Hydration and Electrolytes for Runners

How much to drink

On runs under an hour, drink when you are thirsty and plan nothing more. You do not need an electrolyte drink for a run this short.

Past an hour, start from about 500 milliliters (17 ounces) an hour in cool weather and up to a liter an hour in heat, and let thirst steer from there. More than a liter an hour after the first hour is almost always too much, and replacing sweat one for one does not work. A loss near 2 percent of body weight over a long run is normal.

That fits the evidence. A 2025 review of six studies in ultra runners found drinking to thirst safe and effective for many experienced athletes, with no rise in hyponatremia and no measurable performance loss despite moderate body mass loss. It can fall short for runners with high sweat rates or a blunted thirst, and the review concluded that sweat testing, the weather and thirst together work best¹.

To find your own number, weigh yourself before and after a 30-minute easy run in conditions like your race, add what you drank and subtract any pee. In warm weather, recreational runners sweat on average about 25 ounces an hour (women) to 32 (men), so your result tells you where you sit. Then check the plan against what you will actually drink: if it says 1.2 liters an hour and you will not drink that, change the plan. Women need no different plan by cycle phase.

Drinking too much

Overdrinking dilutes the sodium in your blood, a condition called hyponatremia, and it can happen in a long run or race. In a 2005 study of 488 Boston Marathon runners who gave a finish-line blood sample, 13% had low blood sodium, and it was about four times as likely in those who gained weight during the race and more likely with a finish time over 4 hours². The sign is weight gained across the run. Runners who fear dehydration drift into overdrinking. David and Megan Roche, who coach trail, ultra and road runners, set a starting rate from the sweat test, let feel steer it, and plan the later hours below the rate tested fresh, because sweat rate falls after the first hour.

Sodium

Sodium is replaced as a concentration in what you drink, not as a number per hour, because it follows the fluid you take. For long runs and hot days, a drink with about 500 to 1,000 milligrams of sodium per liter suits most runners, toward the top for salty sweaters and in summer, when training hard makes electrolytes matter most. A good starting place is 800 to 1,000 milligrams per liter.

Sweat sodium varies widely. In sweat tests results run from about 200 milligrams per liter of sweat to 1,500 or even 2,000. White salt on your forehead or clothes means you probably sweat a lot of it. Do not chase the exact figure: put yourself on a product in the same range, like picking a small, medium or large t-shirt. Sodium capsules can upset the stomach, so test them on long runs before you rely on them in a race.

Cramps

Late cramps have more than one cause. Running farther or faster than you prepared for is one, and Shorenstein, a sports dietitian, reads cramping beside strong thirst and gut trouble as a sign of too little sodium. A 2022 review of exercise-associated muscle cramps calls the cause controversial, says recent evidence points to several factors acting together, and says individualized prevention will likely work better than general advice such as drinking more fluid³. If you cramp late, check both whether the run went past what you trained for and whether your drink carries enough sodium for your sweat. When a cramp hits, ease off and stretch the muscle gently until it settles.

Practice drinking before race day

The rate you drink at is new on race day, not only the product: 500 milliliters an hour in training and a liter in the race is a change, and nothing should be new on race day. On long runs use the drink, the bottle and the amount you will use in the race, and practice drinking while running, whether you stop for a cup or grab and go. In a road marathon, carry a hand flask even if you are not elite, because an aid station table is a poor setup for anyone but an elite runner with their own bottles.

Bloating from drinking can be trained away in two to three weeks by taking more fluid at once than usual. See the guide on training your gut.

Sort the fluid before you push carbohydrate. Fueling hard while behind on fluid is likely to cause stomach trouble. Keep the drink dilute as runs get longer, and see the guide on carbohydrate during exercise.

Racing in the heat

Heat takes three steps: get used to it, slow down for it, and cool during the race.

Getting used to it is the biggest lever for a hot race, and it takes about a dozen to 20 days, not months. Most of the benefit comes in the first nine to eleven sessions, and it fades quickly. Across 211 papers, an average of 8 sessions of about 90 minutes at about 39 degrees C lowered heart rate at the end of exercise by 17 beats a minute and core temperature by 0.43 degrees C⁴. Rather than 14 days in a row, start two or three months out with one or two sessions, then do two or three a week with a few top-ups in lighter weeks, and stop short of anything that wrecks your training. See the guide on passive heat training.

Slowing down for it means pace is a poor measure on a hot day and effort is the measure. A mid-70s F morning with 85 to 90 percent humidity costs 20 to 30 seconds a mile for the same effort. Start a hot marathon more conservatively and adjust the goal. See the guide on race-day pacing.

Cooling works on the body's core, and wet skin cools by evaporating like sweat. Cooling the forehead and the back of the neck does little for core temperature, and menthol changes how hot you feel and nothing else. Try anything on a long run in similar heat first, because your gut behaves differently six to eight hours in.