Which Delivers Superior Hydration Under Sweat-Induced Transepidermal Water Loss?

KEY INSIGHTS

Exercise elevates TEWL significantly — elite swimmers showed measurable barrier disruption after a single 2-hour session. Glycerin is irreplaceable: it is the only humectant transported into epidermal cells via Aquaporin-3 (AQP3), directly fuelling ceramide synthesis and lipid barrier repair. At 60–70 wt%, glycerin reaches thermodynamic equilibrium — zero water evaporation — and above 70 wt%, it actively pulls moisture from surrounding air. 1,3-Propanediol (PDO) cannot use AQP3 transport; it works extracellularly by spacing lipid head groups, solubilising actives, and improving sensory profile. The combination of 15% PDO + 5% glycerin delivers the highest skin hydration capacitance and the steepest TEWL reduction — proven in a 2024 peer-reviewed clinical trial. Projekt Clarity’s formulations embed this dual-humectant strategy in both the Pre-Game Defense Spray (PWO-BS-001) and Advanced Purifying Body Wash (BW-SH-004B). The optimal synergistic ratio neutralises glycerin’s notorious tackiness while preserving its biological depth — a non-negotiable for athlete compliance.

The Problem No One Talks About: Sweat Is Destroying Your Skin Barrier

Every serious athlete knows the physical cost of training. Muscle soreness, fatigue, caloric deficit — these get tracked obsessively. What goes entirely unmonitored is what intense sweating does to the skin barrier, specifically the rate at which it drives Transepidermal Water Loss (TEWL) — the passive, continuous evaporation of water through the outermost skin layer, the stratum corneum.

During exercise, core body temperature rises. The hypothalamus fires a sympathetic sudomotor response, activating millions of eccrine sweat glands across the dermis. These glands secrete a hypotonic fluid onto the skin surface to facilitate evaporative cooling. [1]

But eccrine sweat is not pure water. It is a complex electrolyte solution. Heavy exercise in a hot, humid environment generates sweat containing approximately 50.8 mM sodium, 46.6 mM chloride, 4.8 mM potassium, and lactate concentrations reaching up to 110.3 mM. [2]

As the aqueous phase of sweat evaporates post-exercise, what remains is a concentrated crystalline salt residue directly on the stratum corneum. This triggers a cascade:

• Residual NaCl creates a hyperosmolar microenvironment that forcibly draws water out from deeper, hydrated layers of the epidermis.
• Sweat-induced alkaline pH shifts (healthy skin: pH 4.5–5.5) impair lipid-processing enzymes like β-glucocerebrosidase and acid sphingomyelinase — the very enzymes that synthesise barrier ceramides. [3]
• Rapid evaporation strips the skin of Natural Moisturising Factors (NMF) and disrupts the hydrolipid film.

Clinical data confirms this is not theoretical. In a controlled study on elite swimmers, a two-hour training session produced a statistically significant increase in TEWL on the volar forearm and antecubital flexure — validating that extreme water exposure, physical activity, and sweat compound to fundamentally impair barrier function. [4]

To counteract this, a performance skincare formulation must deploy humectants that do not merely bind water passively. They must actively stabilise the lipid matrix against hyperosmolar stress and facilitate rapid biological repair. That narrows the field to two candidates: glycerin and 1,3-propanediol.

Table 1: Key Sweat Electrolytes and Their Impact on the Stratum Corneum [2]

Sweat Constituent	Avg. Concentration (Exercise-Induced)	Impact on Stratum Corneum
Sodium (Na⁺)	50.8 ± 16.5 mM	Creates hyperosmolar gradient; dehydrates epidermis
Chloride (Cl⁻)	46.6 ± 13.1 mM	Disrupts local ionic balance
Potassium (K⁺)	4.8 ± 1.6 mM	Alters cellular ion channels
Lactate	Up to 110.3 mM	Modulates skin surface pH
Calcium (Ca²⁺)	1.3 ± 0.9 mM	Influences corneodesmosome degradation

Glycerin: The Biological Heavyweight

Glycerin (propane-1,2,3-triol, C₃H₈O₃, MW ≈ 92.09 g/mol) has been the undisputed gold standard of humectancy for good reason. But its advantage over other polyols is not just thermodynamic — it is fundamentally biological.

The AQP3 Transport Mechanism: Why Glycerin Is Irreplaceable

The most critical differentiator is this: glycerol is actively transported into epidermal keratinocytes by Aquaporin-3 (AQP3) — a specialised transmembrane protein abundantly expressed in the basal and suprabasal layers of the epidermis. This is not passive diffusion. It is an active, biological transport mechanism that 1,3-propanediol fundamentally cannot replicate. [5]

In landmark studies using AQP3-deficient mice, researchers observed a 3-fold reduction in stratum corneum glycerol content, directly corresponding to severely reduced skin hydration, impaired elasticity, and delayed barrier recovery after physical insult. When these mice were treated with topical glycerol, all defects were corrected. When researchers substituted propanediol — same molecular weight, same polarity class — it completely failed to correct the hydration deficit. [5]

Once inside the cell, glycerol participates in transphosphatidylation via phospholipase D2 (PLD2), generating phosphatidylglycerol — a signalling molecule that promotes keratinocyte differentiation and stimulates lamellar body secretion, literally regenerating the lipid barrier from the inside out. [5]

Thermodynamic Superiority: The RERW Data

Glycerin’s water retention follows a clear thermodynamic profile, quantified by the Relative Evaporation Rate to Water (RERW):

Table 2: Thermodynamic Behaviour of Glycerin vs. Water (RERW Data) [6]

Glycerin Concentration	RERW (%)	Evaporation Status	Clinical Significance
0–60 wt%	>0% to <100%	Slowed evaporation	Prolongs surface hydration
60–70 wt%	0%	Zero evaporation	Total blockage of water loss
>70 wt%	<0% (Negative)	Active moisture sorption	Actively pulls water from air

Differential Scanning Calorimetry (DSC) corroborates this: above 70 wt% glycerin, the DSC signal for ‘free water’ vanishes entirely. All water present is bound into a stable ‘nonfreezable’ state that cannot evaporate, freeze, or crystallise. [6]

Glycerin + Sweat: A Counterintuitive Finding

In vitro studies measuring skin moisturisation demonstrated something remarkable. Application of salt alone to the skin lowered hydration readings, confirming hyperosmolar dehydration. But the combination of 1.5% NaCl and 5% glycerin paradoxically increased hydration readings — the maximum synergistic hydrating effect observed in the dataset. Glycerin’s hygroscopic strength is sufficient to neutralise and reverse the osmotic gradient created by sweat electrolytes. [7]

1,3-Propanediol: The Delivery System

If glycerin is the biological payload, 1,3-propanediol (PDO, CH₂(CH₂OH)₂) is the delivery system. Its limitations are real and well-documented. It cannot use AQP3 transport. Its absolute water-binding capacity is slightly inferior to glycerin in gram-for-gram comparisons. [8]

But in the context of a performance skincare formulation — one that must survive a gym bag, absorb instantly on sweaty skin, and actually get used — PDO solves problems that glycerin creates.

What PDO Actually Does

Mechanism: PDO penetrates the superficial layers of the stratum corneum and physically inserts itself between the hydrophilic head groups of extracellular ceramides and free fatty acids. This slight disruption of rigid lipid packing creates interstitial space that traps water molecules. [8]

Solvency: PDO is fully miscible with water yet exhibits a more lipophilic character than glycerin. This allows it to dissolve notoriously difficult actives — salicylic acid, niacinamide, ferulic acid — that would otherwise require elevated temperatures or separate solubilisation steps. [9]

Sensory Modification: PDO’s polarity disrupts the dense intermolecular hydrogen bonding that causes glycerin’s characteristic tackiness. On sweaty, vasodilated athlete skin, this is not a cosmetic preference — it is a compliance requirement. A sticky product does not get used consistently. [10]

Preservative Boosting: PDO lowers free water activity (aw) in the formulation, working synergistically with traditional preservatives and allowing up to 50% reduction in total preservative load without compromising microbiological safety. [9]

Safety: In modified Draize Repeated Insult Patch Tests on humans, 1,3-propanediol produced zero instances of irritation or sensitisation, even under occlusion at 75% concentration. Propylene glycol, by contrast, showed clinical irritation at 25%. [10]

Low-Humidity Stress Testing: Regimen Lab

Independent stress testing across three RH conditions (46%, 24%, 21%) using pure glycerin as the benchmark produced these findings:

• At 46% RH (moderate), glycerin produced the highest average capacitance change of +19 arbitrary units, outperforming betaine and lactic acid. Urea and Sodium PCA actually decreased hydration. [8]
• At 24% RH (low), 1,3-propanediol demonstrated excellent resilience. [8]
• At 21% RH (very low/arid — think air-conditioned gym or outdoor winter training), glycerin reclaimed the top position. Hyaluronic acid polymers paradoxically caused decreased hydration, pulling water from deeper skin layers when ambient moisture was insufficient. Glycerin maintained a positive hydration curve. [8]

For Indian athletes training in air-conditioned facilities or outdoors in dry conditions, this matters directly.

How Projekt Clarity Formulates Around This Science

Understanding the biochemistry is one thing. Building it into products that athletes will actually use — pre-training, post-training, daily — is a different engineering problem. Here is how Projekt Clarity applies this research.

Performance Series 01: Pre-Game Defense Spray (PWO-BS-001)

The Pre-Game Defense Spray is a low-viscosity aqueous spray (target: <100 cP) designed for application before training. The humectant system includes:

• Glycerin at 3.0%: Provides the AQP3-mediated biological transport and barrier hydration foundation.
• 1,3-Propanediol (as Propanediol) at 5.0%: Solubilises the Salicylic Acid (1.0% BHA) that would otherwise require elevated temperature processing without it. Also boosts the Sodium Benzoate / Potassium Sorbate preservative system, reducing total preservative load while maintaining full microbiological safety.

The formulation pH is held at 4.0–4.5 — directly within the range that supports the acid mantle enzymes required for ceramide biosynthesis, exactly the enzymes disrupted by sweat-induced alkaline shifts. The deodorant actives — Triethyl Citrate (2.0%) and Zinc Ricinoleate (0.5%) — address the odour problem that causes athletes to reach for antiperspirants, which are the wrong tool for athletic use.

Advanced Purifying Body Wash (BW-SH-004B)

The Advanced Purifying Body Wash runs a parallel dual-humectant system:

• Glycerin at 2.5%: Retained at its full concentration from the previous formulation iteration (BW-SH-004A). No change to the humectant core.
• 1,3-Propanediol at 2.5%: Serves as both humectant and preservative booster within the mild surfactant chassis.

The BW-SH-004B formulation was stability-hardened after phase separation was observed in BW-SH-004A. The root cause was insufficient micellar solubilisation capacity (~7.6% Active Surfactant Matter) for the Glyceryl Oleate at 3.0%. The revised formulation added Caprylyl/Capryl Glucoside at 2.0%, bringing total ASM to ~8.84% and the solubilisation ratio to approximately 5.5:1 — comfortably above the safe threshold of 5:1.

The Verdict: It Is Not a Competition

Framing glycerin versus 1,3-propanediol as a binary competition is the wrong question. The correct question is: what ratio maximises both biological efficacy and athlete compliance?

The answer from the clinical data — 15% PDO paired with 5% glycerin for the maximum sustained TEWL reduction and hydration capacitance — establishes a clear formulation principle. Glycerin provides the irreplaceable AQP3-mediated biological repair that no alternative polyol can replicate. PDO solves glycerin’s sensory limitations, expands the solvency window for actives, and strengthens microbiological safety.

For athletes specifically, the post-exercise hyperosmolar environment — concentrated sodium chloride on the stratum corneum, pH elevated above the acid mantle optimum, NMF stripped by evaporation — demands both. Glycerin counteracts the osmotic dehydration at a cellular level. PDO stabilises the physical lipid architecture and ensures the product is actually applied consistently.

Regular consumer skincare is designed for a controlled environment: post-shower, ambient temperature, no active physiological stress. Performance skincare operates in a fundamentally different context. The science is not different — it just needs to be applied with the right priorities.

Projekt Clarity’s product architecture — spray before training, wash after — is built precisely around this protocol. Performance care is not a product category. It is a problem-solving discipline.

References

• Eccrine sweat gland activation and thermoregulation. Eccrine Sweat Gland Activation and Solute Secretion section. Skincare Humectant Research Request document (internal research compilation).
• Sweat electrolyte concentrations in exercise-induced sweating. Table 1: Key electrolyte and metabolite concentrations in exercise-induced human eccrine sweat. Skincare Humectant Research Request document. Original source: Can Wearable Sweat Lactate Sensors Contribute to Sports Physiology? ACS Sensors (2021).
• Sweat-induced alkaline shift and barrier enzyme impairment. Post-Exercise Hyperosmolar Stress and TEWL Spikes section. Skincare Humectant Research Request document.
• Exercise-induced TEWL in elite swimmers. Effects of Exercise on the Skin Epithelial Barrier of Young Elite Athletes — Swimming Comparatively to Non-Water Sports Training Session. MDPI International Journal of Environmental Research and Public Health (2021).
• AQP3-mediated glycerol transport and barrier repair. The Aquaporin-3 (AQP3) Cellular Transport Mechanism section. Skincare Humectant Research Request document. Original source: Glycerol replacement corrects defective skin hydration, elasticity, and barrier function in aquaporin-3-deficient mice. PNAS (2003).
• Glycerin RERW thermodynamic data. Relative Evaporation Rate to Water (RERW) and Differential Scanning Calorimetry section. Skincare Humectant Research Request document. Original source: Moisture retention of glycerin solutions with various concentrations: a comparative study. PMC (2022).
• NaCl–Glycerin synergistic hydration effect. The Impact of Salinity section. Skincare Humectant Research Request document. Original source: The ability of electrical measurements to predict skin moisturization. I. Effects of NaCl and glycerin on short-term measurements. PubMed (2001).
• 1,3-Propanediol humectancy, low-humidity stress testing. Molecular Mechanisms of Humectancy: 1,3-Propanediol; Low-Humidity Stress Testing: The Regimen Lab Data sections. Skincare Humectant Research Request document. Original source: Ultimate Humectant Comparison. Regimen Lab (2023).
• PDO solvency, penetration enhancement, preservative boosting. Superior Solvency and Penetration Enhancement; Preservative Boosting sections. Skincare Humectant Research Request document.
• PDO safety profile — zero irritation at 75% concentration. Thermal Stability and Uncompromising Safety Profile section. Skincare Humectant Research Request document. Original source: Performance is in our nature. Essential Ingredients / Zemea® Skin Care Brochure.
• Pinto et al. (2024) clinical trial: 1,3-propanediol with glycerol combinations. Clinical Efficacy: Quantitative Comparative Analysis section. Skincare Humectant Research Request document. Original source: Effects of 1,3-propanediol associated, or not, with butylene glycol and/or glycerol on skin hydration and skin barrier function. International Journal of Cosmetic Science (2024).