Electrolytes in Sport: When You Really Need Them – the Situation Matrix for Tennis, Running & Co.

There are those matches in midsummer when, after the second set, you notice that your shirt no longer dries and your cap is getting white edges. That's exactly when "drink a bit of water" turns into a real question: do I need electrolytes now – and if so, which ones and how much?

How much you personally sweat per hour takes two weigh-ins – the free sweat rate calculator turns them into sweat rate, sodium loss and a drinking plan.

The honest answer is: much less often than advertising and social media suggest. For a large part of training, water is completely enough. But there are situations in which missing sodium costs performance and, in extreme cases, can even become dangerous. This article sorts out what makes sense when – with a situation matrix by duration and heat, a sweat test for home and an honest assessment of our own products.

What electrolytes are – and what is really lost in sweat

Electrolytes are minerals dissolved in the body as charged particles: sodium, chloride, potassium, magnesium and calcium. They regulate fluid balance, the excitation of nerves and muscles and many metabolic processes.

When you sweat, they are lost in very different amounts:

The range for sodium is enormous: some people lose almost nine times as much per litre of sweat as others. On top of that comes the amount of sweat itself – depending on heat, intensity and training, roughly 0.5 to over 2 litres per hour. A heavy sweater with salty sweat can lose several grams of sodium on a tournament day.

The situation matrix: water, electrolytes or sports drink?

The following overview combines duration and conditions. It doesn't replace individual advice, but it covers most training situations in club and recreational sport.

A drink "with sodium" means, according to the recommendation of the American College of Sports Medicine: around 20 to 30 mmol of sodium per litre, i.e. roughly 460 to 690 mg, plus some potassium (2 to 5 mmol, roughly 80 to 200 mg per litre) and carbohydrates. In sessions longer than 60 to 90 minutes, the carbohydrates provide energy, which often limits performance more than the minerals do.

Typical situations from club life

Tennis practice in the evening, 90 minutes, 20 degrees: water is enough. Afterwards a normal meal, done.

League match in July, singles plus doubles, four hours on court: a plan pays off here – a drink with sodium and carbohydrates, something salty between matches, weighing yourself before and after.

Easy run, 45 minutes: water before and after is enough, usually nothing at all on the way.

Half marathon in late summer: a sodium-containing drink at the aid stations, not litres of water. How to structure long runs in general is covered in The long run.

The sweat test: your personal number

Whether you are a heavy or light sweater can be found out in one training session:

Determining your own sweat rate

1. 1

   Step 1

   Weigh yourself beforehand

   Right before a typical session of about 60 minutes, weigh yourself naked or in dry underwear, after using the toilet.

2. 2

   Step 2

   Note your drinks

   During the session, drink only from a bottle with a known volume and note at the end how much you drank.

3. 3

   Step 3

   Weigh yourself afterwards

   Right after the session, towel off and weigh yourself again under the same conditions, without going to the toilet in between.

4. 4

   Step 4

   Calculate your sweat rate

   Weight before minus weight after plus the amount you drank gives your sweat loss; 1 kg is roughly 1 litre. Convert to one hour.

5. 5

   Step 5

   Repeat under different conditions

   Repeat the test in the heat, indoors and in the cold – your sweat rate can double depending on conditions.

Example calculation: 76.0 kg before, 74.9 kg after, 0.5 litres drunk, 60 minutes of tennis. Sweat loss: 1.1 kg + 0.5 l = 1.6 litres per hour. With an average sodium content of around 900 mg per litre, that's a good 1,400 mg of sodium per hour – a relevant loss on a long tournament day.

Signs of salty sweat: white edges on your cap and shirt, stinging eyes, a salty taste on your lips, cravings for salty food after exercise. If that sounds familiar, plan sodium more deliberately on long days.

How much to drink? Less dogma, more scales

Performance drops noticeably when you lose more than about 2 percent of your body weight in fluid during exercise – at 75 kg, from around 1.5 kg. That doesn't result in a fixed number of litres but in a goal: keeping the loss within this range during long sessions.

Too much is also a problem. If you drink considerably more than you sweat during sessions lasting several hours – especially plain water – you dilute the sodium in your blood. Such hyponatraemia shows up as headache, nausea and confusion and can be life-threatening. It occurs mainly in slow marathon and ultra runners who drink "just in case" at every station. Rule of thumb: drink to thirst and don't gain weight during exercise.

After exercise: drink about 1.25 to 1.5 litres per kilogram of weight lost over the next few hours, together with a meal containing some salt. Bread with cheese, soup or a normal main meal replaces sodium more reliably than any powder. General information on drinking can be found in How much should you drink a day?

The Syrax electrolyte products: what's inside and what they're for

We stock two electrolyte products. Because sodium content is so decisive in sport, I convert both to the finished drink. All values come from the labels, prices from the shop (October 2026).

My honest assessment

The powder is a potassium-focused mineral drink, not a classic sports drink. For sodium, at around 465 mg per litre of finished mix, it is at the lower end of the ACSM recommendation. For potassium, on the other hand, it is far above it – around 2 grams per serving equals about half the daily requirement (DGE estimate for adults: 4,000 mg) and four times what Germany's Federal Institute for Risk Assessment proposes as the maximum for food supplements (500 mg). On top of that come 242 mg of magnesium, practically the entire BfR recommendation for magnesium from supplements.

For healthy people with normal kidney function, one serving a day is no acute risk – potassium is excreted via the kidneys, and many people eat fewer fruit and vegetables than recommended anyway. But it also means:

• No more than one serving a day, as stated on the label – not several bottles spread over a tournament.
• No other magnesium supplement that day, otherwise it quickly becomes too much.
• Not with kidney disease and not with medication such as ACE inhibitors, sartans or potassium-sparing diuretics without consulting a doctor.

The tablets are a mineral base for everyday life. With 300 mg of potassium and 75 mg of magnesium they are moderate, plus a B-vitamin complex. They aren't intended for the bottle during exercise – you swallow them with a glass of water, for example after training.

For a long tournament day in the heat, neither product alone is enough to replace sodium in the amounts heavy sweaters lose. Here is my practical approach:

• 750 ml of water, a portion of carbohydrates (e.g. 300 ml of apple juice or 30–40 g of sugar) and a scant quarter teaspoon of salt (about 1.5 g of salt ≈ 600 mg of sodium) – that makes a simple, sodium-focused sports drink.
• Plus something salty between matches: pretzels, a cheese sandwich, a broth.
• On such days, the Syrax powder fits in once, for example as a mineral drink after the last match – not as a drink all day long.

How to compare any electrolyte product yourself by sodium per litre and price per litre – with a label check to work through – is shown in Electrolyte powders compared.

Who should be careful

Electrolyte products seem harmless, but they aren't for everyone:

• Kidney disease: potassium and magnesium are excreted via the kidneys – consulting a doctor is a must.
• High blood pressure and cardiovascular disease: coordinate sodium and potassium specifically with your doctor.
• Medication such as ACE inhibitors, sartans or potassium-sparing diuretics raises potassium levels – potassium-rich products only after consultation.
• Children: don't need electrolyte products for normal sport; water and food are enough.

Common mistakes

1. Electrolytes with every session. For an hour of training they are usually superfluous – and the money is better spent on a good meal.
2. Focusing on magnesium instead of sodium. The relevant loss in sweat is sodium.
3. Litres of water on long runs. The risk of hyponatraemia rises if no sodium is added.
4. Combining several products. Electrolyte powder plus magnesium supplement plus multivitamin – you can quickly exceed sensible amounts of magnesium and potassium.
5. Automatically blaming cramps on electrolytes. Fatigue is the more common cause. More on that in Dosing magnesium correctly.

Conclusion: electrolytes are a tool for long, hot days

Electrolytes aren't an everyday drink but a tool for specific situations: long sessions, heat, tournament days and people who sweat a lot and saltily. For everything else, water and a normal meal are the best combination. If you know your own sweat rate, planning the rest is easy – and in the third set you'll notice your legs lasting longer.

Next, I'll look at how creatine works in endurance and racket sports and why it draws water into the muscles in Creatine for endurance and racket sports. How to check supplement labels in general is explained by Eduard in How to assess supplements.

Sources: American College of Sports Medicine: Exercise and Fluid Replacement (Sawka et al., 2007) · Baker: Sweating Rate and Sweat Sodium Concentration in Athletes (Sports Medicine, 2017) · BfR: Maximum level proposals for potassium (German) · Verbraucherzentrale: BfR maximum level proposals (statement 006/2024, German) · Labels and lab reports of the Syrax products (as of October 2026)