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Sweat loss during training

The easiest way to gauge sweat loss is a nude weigh-in: sweat (L) ≈ weight before (kg) − weight after (kg) + fluid consumed (L). Because of water density, 1 kg lost = ~1 L of sweat. Rates vary a lot. Expect 0.5–1.0 L/h at moderate intensity in a mild climate and 1.0–2.0 L/h when you push hard or the weather is hot. Elite endurance athletes can hit 3 L/h in tropical races, and NBA players routinely shed 3 L per game. ACSM advises you to replace 150% of fluid lost within 4 hours of finishing, so a 1 L deficit means drinking about 1.5 L, since some of it ends up as urine. Don't ignore the salts either. Electrolytes matter, because sweat carries roughly 460 mg Na, 200 mg K, 5 mg Mg per liter. Plain water covers sessions under 60 min; for anything longer or hotter, reach for an isotonic drink (50–80 g carbs + 500–700 mg Na per liter).

When it matters

Keep an eye on it during marathons and ultras, where most "wall" episodes after km 30 come down to dehydration on top of glycogen depletion. It matters for cycling in heat, where the sweat rate can outrun what your gut absorbs, so you drink ahead of time. The same goes for tropical team sports and cramp prevention, since the usual trigger is sodium loss rather than dehydration alone. Once you lose more than 2% of body weight your aerobic performance drops measurably, and past 4% the heatstroke risk climbs.

FAQ

Can I just drink to thirst? Under 60 min, that works fine. Go longer and thirst trails behind your actual losses, so you cross the line underhydrated.

Are isotonic drinks worth it? Past about 90 min they earn their keep, speeding gastric emptying and putting sodium back. Short sessions don't need more than water.

Is salty sweat a thing? It is. "Salty sweaters" can lose 1500+ mg Na/L against the 460 mg average, and the telltale sign is white stains showing up on dark clothes.

Does sweating more mean better fitness? Up to a point. Trained athletes start sweating sooner and more heavily, which is an efficient cooling adaptation. It says nothing about burning more fat.