What is Dehydration?

Dehydration occurs when you lose more fluids (mostly through sweat) than you take in. For runners, even mild dehydration can significantly impair performance.

Signs & Symptoms

• Thirst: By the time you feel thirsty, you are already slightly dehydrated.
• Dark Urine: Should be pale yellow (like lemonade), not dark (like apple juice).
• Fatigue: Running feels harder than usual.
• Headache: A classic post-run sign.
• Elevated Heart Rate: Blood volume drops, making the heart pump faster to deliver oxygen.

The 2% Rule

Performance reportedly starts to drop when you lose more than 2% of your body weight in fluid. For a 150lb (68kg) runner, that's just 3lbs (1.3kg).