Exercise Begins — Baseline Hydration
A well-hydrated athlete starts with full fluid and electrolyte reserves.
- ~40 L: Total body water (roughly 60% of body mass)
- 140 mmol/L: Plasma sodium (normal serum concentration)
- 2–4 M: Active sweat glands (eccrine glands across skin)
- 5–7 mL/kg: Pre-hydration target (consumed hours before start)
Baseline stores
Body starts fully hydrated with normal electrolyte levels.
Sodium's role
Sodium retains water and drives fluid balance.
Pre-hydration habit
Athletes pre-load fluids before intense sessions.
Sweat Loss Accumulates
Exertion drives sweat output, carrying sodium and potassium out of the body.
- 200–2000 mg/L: Sweat sodium range (varies widely by athlete)
- 150–200 mg/L: Sweat potassium range (far lower than sodium)
- 0.5–2.5 L/hr: Sweat rate range (intensity and heat dependent)
- up to 2×: Salty sweater loss (genetic and acclimation driven)
Sweat composition
Sweat is hypotonic, mostly water with dissolved salts.
Sodium dominates
Sodium is lost far faster than potassium.
Individual variation
Salty sweaters lose two to three times more sodium.
Fluid Deficit Builds
Without matched replacement, water and electrolyte losses compound over the session.
- 2%: Performance threshold (body mass loss impairs output)
- >2%: Dehydration onset (thirst already lagging behind)
- 5–10%: Plasma volume drop (reduces cardiac stroke volume)
- +0.3°C: Core temp rise (per percent water lost)
Deficit accumulates
Losses outpace sips of plain water.
Performance drops
A two percent water loss slows endurance output.
Thirst lags
Thirst signals arrive after the deficit begins.
Sports Drink Formulated
Sodium, potassium, and carbohydrate concentrations are tuned to this athlete's losses.
- 500–700 mg/L: ACSM sodium range (general athlete recommendation)
- 6–8%: Carbohydrate target (optimal gastric emptying rate)
- 150–225 mg/L: Potassium range (replaces smaller sweat losses)
- 200–330 mOsm/kg: Osmolality target (isotonic to slightly hypotonic)
Matching sodium
Drink sodium is tuned to the athlete's sweat losses.
Adding carbohydrate
Carbohydrate concentration is set by exercise intensity.
Balancing osmolality
The mix stays isotonic for fast gastric emptying.
Optimal Rehydration
The matched formulation restores fluid and electrolyte balance efficiently.
- +40%: Fluid retention gain (vs plain water alone)
- 1.5 L / kg: Rehydration ratio (accounts for ongoing urine loss)
- ~60 g/hr: Carb oxidation rate (multiple transportable carb sources)
- 2–3%: Performance benefit (time-trial gain with matched drink)
Fluid returns
Sodium pulls water back into circulation quickly.
Electrolytes restored
Sodium and potassium stores return toward baseline.
Ready to continue
The athlete resumes exertion without cramping or fatigue.