Sweat, Science, and Performance: The Truth About Hydration

Imagine two athletes run the same 5 km race on a hot day. Athlete A drinks nothing. Athlete B sips water throughout. At the finish line, Athlete A's time is noticeably slower — and they feel dizzy. Here is the surprising part: Athlete A lost only about 2% of their body mass in sweat. That tiny percentage was enough to measurably hurt their performance. Why would such a small change make such a big difference? Think about what your body is actually made of and what it needs to keep working. Jot down your best guess before reading on.

Your body is roughly 60% water. During exercise, your muscles generate heat. To cool down, your body sweats — releasing water and electrolytes through your skin. This process is called thermoregulation. As you lose fluid, your blood volume decreases. Lower blood volume means your heart has to beat faster to deliver the same amount of oxygen to working muscles. This state — too little fluid in the body — is called dehydration. Research consistently shows that losing as little as 2% of body mass through sweat causes a measurable drop in aerobic performance, concentration, and reaction time. At 3–4% loss, muscle cramps become likely and coordination suffers. Beyond 5%, the risk of heat exhaustion rises sharply. Electrolytes — minerals such as sodium, potassium, and chloride — are also lost in sweat. They control the movement of fluid into and out of cells and help muscles contract properly. Replacing water without replacing electrolytes during very long sessions can actually be dangerous, causing a condition called hyponatraemia (low blood sodium). So how do you know how much fluid to replace? Sports scientists use sweat rate. The formula is straightforward: Sweat rate (mL/h) = (Pre-exercise mass − Post-exercise mass) × 1000 + Fluid consumed (mL), all divided by exercise duration (hours). For example: if you weigh 60.0 kg before exercise, 59.4 kg after a 1-hour session, and you drank 300 mL during that session: Fluid lost from sweat = (60.0 − 59.4) × 1000 = 600 mL Total fluid output = 600 + 300 = 900 mL (the 300 mL you drank replaced some loss but did not fully offset it) Sweat rate = 600 mL + 300 mL ÷ 1 hour = 900 mL/h Knowing your sweat rate lets you plan how much to drink before, during, and after exercise — a strategy called a hydration protocol. General guidance from sports scientists: • Before exercise: drink 400–600 mL of water 2–3 hours before. • During exercise: aim to match fluid loss without over-drinking; roughly 150–250 mL every 15–20 minutes for most activities. • After exercise: drink 1.2–1.5 times the fluid lost (the extra amount accounts for continued sweating and urine losses during recovery). Urine colour is a practical real-time indicator: pale straw yellow = well hydrated; dark amber = dehydrated.