Hydration and electrolytes — individualized, not prescriptive
Sweat rates span 0.3 to 2.5 L/hr and sweat sodium varies ten-fold between runners. One-size hydration advice is useless. Here is how to personalize it.
The 2007 ACSM "drink to prevent >2% body mass loss" prescription was critiqued by Tim Noakes in Waterlogged (2012) and refined by subsequent consensus (Hew-Butler et al., CSMR, 2015; CJSM, 2017). Current synthesis: ad libitum drinking by thirst is safe and effective for most runners in races of 2 hours or less; programmed sweat-rate-informed hydration is superior for hot conditions, long events, and salty sweaters.
The individual variability is huge. Sweat rates span 0.3–2.5 L/hr; sweat sodium concentration spans 200–2,000 mg/L. A 70 kg runner could lose anywhere from 300 mg of sodium in a hot hour (low sweater, low-sodium sweat) to 3,000+ mg (high sweater, salty sweat). Blanket "drink 500 mL per hour with 500 mg sodium" advice is wrong for more than half the runners it is given to.
Sweat rate is trivially measurable. Weigh nude before a 60-minute run at race pace in representative conditions, track any fluid consumed, weigh nude after. Sweat rate (L/hr) = (pre-weight − post-weight in kg + fluid consumed in kg) / hours. Repeat in cool and hot conditions to see the variance. Many runners discover they lose 1.2 L/hr in summer and 0.6 L/hr in winter — same pace, half the fluid need.
Sweat sodium is commercially testable through Precision Hydration (absorbent patch, lab analysis), Levelen, and real-time wearables like Nix and Flowbio. Most amateur runners will never test this formally; the field observation "my skin tastes of salt after a long run" and "my clothes are white with rings" are rough proxies.
Exercise-associated hyponatraemia (EAH) killed runners before it was understood. Almond et al. (NEJM, 2005) analysed the 2002 Boston Marathon: 13% of runners had post-race sodium below 135 mmol/L; 0.6% were in the dangerous range. The mechanism is overdrinking hypotonic fluid combined with inappropriate antidiuretic hormone release. Risk factors: smaller body size, slower race pace (more hours of drinking), female sex, NSAID use, hot conditions, and inexperienced runners who overhydrate preemptively. The consensus: drink to thirst, not on a schedule; never exceed sweat rate; include sodium in events over 4 hours and in the heat.
Electrolyte products lie on a sodium-concentration spectrum. Standard offerings — Nuun (~300 mg), Skratch Sport (~380 mg), Gatorade Endurance (~300 mg), Tailwind (~310 mg) — suit typical runners in moderate conditions. High-sodium offerings — Precision PH1000/PH1500 (~1,000–1,500 mg), LMNT (~1,000 mg) — serve salty sweaters, heat acclimatisation, and long-duration events. Potassium and magnesium losses during exercise are modest; the sodium-centric formulation of most products is evidence-aligned.