How Much to Drink While Running: Find Your Own Number

Search "how much to drink while running" and you'll find a fixed amount everywhere, to take every 15 or 20 minutes. The problem: two runners at the same level don't sweat the same way, and the pages don't agree with each other.

Official guidelines don't give a universal amount, only limits not to exceed in either direction. Here's how to read them, and how to find your number with a simple scale.

What sweat costs you

You lose water through sweat, and the amount varies a lot. In an analysis of 506 athletes (various sports, not just runners), the average sweat rate was 1.21 L/h.

The official benchmark comes from the American College of Sports Medicine (ACSM, 2007): drinking during exercise helps you avoid losing more than 2% of your body weight through fluid deficit. For 70 kg, 2% is 1.4 kg.

According to the Kenefick review (2018), beyond that loss, temperature regulation becomes less efficient, your heart works harder, and in many conditions (heat, long effort, high intensity), endurance performance drops. The drop takes time to build up.

Drinking to thirst or on a plan?

This is the real debate, and it is still open: both sides are backed by the scientific literature.

For a plan. Kenefick (2018) notes that in humans, thirst fades before full rehydration. If you sweat a lot and only drink when thirsty, a water deficit can build up little by little. He places the value of a plan in efforts over 90 minutes, especially in hot weather, in intense efforts with heavy sweating, and when you want to take in a lot of carbs.

For drinking to thirst. The 2015 international consensus on exercise-associated hyponatremia, which brought together 17 experts, estimates that thirst is enough to prevent excessive dehydration and greatly reduces the risk of drinking too much, in all sports. A meta-analysis (Goulet and Hoffman, 2019) points the same way: over performances of 1 to 2 hours, running or cycling, both strategies give a similar effect. Yet subjects drank on average 505 mL/h when drinking to thirst, versus 1,073 mL/h with a plan.

The two major official position papers, the ACSM (2007) and the National Athletic Trainers' Association (NATA, 2017), are American and agree: don't lose more than 2% of your body weight, and don't gain any. In practice, thirst is enough for runs under 1 hour to 90 minutes in cool weather or at an easy pace. Beyond that, in strong heat or heavy sweating, or when performance matters, a tested plan becomes useful.

The opposite risk: drinking too much

People talk a lot about dehydration, much less about the opposite risk. Drinking more than you lose doesn't help you: the NATA (2017) sees no physiological or performance benefit to superhydration.

It can also be dangerous. Drinking too much dilutes the sodium in your blood: this is exercise-associated hyponatremia. Kenefick advises never drinking to the point of gaining weight during exercise.

Calculate your own needs with a scale

The studies set the 2% benchmark and the weight-based logic. The weigh-in method below comes from coaching practice. It draws on the NATA, which recommends body weight as an objective measure of hydration, and on Kenefick, who advises measuring your sweat rate under conditions close to competition.

1. Weigh yourself before the run, in light clothing.
2. Run 1 hour at the pace you hold in a race, noting what you drink.
3. Dry off and weigh yourself again right after, in the same clothing.
4. Add the weight lost and the volume drunk.

Example: you lost 0.8 kg and drank 0.4 L in 1 hour. Your sweat rate is 1.2 L/h. Your net loss is 0.8 kg: on 70 kg, that's a bit over 1% of your body weight, below the 2% limit.

What you can do on your next run

• ✓Weigh yourself before and after a run of about 1 hour.
• ✓Note the duration, the heat, the volume drunk, and the weight lost.
• ✓On a short, cool run, drink to thirst without forcing it.
• ✓On a run over 90 minutes or in strong heat, test a plan in training, never for the first time on race day.
• ✓Check at the finish that you haven't gained weight.

ELYO reads your watch workouts and analyzes your pace and heart rate. You can compare this data yourself with your weigh-in notes to review each run with your own numbers. To connect fluids and food, also read how to tell if your fueling worked.

Written with the help of AI.