Hydration for Runners: How Much, When, and What to Drink

Hydration advice for runners swings between two bad extremes: "drink constantly or perish" and "real runners don't need water for anything under two hours." The physiology is more useful than either slogan.

What dehydration actually costs

As you sweat, blood plasma volume drops, so your heart works harder to deliver the same oxygen — you feel it as rising heart rate and effort at the same pace. Performance measurably declines somewhere around 2–3% of body weight lost as fluid, sooner in heat. But mild fluid loss during a one-hour run is normal and harmless; you do not need to replace every drop in real time.

Know your sweat rate

Weigh yourself before and after a one-hour run without drinking. Each kilogram lost is roughly a litre of sweat. Most runners land between 0.5 and 1.5 litres per hour — heavy sweaters in heat can exceed 2. This single number turns hydration from guesswork into arithmetic.

Practical rules that cover most situations

• Under 60–75 minutes, cool weather: starting well-hydrated is usually enough. Carrying water is optional comfort, not necessity.
• Longer or hotter: drink to thirst, aiming to replace roughly half to two-thirds of your sweat rate — small regular sips beat occasional gulps.
• Add sodium when sweat is heavy: sweat carries salt (typically 500–1500 mg sodium per litre). For long, hot sessions, use an electrolyte drink or salted fueling rather than plain water alone.
• Check your morning baseline: pale-straw urine and a stable morning weight mean you are starting topped up — which matters more than mid-run heroics.

The overlooked danger: too much plain water

Hyponatremia — dangerously diluted blood sodium from overdrinking — hospitalizes marathoners every year, and it is entirely self-inflicted. Slower runners drinking large volumes of plain water at every station are the classic case. Thirst is a better guide than fear: drink when thirsty, include sodium on long efforts, and do not force fluids you do not want.

Thirst evolved over a few million years. The "drink before you're thirsty" slogan came from beverage marketing. Trust the older signal.

Race day

Rehearse it like everything else: know what the course serves, practice drinking from cups on the run (pinch the rim), and pair fluids with your fueling plan so both systems have been tested together before it counts.