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Water intake by activity: hydration that scales with effort

Start from a baseline of 35 mL of water per kg of body weight per day, which covers a sedentary adult living in a temperate climate. Then layer effort on top. Physical exercise adds 500–750 mL per hour when the activity is moderate, and as much as 1 L per hour when it's intense. A hot or humid environment tacks on another 25–50%. Take an example: a 70 kg person who trains 1 h/day works out to 70 · 35 = 2,450 mL, plus 600 mL for the workout, so roughly 3.05 L/day. Losing just 2% of your body weight to hypohydration is enough to drop performance by 10–20% and dull your thinking. The American College of Sports Medicine (ACSM, 2007) suggests weighing yourself before and after long sessions and drinking back 150% of the weight you lost over the next few hours (every kg lost means about 1.5 L to replace). The opposite mistake is also dangerous. Hyponatremia, which comes from drinking more than 1 L/h for a long stretch without replacing sodium, can kill you, and it shows up most often in marathon and ultramarathon runners.

Applications: athletes, marathons, cycling, soccer, aquatic sports

Hydration tied to activity matters most for endurance athletes (marathon, ultramarathon and triathlon protocols schedule a drink every 15–20 min) and for cyclists, who go through at least one bottle an hour and more when it's hot. Soccer players have to grab whatever they can at halftime and stoppages, since the run of play leaves no room to drink. Even swimmers and water-polo players need a plan, because being in the water hides how much fluid they're losing while they still sweat heavily. Brazil's heat makes every one of these cases worse, and Carnival blocos, summer training or beach soccer can push a person past 5 L/day without much trouble.

FAQ

Is the 35 mL/kg rule scientific? Treat it as a working rule of thumb. The Institute of Medicine (2004) puts total water at roughly 3.7 L/day for men and 2.7 L for women, food included, and what you actually need shifts with climate, body composition and diet.

Do coffee and tea count? They do. Maughan et al. (2016) found that moderate caffeine (< 400 mg) doesn't cause a net loss of fluid, so coffee, tea, juice and milk all add to your total.

How do I know if I'm hydrated? The easiest check is your urine: pale yellow, 5–6 times a day, means you're on track. Dark amber says drink more. Completely clear can mean you're overdoing it.

Do I need isotonic drinks? Mostly for sessions that run past 60–90 min in the heat or that leave you drenched in sweat. Anything shorter, and plain water alongside your usual meals puts the electrolytes back just fine.