Electrolyte Management During Long-Distance Races

In a long race, the electrolyte you lose most is sodium, which leaves your body in sweat. Potassium losses are small.^[1][7] Expert position stands agree on three basics. Drink enough to avoid large fluid losses, but never more than you sweat. Plan for sodium in long or hot events. And take in 30 to 90 grams of carbohydrate an hour, depending on how long the race lasts.^[4][5][9]

Key Takeaways

Sweat rates usually run 0.5 to 2.0 liters an hour, and sweat sodium differs several-fold between athletes, so plans need to be individual.^[7]
The ACSM advises avoiding water losses of more than 2% of body weight. A pre- and post-run weigh-in is the simplest way to learn your sweat rate.^[4][7]
Drinking too much is the strongest driver of dangerously low blood sodium in endurance events. Consensus advice is to drink to thirst and not drink more than you sweat.^[5]
Aim for 30 to 60 g of carbohydrate an hour in longer events, and up to 90 g an hour beyond 2.5 hours, using mixed carbohydrate sources.^[9][10]
In 210 Ironman triathletes, cramping was linked to faster pace and a history of cramps, not to fluid loss or blood sodium.^[11]
Test every drink, gel and salt plan in training before race day.

Evidence grade: Moderate. Fluid and sodium guidance comes mainly from expert consensus statements (ACSM, IOC and the international hyponatremia panel) and field studies of endurance athletes. Carbohydrate guidance rests on more controlled human trials.

Which Electrolytes Matter Most in a Long Race?

Sweat begins with roughly the same sodium content as blood. As it passes through the sweat duct, much of that sodium is reabsorbed, so the sweat on your skin is more dilute. How dilute depends on the person and on how fast they sweat. Local sweat sodium typically ranges from about 10 to 90 mmol per liter, while sweat potassium stays low, around 2 to 8 mmol per liter.^[7] The NIH Office of Dietary Supplements likewise notes that only a very small amount of potassium is lost in sweat.^[1]

In practice, sodium is the electrolyte to plan for. People who are not used to the heat and who exercise for long periods in hot weather can lose enough sodium in sweat that their needs rise above usual intake levels.^[2] Potassium still matters for normal muscle and nerve function, but day to day it comes mainly from food.^[1] See our guide to potassium benefits.

How Much to Drink: Balancing Two Risks

A long race carries two opposite fluid risks. One is losing too much water. The other is drinking far more than you sweat. The American College of Sports Medicine's position stand aims to prevent water losses of more than 2% of body weight, along with large shifts in electrolytes. Because sweat rates vary so widely, it recommends individual plans.^[4]

The 2015 international consensus on exercise-associated hyponatremia (low blood sodium during or after exercise) puts more weight on the second risk. Its first recommendation is to drink to thirst. Its alternative is to estimate your hourly sweat loss and never drink more than that, aiming to finish at the same weight or slightly lighter. The panel notes that mild losses of up to 2% to 3% of body weight are generally well tolerated.^[5]

Field data from ultramarathons support that balance. In 887 finishers of 161-km (100-mile) races in northern California, the top finishers ranged from about a 1% weight gain to about a 6% weight loss. Losing more than 3% did not appear to hurt performance. The researchers concluded that heavy sodium supplementation and drinking beyond thirst are ill advised.^[6]

How to estimate your sweat rate

Weigh yourself with no clothes on before a training run of about an hour, in conditions close to race day.^[7]
Record everything you drink, and note any bathroom stops.
Towel off and weigh again. Weight lost plus fluid drunk, minus any urine passed, approximates your sweat loss for that session.^[7]
Repeat in different weather. Sweat rate changes with heat, intensity and fitness.^[7]

Sodium During the Race

The IOC consensus statement on supplements gives typical compositions for the main race products.^[8] Sports drinks usually contain 5% to 8% carbohydrate, 10 to 35 mmol/L of sodium and 3 to 5 mmol/L of potassium. Electrolyte replacement products contain about 50 to 60 mmol/L of sodium and are used for large sodium losses in ultra-endurance events and for rapid rehydration. Many gels and chews also add electrolytes or caffeine.^[8]

Both water balance and sodium matter for low blood sodium. In the ultramarathon data above, 15.1% of finishers had low blood sodium. The rate ranged from 4.6% to 51% depending on the year and was higher in hotter years. Of those affected, about a quarter had gained weight, while the rest were at stable weight or had lost weight.^[6] Low blood sodium can come from too much fluid, too little sodium replacement, or both.^[2] That is why the safest plan avoids overdrinking first and then matches sodium to your measured losses, rather than relying on salt alone.^[5][6]

Carbohydrate During the Event

Fluids and electrolytes are only part of race fueling. A paper prepared for the IOC's sports nutrition consensus recommends scaling carbohydrate to the length of the event. For efforts of about an hour, small amounts, or even a carbohydrate mouth rinse, can help. Longer events call for 30 to 60 grams an hour. Events lasting more than 2.5 hours may benefit from up to 90 grams an hour.^[9]

The body can use only about 60 grams an hour from a single type of sugar. Reaching 90 grams an hour requires blends of different types of carbohydrate, which also helps keep unabsorbed carbohydrate from building up in the gut. The carbohydrate can come from drinks, gels or solid food. These targets do not depend on body weight, and they may need to be lower at easy intensities.^[10] Eating carbohydrate in the hours and days before a race also tops up your fuel stores.^[9] For more, see the truth about carbs and athletic performance.

Race Planning Table

Event length	Fluid	Sodium	Carbohydrate
About 1 hour	Start well hydrated; drink to thirst^[4][5]	Start with normal blood electrolytes from usual meals^[4]	Small amounts or a mouth rinse^[9]
1 to 2.5 hours	Drink to thirst, or to a plan that keeps losses under about 2% of body weight without exceeding your sweat rate^[4][5]	A sports drink (10 to 35 mmol/L sodium) supplies fluid, sodium and carbohydrate together^[8]	30 to 60 g per hour^[9]
More than 2.5 hours, including ultras	Same rules; never gain weight during the race^[5][6]	Higher-sodium electrolyte products (50 to 60 mmol/L) are used for large losses; avoid heavy salt dosing^[6][8]	Up to 90 g per hour from mixed carbohydrate sources^[9][10]
After the race	Replace the remaining fluid deficit^[4]	Replace the electrolyte deficit with salted food or drinks^[4]	Carbohydrate to refuel, plus 20 to 40 g of protein^[9][12]

Cramps: What the Research Shows

Cramps are often blamed on low electrolytes, but a prospective study of 210 Ironman triathletes found otherwise. The 43 athletes who cramped had the same blood electrolyte levels and body-weight changes as those who did not. The only independent risk factors were a faster race pace and a history of cramping.^[11] That points to pacing and fatigue, not salt or fluid alone, as the place to focus.

Recovery After the Race

After you finish, the goal is to replace the fluid and electrolytes you are still missing.^[4] The International Society of Sports Nutrition suggests protein doses of about 20 to 40 grams (roughly 0.25 g per kg of body weight), spread every 3 to 4 hours. It treats exact timing as a matter of personal tolerance.^[12] Our article on hydration and its role in recovery covers rehydration in more detail, and hydration strategies for sports performance covers shorter events.

Safety and Interactions

Low blood sodium. When blood sodium falls, cells swell, and brain cells are especially sensitive. Symptoms include confusion, headache, nausea, vomiting, fatigue, muscle weakness or cramps and, in severe cases, seizures.^[3] Exercise-related cases are typically linked to more than six hours of hard exercise.^[2] Healthy kidneys can clear only about 0.7 to 1.0 liter of water an hour, so steady overdrinking adds up.^[2] Symptomatic cases are uncommon, under 1% in most endurance studies, but they can be fatal.^[5] An athlete who is confused, vomiting or has a severe headache during or after a race needs medical assessment.

Medications and medical conditions. Talk to your doctor or pharmacist before a race-fueling plan if you take diuretics (water pills), which can lower blood sodium.^[3] Ask the same before using potassium-containing products if you take ACE inhibitors, angiotensin receptor blockers or potassium-sparing diuretics such as spironolactone or amiloride. The potassium intake guidelines do not apply to people whose kidneys do not clear potassium normally.^[1]

Product quality. Sports foods can contain added caffeine or ingredients that carry a contamination risk. Athletes in tested sports can use third-party audited products to manage that risk, though no scheme guarantees a product is completely safe.^[8]

Frequently Asked Questions

How much sodium do I need per hour in a marathon?

There is no single number, because sweat sodium varies several-fold between runners. For reference, sports drinks typically contain 10 to 35 mmol of sodium per liter, and higher-sodium electrolyte products contain about 50 to 60 mmol per liter. Learn your sweat rate, test a product in training, and avoid large salt doses.

How can I tell if I am drinking too much during a race?

Gaining weight during a race means you are drinking more than you sweat. Consensus guidance is to drink to thirst, or to stay at or slightly below your starting weight. Confusion, vomiting or a worsening headache are warning signs that need medical attention.

Do electrolytes stop muscle cramps?

The evidence does not show that. In a study of 210 Ironman triathletes, those who cramped had the same blood electrolytes and fluid losses as those who did not. Faster pace and a history of cramps predicted cramping.

How many carbs per hour should I take in a long race?

About 30 to 60 grams an hour for events of roughly 1 to 2.5 hours, and up to 90 grams an hour beyond 2.5 hours. The higher intakes work best with blends of different carbohydrates. Practice your plan in training to check your stomach tolerates it.

Is water enough for a long race?

For about an hour, water and drinking to thirst are usually enough. In longer events, the ACSM notes that drinks containing electrolytes and carbohydrate can offer benefits over water alone. These drinks supply the sodium and fuel that water does not.

What should I eat and drink after a long race?

Replace the fluid and sodium you are still missing with drinks and salted food. Eat carbohydrate to refuel, and have about 20 to 40 grams of protein, spread over the following hours.

Sources

Disclosure: OPA Nutrition sells supplements, including products mentioned in this article. This content is for education only and is not medical advice. Statements about dietary supplements have not been evaluated by the Food and Drug Administration. These products are not intended to diagnose, treat, cure, or prevent any disease. Talk to your healthcare provider before using any supplement, especially if you take medication or have a medical condition.

Written and reviewed by Michael Gonzales, Founder, OPA Nutrition · Last reviewed: October 17, 2026 · How OPA researches and writes

Electrolyte Management During Long-Distance Races: An Evidence-Based Guide

Key Takeaways

Which Electrolytes Matter Most in a Long Race?