch biopsy wounds. Perceived stress correlated with healing speed at r = −0.59; morning cortisol response at r = −0.55. Higher stress, slower healing.
A small, male-only sample, and correlational rather than causal. It is nonetheless one of the most direct demonstrations available that a psychological state changes the rate at which skin repairs itself.
Centella asiatica compounds reduce mast cell activation.
Jiang JZ, Ye J, Jin GY, et al. (2017). Asiaticoside Mitigates the Allergic Inflammation by Abrogating the Degranulation of Mast Cells. Journal of Agricultural and Food Chemistry 65:8128–8135.
Asiaticoside reduced mast cell degranulation, histamine release and inflammatory cytokine output.
Cellular and animal work, not a human topical trial. The mechanism is well demonstrated; the size of the effect on a human face applying a topical product is not.
Domain three
Circadian rhythm & sleep recovery
18% of the Skin State Decoder™
Skin does different work at different hours. This is the best-evidenced part of the whole framework, and the reason timing matters as much as ingredients.
Skin cells carry their own circadian clocks, which schedule repair, proliferation and immune activity.
Duan J, Greenberg EN, Karri SS, Andersen B. (2021). The circadian clock and diseases of the skin. FEBS Journal.
Peripheral clocks in keratinocytes and fibroblasts gate DNA repair, cell-cycle timing, barrier function and immune response across the day.
Barrier permeability is measurably different at night than in the morning.
Yosipovitch G, Xiong GL, Haus E, Sackett-Lundeen L, Ashkenazi I, Maibach HI. (1998). Time-dependent variations of the skin barrier function in humans: transepidermal water loss, stratum corneum hydration, skin surface pH, and skin temperature. Journal of Investigative Dermatology.
Sixteen healthy adults measured every two hours across twenty-four hours. Skin permeability was higher in the evening and at night than in the morning.
Sleep restriction delays the skin's ability to repair itself.
Smith TJ, Wilson MA, Karl JP, et al. (2018). Impact of sleep restriction on local immune response and skin barrier restoration. Journal of Applied Physiology.
Controlled human study. Barrier recovery took 5.0 days after seventy-two hours of sleep restriction, against 4.2 days with adequate sleep.
The standard deviations around those figures overlap, so this is a real but modest difference — not a dramatic one. It is quoted here because it is one of the few direct human measurements of sleep changing the rate of skin repair.
Topical melatonin protects skin against ultraviolet damage.
Dreher F, Gabard B, Schwindt DA, Maibach HI. (1998). Topical melatonin in combination with vitamins E and C protects skin from ultraviolet-induced erythema: a human study in vivo. British Journal of Dermatology.
Randomized, double-blind human study showing dose-dependent protection against UV-induced erythema.
Domain four
Cellular energy & nutrient availability
16% of the Skin State Decoder™
Everything the skin builds costs energy to build. When cellular energy is depleted, structural work is what gets postponed.
Coenzyme Q10 protects skin cells under energy stress.
Abi Nahed R, et al. (2025). Coenzyme Q10 protects keratinocytes against oxidation-induced energy stress as revealed by spatiotemporal analysis of cell energetics. Scientific Reports 15:14501.
Direct measurement of keratinocyte energetics showing CoQ10 preserving cellular energy production under oxidative stress.
Alpha lipoic acid functions as an antioxidant that regenerates other antioxidants.
Packer L, Witt EH, Tritschler HJ. (1995). Alpha-lipoic acid as a biological antioxidant. Free Radical Biology & Medicine.
Establishes the recycling role — alpha lipoic acid regenerates oxidized vitamins C and E rather than acting alone.
Podda M, Grundmann-Kollmann M. (2001). Activity of alpha-lipoic acid in the protection against oxidative stress in skin.
The same activity demonstrated specifically in skin.
Vitamin C is required for carnitine synthesis, which cells use to move fatty acids into energy production.
Rebouche CJ. (1991). Ascorbic acid and carnitine biosynthesis. American Journal of Clinical Nutrition.
Establishes vitamin C as a required cofactor in carnitine biosynthesis.
This is general human biochemistry, not a study of topical vitamin C in skin. It supports the biochemical role and not a claim about what a serum does to your energy metabolism.
Domain five
Hormonal rhythm & cyclical influence
13% of the Skin State Decoder™
Skin carries hormone receptors. What changes cyclically in the body changes measurably in the skin.
Skin has estrogen and androgen receptors, and hormonal change alters collagen, thickness, sebum and healing.
Brincat MP. (2005). Estrogen and skin: the effects of estrogen, menopause, and hormone replacement therapy on the skin.
Estrogen deprivation is associated with dryness, atrophy, epidermal thinning, collagen decline and impaired healing.
Son ED, et al. (2011). Estrogen and skin: therapeutic options.
Estrogen's effects on collagen synthesis, dermal thickness, water-holding capacity, sebum, barrier function and re-epithelialization.
Azelaic acid inhibits the enzyme that drives androgen-mediated sebum production.
Stamatiadis D, Bulteau-Portois MC, Mowszowicz I. (1988). Inhibition of 5 alpha-reductase activity in human skin by zinc and azelaic acid. British Journal of Dermatology 119:627–632.
Both azelaic acid and zinc inhibited 5-alpha-reductase activity in a human skin assay.
An in-vitro assay on human skin, not a clinical trial. It establishes the enzyme mechanism, not a predicted result on a person's sebum output.
Salicylic acid acts on sebum production through a specific cellular pathway.
Lu J, Cong T, Wen X, et al. (2019). Salicylic acid treats acne vulgaris by suppressing AMPK/SREBP1 pathway in sebocytes. Experimental Dermatology 28:786–794.
Human sebocytes: salicylic acid suppressed lipogenesis via AMPK/SREBP-1 and reduced inflammatory signaling.
Domain six
Environmental load & recovery capacity
11% of the Skin State Decoder™
The best-evidenced interception layer on this page. What the air does to skin, and what measurably reduces it.
The exposome — sun, pollution, nutrition, hormones, psychological factors — has documented, measurable effects on skin.
Passeron T, Kraheva A, Andres P, et al. (2020). Clinical and biological impact of the exposome on the skin. Journal of the European Academy of Dermatology and Venereology.
Consensus review of the full set of external and internal exposures affecting skin over a lifetime.
Kim BE, Kim J, Goleva E, et al. (2021). Particulate matter causes skin barrier dysfunction. JCI Insight.
Airborne particulate matter directly causes barrier dysfunction — demonstrated, not associated.
Ferulic acid stabilises vitamins C and E and roughly doubles their photoprotection.
Lin FH, Lin JY, Gupta RD, et al. (2005). Ferulic acid stabilizes a solution of vitamins C and E and doubles its photoprotection of skin. Journal of Investigative Dermatology 125:826–832.
Adding ferulic acid stabilized the formulation and approximately doubled its photoprotective effect.
Murray JC, Burch JA, Streilein RD, et al. (2008). A topical antioxidant solution containing vitamins C and E stabilized by ferulic acid provides protection for human skin against damage caused by ultraviolet irradiation. Journal of the American Academy of Dermatology 59:418–425.
The same combination tested on human skin against UV damage.
Astaxanthin reduces UVB-driven inflammatory signaling and matrix breakdown.
Tominaga K, Hongo N, Fujishita M, Takahashi Y, Adachi Y. (2017). Protective effects of astaxanthin on skin deterioration. Journal of Clinical Biochemistry and Nutrition.
Reduced UVB-induced cytokine production in keratinocytes and MMP-1 in fibroblasts, alongside a sixteen-week study in sixty-five women.
What this page does not claim
The limits, stated by me rather than found by you.
A sources page that only lists supporting evidence is an advertisement. These are the boundaries of what the research above establishes.
No fixed hierarchy
The literature establishes that repair is metabolically expensive and that inflammation and repair are distinct metabolic states. It does not establish a fixed order in which the skin ranks its jobs. That ordering is my model for making the biology usable, not a finding.
Mechanism is not outcome
Many ingredient citations here are mechanistic — cells, tissue, or animal models. A demonstrated mechanism is a good reason to expect something; it is not a prediction of what a product will do on your face.
Small samples
Several of the most relevant human studies involve twenty-five people or fewer, and one is male-only. They are cited because they are direct, not because they are large.
Under review
Some claims I have made elsewhere are still being sourced, and some have been withdrawn from my own materials after review. This page is updated as that work continues.
Not medical care
Nothing here diagnoses, treats or prescribes. It is context, and context is not a substitute for a dermatologist.
Every reference above carries a PubMed ID. Look up anything you doubt — that is what the identifiers are for.