Endurance sports place heavy demands on our bodies. When you exercise for hours, your fueling, recovery, and hydration (including electrolytes) need to be dialed in — not just to make it through a training session or event, but to feel well doing it.
But you don’t need to calculate and replace your precise water and salt losses to feel and perform your best. It can be helpful to learn how much water to carry, or how many aid stations to hit along the course, but it’s time and labor-intensive to calculate. I’m a hydration nerd, but I’m also big on simple guidelines that are easy to follow.
That’s where the golden rule of endurance hydration comes in: Include 1 gram of sodium in 16–32 ounces of water, and then drink that electrolyte water to thirst before, during, and after your activity.
Following this science-backed rule keeps your fluid intake at a Goldilocks level — not too little, not too much — while effectively replacing sweat sodium losses. It’s a solid rule of thumb to get the most out of your training or crush your next race.
That’s the big takeaway today, but there’s lots more science on endurance hydration if you have the stamina for it (pun definitely intended). Let’s start with some background on how fluids and electrolytes help your body during exercise.
Does Dehydration Impair Endurance?
Dehydration is defined as net water loss from the body. It’s common during endurance sports because athletes lose considerable fluids through sweat — up to 10 liters/day in warm climates, according to a paper from the Journal of Sports Sciences. And yes, the bulk of the science suggests dehydration does impair performance.
- A 2010 meta-analysis of 29 studies concluded that losing 3% or more of one’s body weight in water decreases power output during endurance exercise.
- A 2015 study found that dehydrated cyclists (2–3% body weight loss) had higher heart rates, rates of perceived exertion, and core body temperatures, and performed 13% more slowly on time trials.
The latter study helps explain why dehydrated athletes don’t perform as well. Their hearts are working harder and their cooling mechanisms are impaired. Why is that?
First, being dehydrated means your blood volume decreases. Consequently, your heart must beat faster to deliver the same amount of blood (which is carrying oxygen and other nutrients) to your muscles. Your heart working harder means you tire more quickly. Dehydration also decreases sweat rate, impairing your body’s ability to cool itself and potentially decreasing performance.
So, the main hydration goal for many athletes is to prevent dehydration. But over indexing on that goal can lead to overhydration.
Overhydration in Endurance Sports
Endurance athletes often drink water on a set schedule. That’s why you see watering stations on the marathon course every few miles. The goal is to prevent net water loss from the body, a practice encouraged by institutions like the American College of Sports Medicine.
Unfortunately, drinking plain water beyond thirst can create a separate problem: overhydration. Too much sodium-free water dilutes blood sodium levels, causing a condition called hyponatremia that leads to fatigue, confusion, headaches, brain fog, and muscle cramps. Not only do the symptoms of hyponatremia impair exercise performance, but the condition can be extremely dangerous, and is why elite endurance athletes sometimes need medical attention (usually intravenous salt water) after crossing the finish line. We break down exercise-associated hyponatremia here if you want to learn more.
This leads us to the missing link in many rehydration strategies.
Sodium for Endurance Hydration
Athletes lose a boatload of sodium through sweat. According to that paper from the Journal of Sports Sciences, vigorous exercise in warm climates can also provoke sodium losses of 3.5–7 grams per day. Replacing the depleted sodium helps performance for two reasons:
- Sodium supports fluid balance, which supports blood flow, sweating, brain function, and more.
- Sodium drives the sodium-potassium pump, a part of every human cell that allows nerve impulses to fire. This allows cells to “talk” to each other, contract muscles, and more.
With this solid understanding of water and sodium’s role in performance, let’s talk about how to get enough of both now.
How Much Water and Sodium Do You Need for Endurance Sports?
When exercising, you need enough water and sodium to replace sweat losses. How much of each depends on your sweat rate and sweat sodium concentration, but the golden rule of exercise hydration will help keep your strategy simple: Drink electrolyte water to thirst before, during, and after exercise. Let’s break down why this rule works.
Drink to thirst
Dehydration decreases blood volume, and decreased blood volume triggers thirst. When you drink to thirst (and not beyond), you restore blood volume to healthy levels, and you avoid dehydration and overhydration in one go. In most cases, thirst is all you need to guide fluid needs. Exceptions to this rule are older adults and folks who spend time or exercise in cold weather and at high-elevation.
Add electrolytes to your water
For the electrolytes part of the equation, studies suggest athletes can lose approximately 0.5–2 grams of sodium per liter of sweat, with an average of about 1 gram/liter. To ensure adequate sodium replacement, include ~1 gram of sodium per 32 ounces (~1 liter) of fluid, and consider increasing it to 1 gram per 16 ounces of fluid if you think you’re a salty sweater. How do you figure this out without sweat testing? Play with your sodium-to-liquid ratio. You’ll feel the difference when you get it right.
Factors increasing fluid and electrolyte needs
Sweat rate influences your hydration needs during exercise, and sweat sodium concentration influences sodium needs. Both of these are affected by a variety of inputs.
Factors that increase sweat rate include greater ambient temperatures, humidity, exercise intensity, aerobic fitness, body weight, heavy clothing, and heat acclimation. Factors that decrease sweat rate include airflow (as in, oh yeah, that breeze feels nice) and dehydration.
What about sweat sodium concentration? Anything that increases sweat rate (like intense exercise) makes sweat saltier, increasing sodium losses. There’s also lots of individual variability. The saltiest sweaters have 4–5 times more sodium in their sweat than the least salty.
Planning your Endurance Hydration
For many folks, including 1 gram of sodium in 16–32 ounces of water and ensuring you have access to your water bottle while you work out — or have the ability to refill it along the way — will do the trick. Your body will guide your hydration needs.
For those who are going longer distances and carrying all their water with them, planning how many bottles to stage along a course, or want an extra edge, you may want to determine your exact sweat losses. We provide in-depth guides for how to calculate your sodium and fluid losses in our articles on sweat rate, sweat sodium concentration, and performance hydration. Our Sodium Intake Calculator can also provide a quick estimate of your sodium needs, including factoring in your activity intensity and climate.
Precise calculations will help you estimate your individual needs along the course, but it’s important to note you’ll still want to rely on your thirst signals to determine when and how much you consume. Sweat losses vary considerably by activity and environment, and drinking to thirst rather than on a set schedule allows your body to adjust your hydration strategy to the situation.
We designed LMNT with 1 gram of sodium in each stick pack and can to provide an easy option to hit that 1 gram per 16–32 ounces of water target. DIY fans can also try our electrolyte homebrew recipes, or keep it as simple as shaking ½ teaspoon of salt (~1 gram sodium) into a liter of fluid. It’ll taste like mildly salty water, but some folks don’t mind that, and it’ll get the job done.
However precise you get with your calculations and whichever rehydration strategy you choose, Stay Salty in your training and on the race course, and don’t forget to have fun!
