Why sweat sodium varies between individuals
Sweat begins as a fluid close to the sodium concentration of blood plasma. As it moves through the sweat gland duct, sodium is reabsorbed back into the body. How efficiently that reabsorption happens differs from person to person based on genetics, heat acclimatization, and training status — which is why measured sweat sodium concentration ranges roughly 5–10x across the population (commonly cited from about 200 to 2,000+ mg/L). TODO: cite primary literature for population ranges.
Sweat rate and fluid loss
Separate from concentration is your sweat rate — how much total fluid you lose per hour, typically estimated from body-weight change and fluid intake over a session. Combined with sweat sodium concentration, sweat rate determines your actual hourly sodium and fluid loss, which is what a formula needs to replace. Two athletes with identical sweat sodium can have very different total sodium losses if their sweat rates differ.
Population variation
Illustrative only. Your Sweatprint™ Test places you on your own curve with measured data.
The risk runs both ways
Under-replacing sodium and fluid is the familiar failure mode — associated with cramping and performance decline. But over-drinking plain water relative to sodium losses carries its own risk: dilutional hyponatremia, a dangerous drop in blood sodium concentration. Matching intake to measured losses is about staying in the safe, effective middle — not maximizing one number.
How sodium supports fluid balance and performance
Sodium is the primary electrolyte governing extracellular fluid balance. Adequate sodium intake supports fluid retention and the drive to keep drinking during long efforts, and helps keep blood sodium concentration in range as you lose fluid. The right amount is the amount that matches your losses — which is exactly what varies between athletes.
Carbohydrate strategy: multiple transportable carbs
Carbohydrate absorption during exercise is limited by gut transporters, not willpower. Maltodextrin — a glucose polymer — is broken down to glucose and absorbed via SGLT1, which saturates around 60g per hour for most athletes. Fructose uses a separate transporter, GLUT5. Because maltodextrin-derived glucose and fructose are absorbed independently, a maltodextrin:fructose blend can push total carbohydrate uptake above the single-transporter ceiling — commonly cited in the 80–90g/hr range for trained athletes — and often with better GI comfort than an equivalent single-source dose. TODO: cite primary literature for these absorption figures.
Caffeine as an optional add-on
Caffeine is among the most well-studied ergogenic aids for endurance, typically discussed in the ~3–5 mg/kg range and best timed strategically rather than taken indiscriminately. In Solute formulas it’s an optional, athlete-selected add-on — not part of the base — because tolerance, timing, and goals vary. TODO: cite dosing and timing literature.
From one measurement to a predictive model
A single sweat test is a snapshot of one day in one set of conditions. Because sweat rate and sweat sodium respond predictably to heat, intensity, and duration, sampling across a few sessions lets us fit a personal response curve — and estimate your needs in conditions you haven’t directly tested, bounded by what the literature says about thermoregulation and carbohydrate oxidation. Those responses also shift as you heat-acclimatize and progress through training, which is why re-testing is best timed to real adaptation points rather than a fixed calendar. TODO: cite literature on heat acclimatization and sweat-response modeling. See the platform for how we’re building this out.
References
TODO: add a references section linking reputable sports-science literature (e.g. peer-reviewed reviews on sweat sodium variability, multiple-transportable-carbohydrate oxidation, and caffeine ergogenics). Every quantitative claim above should map to a citation here before publication.