How to build your sodium protocol for endurance running

With 20 miles left in the Western States 100 — a 100.2-mile ultramarathon — professional endurance athlete, running coach, and LMNT partner Zach BitterOpens in a new tab was still moving, but barely.

The race had done what it does every year: baked runners through the canyons of Northern California in temperatures pushing 100 degrees. Bitter had spent the day drinking water. But over around 14 hours of running, he was replacing fluid without replacing enough sodium.

The consequences didn't fully hit until he crossed the finish line.

"As soon as I stopped, within less than a minute, I almost passed out," Bitter recalls. Medical staff handed him a cup of salty broth. Within minutes, he felt normal again.

Looking back, he considers it one of the most important lessons of his career.

"I remember thinking right there, I just screwed my race up by not taking enough electrolytes."

Sodium helps you hold onto the fluid you drink. Without enough of it, blood volume drops. In Bitter's case, not getting enough caused his volume to drop. 

“While I was moving, my heart was still pumping and blood was still getting circulated enough to keep me upright and moving.” Once he stopped — boom — that compensation disappeared.

Hydration advice often begins and ends with water. But water is only part of the equation for endurance athletes. Every drop of sweat takes sodium with it. Lose enough of both and you're running with less circulating fluid, a harder-working heart, and fewer resources available for muscles, cooling, and digestion. 

The challenge is that there is a massive range: some athletes lose just a few hundred milligrams of sodium per liter of sweat. Others lose more than 1,500 mg. Heat, humidity, exercise intensity, sweat rate, and individual physiology all influence how much sodium your body needs.

That's why the best sodium protocol is built around your unique sweat losses and training environment, rather than a one-size-fits-all recommendation.

Here's how to figure out what your body actually needs.

How Does Sodium Influence Exercise Endurance? 

Losing as little as 2% of body weight through dehydrationOpens in a new tab can tank performance, while losses of 4–5% are linked to reductions in VO₂ max, shorter time to exhaustion, and decreased capacity for high-intensity efforts.

Low levels of dehydration aren’t unusual: Marathon runners lose an average of 2.3% of their body weightOpens in a new tab during a race. Sub-3-hour finishers lose around 3.1%. 

Replacing fluid losses helps maintain hydration, but sodium is what helps you hold onto the fluid you drink. Blood volume is one of the body's key performance resources. It delivers oxygen to working muscles, moves heat away from the core, and supports normal digestive function. Lose too much fluid and sodium, and all of those systems compete for a shrinking pool of reserves.

Here's what sodium is doing for you during long efforts:

• Makes it easier to keep drinking: A 2025 reviewOpens in a new tab in Performance Nutrition found sodium may increase fluid intake by influencing thirst. That's useful when you're trying to stay ahead of sweat losses over several hours of exercise.
• Helps you hang on to what you drink: Sodium helps maintain blood osmolality, a measure of how concentrated your blood is. When sodium levels fall and blood becomes more diluted, the kidneys respond by getting rid of more water, according to a 2023 paperOpens in a new tab in the journal Nutrients. Maintaining sodium levels helps more of what you drink stay in circulation instead of peeing it out. 
• Supports blood volume and cardiovascular function: Blood plasma is the fluid portion of blood that delivers oxygen and nutrients throughout the body. As sweat losses mount, blood volume shrinks and the heart has to work harderOpens in a new tab to move the same amount of oxygen to working muscles. Maintaining sodium levels helps preserve plasma volume and may reduce some of that cardiovascular strain.
• May help the gut absorb carbs: Sodium helps transport glucose across the intestinal wall through sodium-glucose cotransporters (SGLTs)Opens in a new tab. That means sodium may help your body absorb more of the carbs you consume during long efforts, leaving less sitting in the gut. 
• Reduces GI issues: Few things derail a race faster than a rebellious gut. Nausea, abdominal pain, diarrhea, and vomiting are common complaintsOpens in a new tab among endurance athletes. One reason may be that dehydration can lead to hypoperfusion Opens in a new tab — reduced blood flow to tissues, including the gut, that limits the delivery of oxygen and nutrients needed for normal digestive function, according to a 2019 cross-sectional studyOpens in a new tab in the journal Sports. Supporting hydration and blood volume may help reduce some of that risk.
• Helps prevent hyponatremia: Drinking too much plain water without enough sodium during long bouts of exercise can contribute to exercise-associated hyponatremiaOpens in a new tab, a condition where blood sodium becomes dangerously diluted, leading to muscle cramps, headaches, and fatigue. Hyponatremia affects anywhere from less than 1% to 22%Opens in a new tab of marathon runners. Sodium helps offset the dilution that happens when athletes drink large amounts of water during long events, according to a 2025 reviewOpens in a new tab in Performance Nutrition.

How Much Sodium You Need Depends on How Much You Lose

Endurance athletes often need more sodium than the average person for a simple reason: They sweat more. The American College of Sports Medicine (ACSM) recommendsOpens in a new tab athletes consume 300–600 mg of sodium per hour during bouts of exercise that last more than two hours. For a runner doing a four-hour effort in heat, that’s 1,200–2,400 mg just during the run.

Those guidelines are useful as a starting point but miss the nuance. The amount of sodium lost in sweat varies dramatically from person to person. A 2016 field studyOpens in a new tab in The Journal of the International Society of Sports Nutrition measured sweat sodium in 157 marathoners during a race and found more than a 13-fold difference between the lowest and highest runners. Low-salt sweaters lost about 500 mg of sodium per liter of sweat, typical sweaters lost about 1,000 mg, and salty sweaters lost more than 1,600 mg.

During exercise, most people lose 0.5 to 2 liters of sweatOpens in a new tab per hour. That said, you can lose a liter in 30 minutes in the heat or during very intense exercise, says running coach Bailey LaskoOpens in a new tab.

Race in the heat and your need for sodium climbs because the hotter and more humid the environmentOpens in a new tab, the more sweat — and sodium — you'll lose. 

How to Assess Your Ideal Sodium Needs

Bitter has had his sodium losses tested in a lab, so he knows he can lose about 614 mg of electrolytes per liter of sweat. He uses this information to build his race day hydration strategy.

Most people aren't getting sweat tested. That's fine. Bitter thinks of electrolyte needs as a bell curve. Most people land somewhere around 950 mg per hour in hot conditions, give or take 200 mg in either direction. 

Both Bitter and Lasko suggest starting with about 500 mg of electrolytes per hour (half a stick pack of LMNT) — increasing to about 900 mg for hotter, longer efforts (1 stick pack of LMNT) — then use the following framework to dial in your personal needs.

Estimate sweat losses: Weigh yourself before and after an hour of training in regular conditions. “One liter of water weighs 1 kg so it’d directly translate to 1 kg (or 2.2 pounds) lost if you don’t drink anything during the workout,” Lasko says.

• Note signs you’re a salty sweater: Think white salt deposits on clothing, a salty taste on skin, or stinging eyes.
• Take the weather into account: If you’re training or racing in the heat, Lasko recommends consuming roughly 1.5 to two times more sodium compared to cooler conditions. 
• Consider your diet: Athletes fueling on low-carb or ketogenic dietsOpens in a new tab may need to bump up their sodium intake, especially when first adopting the diet. Restricting carbs lowers insulin, and lower insulin causes the body to excrete more sodiumOpens in a new tab in urine.

If you’re experimenting with a new sodium protocol, Bitter says to monitor your body and be open to dialing it up or down accordingly. 

If every hot-weather long run ends with you feeling wrecked, lightheaded, or spending the rest of the day trying to rehydrate, your sodium target may be too low.

FAQ

Q: How much sodium do endurance runners need per hour?

The ACSM recommendsOpens in a new tab 300–600 mg of sodium per hour during prolonged exercise. But sodium needs vary widely — often ranging from 750 mg to 1150 mg — based on sweat rate, sweat sodium concentration, workout intensity, and weather conditions.

Q: How do I know if I'm a salty sweater?

White salt stains on your clothing, a salty taste on your skin, or stinging sweat in your eyes are signs you’re sweating out a lot of sodium. 

Q: Can drinking too much water cause low sodium levels?

Yes. Drinking large amounts of plain water without enough sodium can dilute the sodium in your bloodstream. Ultramarathoner Zach Bitter learned this the hard way at the 2018 Western States 100, where drinking plenty of water but not enough electrolytes left him severely depleted and nearly fainting after the race.

Q: What is exercise-associated hyponatremia?

Exercise-associated hyponatremiaOpens in a new tab is a condition in which blood sodium levels become abnormally low, usually because an athlete consumes more water than their body can properly balance with sodium. Mild symptoms of exercise-associated hyponatremia can include headaches, fatigue, muscle cramps, and lightheadedness. 

Q: Should runners take sodium supplements during a marathon?

Most marathon runners lose enough sweat and sodium during a race that some form of sodium replacement makes sense. Sodium needs are personal but running coach Bailey Lasko suggests starting with roughly 500 mg of sodium and 500 mL of water per hour. Adjust if it’s a hot day or you’re a salty sweater.