The hydration message in endurance sports has swung between two extremes. For decades, the advice was “drink as much as possible.” Then research revealed that overhydration (hyponatremia — dangerously low blood sodium from excessive water intake) was hospitalizing and occasionally killing marathoners. The advice shifted to “drink to thirst.”

The truth is in the middle, but closer to “drink to thirst” than most runners expect.

How Much You Lose

Sweat rates vary enormously — from 0.5 liters per hour in cool weather at easy pace to 2+ liters per hour in heat at hard effort. Body size, fitness level, heat acclimatization, and genetics all affect sweat rate.

You can measure yours: weigh yourself nude before and after a run. Each pound lost is approximately 16 oz of fluid. Subtract any fluid you consumed during the run. The result is your sweat rate for those conditions.

Most runners don’t need to do this math. For runs under 60 minutes, drink when thirsty and you’ll be fine.

The Guidelines

Before running: 16-20 oz of water 2-3 hours before. This ensures you start hydrated without the sloshing that comes from drinking too much right before.

During runs under 60 minutes: Drink to thirst. For most runners in moderate conditions, this means nothing or a few sips of water. Your body’s thirst mechanism is reliable for this duration.

During runs of 60-90 minutes: Small sips every 15-20 minutes if thirsty. Water is sufficient.

During runs over 90 minutes: 4-8 oz every 15-20 minutes. This is where structured hydration matters — waiting until you’re very thirsty means you’ve already fallen behind. In heat, lean toward the higher end. In cool conditions, the lower end.

After running: Replace lost fluid. A rough guide: 16-24 oz for every pound of body weight lost. Include sodium (food or electrolyte drinks) to help retain the fluid.

When Electrolytes Matter

Your sweat contains sodium, potassium, chloride, and small amounts of other minerals. For most runs, you replace these through normal eating afterward. Dedicated electrolyte replacement during running becomes important in specific scenarios:

Runs over 2 hours. Extended sweating depletes sodium stores significantly.

Heavy sweating in heat. High sweat rates accelerate electrolyte loss.

Salty sweaters. If you notice white residue on your clothes or skin after running, you lose more sodium than average. Electrolyte supplementation during longer runs may benefit you more than most runners.

What to use: Sports drinks (Gatorade, Nuun, LMNT) or salt tabs. The key ingredient is sodium — 300-600mg per hour for runs over 2 hours in heat.

The Overhydration Risk

Hyponatremia occurs when you drink so much water that your blood sodium concentration drops to dangerous levels. Symptoms include nausea, headache, confusion, and in severe cases, seizures and death.

This is primarily a risk for slower marathon runners who drink at every aid station regardless of thirst. The combination of lower sweat rates (less sodium lost) and high fluid intake (diluting remaining sodium) creates the perfect conditions.

Prevention: Don’t force fluid intake beyond thirst. If you’re running a marathon in 4+ hours, use electrolyte-containing drinks rather than pure water for at least some of your fluid intake. Never drink to the point where your stomach feels full.

The Practical Summary

For the vast majority of recreational running: drink when you’re thirsty, drink water, and eat a normal meal with salt after your run. The human thirst mechanism evolved over millions of years for exactly this purpose. It works.

Structured hydration plans with specific ounce-per-hour targets become relevant for runs over 90 minutes, particularly in heat. Below that threshold, your body tells you what it needs.