Electrolytes in heat and sport: what your body needs

Sweat is not chemically neutral. Alongside water it carries mineral ions, the electrolytes, that your body needs to hold its osmotic balance, pass signals along nerve cells and contract muscle.

Sweat losses during exercise run higher than most people assume: 0.5 to 2 litres an hour is the normal range. Push hard in the heat, say a marathon on a 30°C summer day, and it can reach 2.5 litres. What matters is not only how much you sweat, but what leaves with it.

The biggest losses are sodium and chloride, which together make up common salt. They are the dominant ions in sweat by quantity. Potassium, magnesium and calcium go too, in far smaller amounts. The exact loss is highly individual. Some people have remarkably salty sweat (the so-called "salty sweaters"), others barely shed sodium at all.

Electrolytes are not equally important, and they do not all leave your body in meaningful amounts. Here is where the four central ions actually stand:

Sodium (Na+): the dominant cation in sweat by quantity. It regulates water balance, blood volume and nerve signalling. Losses of 400-1,100 mg per litre of sweat are normal, and that spread explains why blanket advice so often misses.

Potassium (K+): mostly sitting inside your cells. Involved in muscle contraction and nerve function. Sweat losses are comparatively small (100-150 mg/L) and rarely critical on a mixed diet.

Magnesium (Mg2+): a cofactor for more than 300 enzymes, essential for ATP production and neuromuscular transmission. Sweat losses are minimal at 5-15 mg/L. The attention magnesium gets in sports nutrition is often out of proportion.

Chloride (Cl-): follows sodium. As its counter-ion it helps regulate pH and forms part of stomach acid. It is seldom discussed on its own, because you replace it together with sodium anyway.

Sodium is the electrolyte endurance athletes fixate on, and the fixation is earned. Sports drinks are typically recommended to carry 400-800 mg of sodium per litre; for long races or heavy sweat losses, up to 1,100 mg/L can make sense.

Individual sweat composition varies enormously. Two runners of the same height, holding the same pace over the same distance, can differ threefold in sodium loss. You can spot a "salty sweater" by white residue on clothing after training, a salty taste on the skin, or frequent cramping despite drinking enough.

If you lose a lot of sodium, salty snacks work as well as commercial electrolyte products, and cost far less: pretzels, salted crackers, olives. On timing, the point worth remembering is that sodium taken during the effort beats sodium taken afterwards. It drives thirst and helps you hold on to water, which keeps hydration steadier over long sessions.

Magnesium is the most talked-about mineral in sports nutrition, usually with expectations it cannot meet. European reference values put adequate intake at around 350 mg a day for men and 300 mg for women. Athletes need slightly more, because magnesium also leaves via urine and sweat, though the sweat side is tiny at 5-15 mg per litre.

The intake side matters more. Studies show a substantial share of the population falls short of the daily reference value. In athletes, a subclinical shortfall can drag on sleep quality and muscle recovery. Pumpkin seeds, cocoa powder, pulses and wholegrain foods are good sources.

Potassium leaves at roughly 100-150 mg per litre of sweat. With a reference intake around 4,000 mg a day, and a single banana supplying some 400 mg, an exercise-driven potassium shortfall is unlikely on a normal diet. In very long races (ultramarathons, multi-day events) some deliberate potassium intake through sodium-containing sports drinks can be worthwhile.

Chloride physiologically follows sodium and is covered by ordinary salt intake. An isolated chloride deficiency in everyday sport is essentially never described.

The table below follows European sports-medicine recommendations for sports drinks and current endurance research. Treat it as orientation. Individual variation, training intensity and climate can move the numbers a long way.

An isotonic drink usually carries 400-800 mg of sodium per litre at 4-8% carbohydrate. That mix empties from the stomach quickly and absorbs efficiently. Hypotonic drinks (under 4% carbohydrate) absorb faster but are less practical when you need energy. Hypertonic drinks (over 8% carbohydrate) slow gastric emptying and are a poor choice mid-effort.