Electrolytes and hydration for athletes: sodium, potassium and cramp

When you train hard you lose both fluid and electrolytes through sweat, sodium first, and on a hot day that can reach 1 to 1.5 litres of sweat per hour. Electrolytes keep your fluid and nerve balance steady, so topping up during long or heavy efforts can make sense. For shorter sessions, water is usually enough.

I think most of the panic around electrolytes and sport is overdone. If you eat normally, you take in most minerals without trying.

What are electrolytes and why do you need them?

Electrolytes are charged minerals such as sodium, potassium, magnesium and chloride, dissolved in your body fluids. They regulate your fluid balance, muscle contraction and nerve signals. During exercise you lose them through sweat, which can shift the balance. A disturbance can affect your performance and recovery, though that is rarely explained by one mineral alone.

Sodium is the most abundant electrolyte in sweat and holds fluid around your cells. Potassium sits mainly inside your cells and works with sodium for good nerve and muscle function. Chloride follows sodium closely and supports your acid-base and fluid balance.

On a varied diet, a healthy athlete rarely runs truly low on electrolytes. According to the RIVM, average sodium intake in the Netherlands sits around 2.9 grams per day, more than most people need. Problems tend to show up in extreme cases: hours of exertion, heavy heat, or heavy sweating without topping up.

How much fluid and salt do you lose through sweat?

It varies a lot between people and situations. As a rough guide you lose between under 0.5 and over 2 litres of fluid per hour, and around 200 to 2000 milligrams of sodium per litre of sweat. Heat, intensity and genetics drive most of it. The figures below are indicative ranges, not a personal measurement.

Why do the numbers spread so wide? Your sweat rate depends on fitness, body weight, humidity and how hard you go. Your sweat sodium is partly genetic too: one athlete loses 300 mg per litre, another well over 1500 mg, in the same session.

To estimate your own loss, weigh yourself before and after a session. Every kilo of weight lost roughly equals a litre of fluid you can replace.

Do electrolytes really help against cramp?

The evidence is mixed. For a long time we assumed cramp came from fluid and salt loss, but recent research finds little consistent support for that. Exercise-associated cramp looks more multifactorial: fatigue, training load and an altered nerve-muscle response all play a part. Electrolytes can help, but offer no guarantee.

Cramp usually has several causes at once.

Pinning cramp on a salt deficit alone probably misses the bigger picture. In practice a mix beats one trick: build up gradually, train through, drink enough, and take salt or food in time. Magnesium comes up often too. Read more in magnesium for athletes, since its effect on cramp is limited as well.

Salt tablets and pickle juice circulate as miracle fixes, but the evidence for them is weak. Treat them as a small aid at most, not a solution.

When should you test your sodium and potassium?

A blood test is not a must, but it can give useful data with persistent complaints. Consider testing your sodium and potassium with stubborn cramp, heavy sweating during long endurance training, or symptoms after exercise in the heat. Always discuss abnormal values with your GP.

Think about a test with:

• persistent cramp that does not improve with rest and food
• heavy sweating during long or intense endurance training
• symptoms after exercise in the heat, such as dizziness or nausea

A single abnormal sodium or potassium says little on its own. It is a snapshot your doctor reads alongside your symptoms, medication and training week. Both low and high potassium can cause muscle complaints, so never interpret one value in isolation.

Endurance athletes carry extra risk here. Read more in blood values for endurance athletes.

How do you top up your electrolytes when you train?

Food first. You usually replace electrolytes fine with normal food: a bit of extra salt, a banana for potassium, and dairy for sodium and potassium. A sports drink with sodium and carbohydrates mainly makes sense above 60 to 90 minutes of exercise or in the heat. Below that, water does the job.

The Voedingscentrum notes that most people get enough fluid and salt from regular food, and that sports drinks often mainly add calories you do not need.

The Gezondheidsraad sets a limit for adults of no more than 6 grams of salt (about 2.4 grams of sodium) per day and a potassium intake around 3500 mg. If in doubt, check with your sports doctor or dietitian. See the whole thing as part of your wider nutrition plan, as in nutrition for muscle growth.

Watch the other side too. Too much water without salt can dilute your sodium, and during very long efforts that can lead to hyponatremia, with nausea and confusion. So drink to thirst and add sodium on long endurance days. A simple homemade sports drink of water, a pinch of salt and some fruit juice can be enough.

My advice: fix it with your plate first and keep it simple. If cramp or symptoms keep coming back, get your values measured, for example through our 360 health test, and discuss the result with your GP.

References

1. Voedingscentrum. Salt and sodium. 2024.
2. Gezondheidsraad. Core advice on sodium and potassium. 2015.
3. RIVM. Salt and sodium intake in the Netherlands. 2023.

Disclaimer

Every blood test result includes a professional assessment by a BIG-registered doctor. This article gives general information and is not a substitute for medical advice. A blood test is a tool to walk into the conversation with your GP better informed, not a diagnosis in itself. For treatment decisions, discuss your results with your GP.