Even a small fluid deficit — around 2% of your body weight — can measurably raise your effort and slow your pace, especially in heat. But over-drinking plain water has its own risk. The goal of smart hydration is to replace roughly what you lose, with enough sodium to keep that fluid where it belongs.

Why hydration drives performance

Sweat cools you, but it also costs you fluid and electrolytes. As you dehydrate, blood volume drops, your heart works harder, and your core temperature climbs — so the same pace feels harder and harder. Replacing fluid steadily keeps your engine running cool and efficient through the back half of a long session.

Hydration isn't about drinking as much as possible — it's about replacing close to what you lose, no more and no less.

How much to drink

Think in three phases around your session:

• Before: drink to a pale-yellow urine color in the hours beforehand; roughly 400–600 ml in the 2–3 hours pre-effort.
• During: a common starting range is 400–800 ml per hour, adjusted up in heat or for heavy sweaters. Sip little and often rather than gulping.
• After: replace about 125–150% of the fluid you lost, since you keep sweating and urinating afterward — so roughly 1.25–1.5 L per kg of weight dropped.

Let thirst guide you within those ranges. Chasing a rigid number can lead to drinking far more than you need.

Electrolytes and sodium

Sodium is the electrolyte that matters most. It helps you retain the fluid you drink and supports muscle function. For efforts longer than about 60–90 minutes, or in heat, aim for roughly 300–700 mg of sodium per hour via a sports drink, electrolyte tablet, or salty food. Drinking only plain water for hours while losing salt in sweat can dilute your blood sodium — a dangerous condition called hyponatremia.

Finding your sweat rate

You can estimate how much you sweat with a simple test:

• Weigh yourself (minimal clothing) right before a one-hour session.
• Train for one hour, noting how much you drink in that time.
• Weigh yourself again afterward, toweled dry.
• Sweat loss per hour ≈ weight lost (in kg, where 1 kg ≈ 1 L) plus the fluid you drank.

Repeat in different conditions — heat and intensity can double your sweat rate. Knowing your number turns hydration from a guess into a plan.

Reading the warning signs

Watch both directions. Signs of dehydration include dark urine, a dry mouth, dizziness, and a creeping heart rate at the same pace. Signs of over-hydration include sloshing in the stomach, swollen fingers, a headache, and nausea despite drinking plenty. If something feels wrong, ease off and reassess — both extremes are worth respecting.

Your hydration habit

Start by finding your sweat rate, then build a simple bottle plan around it and add sodium on anything over an hour. Practice it in training so race day is routine. This is general guidance, not personalized medical advice — sweat rates and sodium needs vary widely, and anyone with heart, kidney, or blood-pressure concerns should consult a qualified professional.

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