Hydration for Athletes: How Much to Drink and When to Add Salt

Hydration advice used to be simple: drink as much as you can, before you get thirsty, because thirst means you're already dehydrated. That advice turned out to be wrong in an important way. Drinking far more than you need during long events can cause a dangerous drop in blood sodium, and some athletes have died from it.

Today's guidance is more balanced. Dehydration can hurt performance, but so can drinking too much. The right amount depends on how long and hard you train, how hot it is and how much you personally sweat, which can vary several-fold between two people doing the same session.

This guide explains how much to drink before, during and after exercise, how to measure your own sweat rate, when electrolytes are worth it, and how to avoid both dehydration and over-drinking.

Why hydration matters for performance

During exercise, your muscles produce heat. Sweating is how you get rid of it. As sweat evaporates from your skin, it cools you down. The cost is fluid and electrolytes, mostly sodium.

If you lose enough fluid, your blood volume drops. Your heart has to work harder to pump blood to your muscles and to your skin for cooling. Your body temperature rises faster. Exercise feels harder at the same pace.

A 2014 review by Samuel Cheuvront and Robert Kenefick concluded that dehydration of more than about 2% of body weight tends to reduce endurance performance, particularly in hot conditions. In cooler weather and shorter events, the effect is smaller. Skill and concentration can also suffer, which matters in team and racket sports.

The 2% guideline

The American College of Sports Medicine's 2007 position stand on exercise and fluid replacement, led by Michael Sawka, recommends trying to prevent fluid losses of more than about 2% of body weight during exercise. For a 70 kg person, that's 1.4 kg.

Importantly, it also warns against drinking so much that you gain weight during exercise. The goal is to limit losses, not to replace every drop of sweat.

How much do athletes sweat?

Far more variable than most people realise. A 2017 review by Lindsay Baker found that sweat rates in athletes commonly range from around 0.5 to over 2 litres per hour, depending on the person, the sport, the intensity, the temperature and the clothing. Sweat sodium concentration also varies widely between individuals.

That's why generic advice like "drink 500 ml an hour" can be too much for one athlete and far too little for another. Your own sweat rate is the most useful number you can know.

How to measure your sweat rate

1. Go to the toilet, then weigh yourself in minimal clothing just before a typical one-hour session.
2. Train as normal. Note exactly how much you drink.
3. Towel off and weigh yourself again in the same clothing, before drinking or going to the toilet.
4. Sweat rate (litres per hour) = weight lost (kg) + fluid drunk (litres).

Example: you weighed 70.0 kg before and 69.2 kg after, and drank 0.4 litres. Sweat rate = 0.8 + 0.4 = 1.2 litres per hour.

Repeat this in different conditions, such as a cool morning run and a hot afternoon session, because your sweat rate will change with the weather and intensity.

Before exercise

The aim is to start training well hydrated, not overloaded. The ACSM recommends drinking about 5 to 7 ml per kg of body weight slowly, at least four hours before exercise. For a 70 kg person, that's 350 to 490 ml, roughly a large glass or two. If you don't pass urine or it's dark, drink a bit more (about 3 to 5 ml/kg) around two hours before.

Including some sodium, from a salty snack or a meal, helps you retain the fluid rather than passing it straight out.

Checking your hydration status

A simple check is urine colour. Lawrence Armstrong's work in the 1990s showed that urine colour tracks reasonably well with other measures of hydration. Pale straw or light yellow usually means you're well hydrated. Dark yellow suggests you need more fluid. Combine it with thirst and morning body weight for a better picture. Some vitamins and foods change urine colour, so don't rely on it alone.

During exercise

Drinking to thirst vs drinking to a plan

For most recreational athletes in sessions of up to about an hour or two, drinking when thirsty works well. Thirst is a reasonable signal. For longer events, hotter conditions or heavy sweaters, a plan based on your known sweat rate is better, because it's hard to drink enough while moving fast, and thirst can lag behind losses.

A practical target for long events: replace enough fluid that you finish with a loss of no more than about 2% of body weight, and never finish heavier than you started.

Electrolytes and sodium

Sweat contains sodium, along with smaller amounts of potassium, chloride and other minerals. Sodium is the one that matters most during exercise.

When do you need electrolytes?

• Sessions longer than about 90 minutes to 2 hours.
• Training or competing in heat and humidity.
• Heavy or salty sweaters.
• Multiple sessions or games in one day.
• When rehydrating quickly after heavy losses.

For shorter sessions in normal conditions, water and your normal meals provide enough sodium.

Are you a salty sweater?

Signs include white, salty streaks on your clothes or skin after training, sweat that stings your eyes or tastes very salty, and cramping or headaches after long sessions. A sweat test can measure your sodium losses accurately if you want precise numbers.

The danger of drinking too much: hyponatraemia

Exercise-associated hyponatraemia is a drop in blood sodium during or after prolonged exercise. It's usually caused by drinking more fluid, especially plain water, than your body loses through sweat and urine. The extra water dilutes your blood sodium.

The 2015 international consensus statement on exercise-associated hyponatraemia, led by Tamara Hew-Butler, identified over-drinking as the main cause. Mild cases cause nausea, bloating, headache and confusion. Severe cases can cause seizures, coma and death.

Risk factors include:

• Events lasting more than four hours, such as marathons for slower runners, ultramarathons and Ironman triathlons.
• Drinking at every aid station regardless of thirst.
• Gaining weight during an event.
• Smaller body size.

After exercise: rehydration

After training, aim to replace the fluid you've lost gradually over the next few hours. The guidance is to drink about 1.25 to 1.5 litres for every kilogram of body weight lost. You need more than you lost because you'll keep losing fluid through urine as you rehydrate.

Including sodium, from food or a drink, helps you retain the fluid. A normal meal with some salt is often enough.

What to drink

Not all drinks hydrate equally. A 2016 study by Ronald Maughan and colleagues compared 13 common drinks and found that some, including milk and oral rehydration solutions, kept people hydrated for longer than water over the following few hours. Their protein, fat, sugar and electrolyte content slows how fast the fluid is lost.

• Water: fine for most situations.
• Milk: good for recovery: fluid, protein, carbohydrate and electrolytes.
• Oral rehydration solutions: useful after heavy losses or illness.
• Sports drinks: fluid, carbohydrate and sodium after long sessions.
• Coffee and tea: count towards fluid intake in moderate amounts.
• Alcohol: best avoided straight after hard training, as it slows rehydration and recovery.

Hydration for different sports

Running

Most runs under an hour need no fluid during. For half marathons and longer, use aid stations or carry a bottle, and practise drinking while running in training. For marathons, know your sweat rate and plan your drinking around it.

Cycling

Cyclists can carry bottles easily, which makes it simpler to drink regularly. Longer rides in heat need planned intake with sodium and carbohydrate.

Team sports

Use breaks, halftime and substitutions to drink. In tournaments with multiple games a day, rehydration between games is important.

Indoor and gym training

Indoor sessions can cause high sweat rates because there's little air movement. Use a fan and keep a bottle within reach.

Hydration myths

Muscle cramps and hydration

Exercise-associated muscle cramps are often blamed on dehydration or low electrolytes. The evidence is mixed. Many researchers now think muscle fatigue and changes in nerve control play a bigger part, especially late in races or when athletes go harder than they've trained for. That said, some heavy, salty sweaters do seem to cramp less when they replace more sodium. If you cramp regularly, look at pacing, training specificity and sodium intake together.

A sample marathon hydration plan

Here's how one runner might plan their drinking for a marathon in mild conditions, based on a measured sweat rate of 1.0 litre per hour and a finish time of around 4 hours:

• Morning: 400 to 500 ml of fluid with breakfast, 3 to 4 hours before the start, plus some salt with food.
• Before the start: a few small sips if thirsty. Go to the toilet.
• During the race: about 400 to 600 ml an hour, from aid stations or a bottle, mixing water and sports drink. This replaces roughly half of sweat losses, keeping total loss well under 2% of body weight for most runners.
• Sodium: from the sports drink, plus a salty snack or electrolyte tablet if they're a salty sweater.
• After: rehydrate gradually with fluid and a meal.

Practise the plan in long training runs before race day.

Hydration for older athletes and children

Older adults tend to have a weaker thirst response, so they may not feel thirsty until losses are greater. A planned drinking routine, particularly in heat, is sensible for masters athletes.

Children have a smaller body size and produce more heat relative to their sweat rate. They also tend to get distracted and forget to drink. Regular drink breaks, cool water and shade during hot training sessions help young athletes stay safe.

Hydration in the heat

• Acclimatise: over 1 to 2 weeks of training in heat, your body sweats earlier and more efficiently, and loses less sodium in sweat.
• Pre-cool: cold drinks, ice slurries or cold towels before exercise can help in very hot conditions.
• Adjust pace: slow down in the heat. Effort matters more than pace.
• Know the signs of heat illness: dizziness, confusion, stopping sweating, nausea. Stop, cool down and get help.

Building your personal hydration plan

1. Measure your sweat rate in cool and warm conditions.
2. Check for signs of salty sweat.
3. For sessions under an hour: drink to thirst.
4. For longer sessions: plan to replace enough to keep losses under 2%, with sodium if needed.
5. Practise in training, including the drinks you'll use in races.
6. Rehydrate gradually after training: 1.25 to 1.5 litres per kg lost, with food or salt.
7. Never finish a long event heavier than you started.

Hydration works hand-in-hand with fuelling. Our guide to eating before, during and after exercise covers carbohydrate intake during long sessions, and our guide to creatine explains why it causes some water gain.

Frequently asked questions

Sources and further reading

1. Sawka MN, et al. American College of Sports Medicine position stand. Exercise and fluid replacement. Medicine and Science in Sports and Exercise, 2007;39(2):377-90. DOI PubMed
2. Hew-Butler T, et al. Statement of the 3rd International Exercise-Associated Hyponatremia Consensus Development Conference, Carlsbad, California, 2015. British Journal of Sports Medicine, 2015;49(22):1432-46. DOI PubMed
3. Cheuvront SN, Kenefick RW. Dehydration: physiology, assessment, and performance effects. Comprehensive Physiology, 2014;4(1):257-85. DOI PubMed
4. Maughan RJ, et al. A randomized trial to assess the potential of different beverages to affect hydration status: development of a beverage hydration index. The American Journal of Clinical Nutrition, 2016;103(3):717-23. DOI PubMed
5. Baker LB. Sweating Rate and Sweat Sodium Concentration in Athletes: A Review of Methodology and Intra/Interindividual Variability. Sports Medicine, 2017;47(Suppl 1):111-128. DOI PubMed
6. Armstrong LE, et al. Urinary indices of hydration status. International Journal of Sport Nutrition, 1994;4(3):265-79. DOI PubMed