Hydration Strategy During Exercise

The goal of an exercise hydration strategy, per the 2007 ACSM position stand on exercise and fluid replacement, is to prevent excessive dehydration (>2% body-weight loss from water deficit) and excessive electrolyte changes, while avoiding overdrinking (drinking more than sweat losses is the main cause of exercise-associated hyponatremia; see Exercise-Associated Hyponatremia).

Before: Arrive euhydrated. The 2007 stand: when prehydrating is needed, drink about 5–7 mL/kg slowly at least 4 hours before exercise; if you pass no urine or it is still dark, drink another ~3–5 mL/kg about 2 hours before. Sodium-containing drinks or salted snacks help retain the fluid. The 2016 AND/DC/ACSM joint position gives a similar range: 5–10 mL/kg in the 2–4 hours before, aiming for pale-yellow urine.

During: Drink to keep the total deficit under 2% of body weight. Losses above 2% degrade aerobic performance, especially in warm-to-hot weather; in cold weather the threshold is nearer 3%, and losses of 3–5% do not appear to impair anaerobic or strength performance (2007). Sweat rates vary enormously: about 0.4–1.8 L/h in running across conditions (2007), and 0.3–2.4 L/h across sports and environments (2016). Many athletes land on an intake of about 0.4–0.8 L/h, customised to tolerance and opportunity (2016). Individualise by weighing before and after sessions: each kg lost is roughly 1 L of sweat (about 16 oz per pound), after accounting for drinks and urine. For hard sessions of about an hour or more, carbohydrate at roughly 30–60 g/h helps sustain performance (see Endurance Fueling). On sodium, the 1996 ACSM stand recommended 0.5–0.7 g per liter for exercise over an hour; the 2007 stand cites sports-drink guidance of about 20–30 mEq/L sodium (≈0.46–0.69 g/L), 2–5 mEq/L potassium and 5–10% carbohydrate, adjusted to the event and weather; the 2016 joint position says to take sodium when sweat sodium losses are large (sweat rate >1.2 L/h, salty sweat, or sessions over 2 hours). The 1996 stand also found little difference between a carbohydrate-electrolyte drink and plain water in exercise under an hour.

After: For rapid, complete recovery, the 2007 stand advises about 1.5 L of fluid per kg of body weight lost; the 2016 joint position gives 125–150% of the deficit (1.25–1.5 L per kg lost), because sweat and urine losses continue. Include sodium (from fluids or food) to retain it, and spread the intake out rather than gulping it at once. Alcohol in recovery is discouraged for its diuretic effect; moderate habitual caffeine (e.g., <180 mg) is not a meaningful problem (2016).

Heat acclimatization increases sweat rate but lowers sweat sodium concentration, because sweat glands reabsorb more sodium (2007; see Exercising in Heat — Illness Prevention and Acclimatization).

• Keep dehydration under ~2% body-weight loss; >2% degrades aerobic performance (especially in heat); avoid overdrinking.
• Pre: ~5–7 mL/kg ≥4 h before, plus ~3–5 mL/kg ~2 h before if needed (ACSM 2007); 5–10 mL/kg in the 2–4 h before (2016).
• During: sweat rates ~0.3–2.4 L/h; typical intake ~0.4–0.8 L/h; carbs ~30–60 g/h in hard sessions of ~1 h+.
• Sodium: ACSM 2007 cites ~20–30 mEq/L (≈0.46–0.69 g/L) (1996 stand: 0.5–0.7 g/L); take sodium when losses are large (>1.2 L/h, salty sweat, >2 h).
• Post: ~1.5 L per kg lost (2007); 125–150% / 1.25–1.5 L per kg (2016), with sodium.
• Personalize via pre/post body-weight weighing (1 kg ≈ 1 L sweat).