Sodium is the most important electrolyte for endurance performance and also the most misunderstood. Athletes either ignore it entirely — relying on whatever is in the gels — or chase sodium obsessively, popping salt tablets every 20 minutes based on a formula they found on a forum. Neither approach is right, and the consequences of getting it wrong range from cramping to something far more dangerous: hyponatremia.
What sodium actually does
Sodium is the primary electrolyte in extracellular fluid. It regulates blood volume, drives fluid retention in the tissues, assists glucose transport across the intestinal wall, and triggers thirst. During exercise, you lose sodium through sweat — and unlike carbohydrates, you cannot store more of it in advance.
The amount you lose varies enormously between individuals. Sweat sodium concentration ranges from 200 to 2,000 mg per litre across the population. A salty sweater — identifiable by white residue on skin and kit after hard sessions — loses four to five times as much sodium as a low-salt sweater at the same exercise intensity and air temperature. This is why blanket prescriptions fail.
How much sodium do you need per hour?
A general starting point for most athletes is 500–1,000 mg of sodium per hour during endurance events longer than 90 minutes. In hot conditions (above 25°C), salty sweaters, or during events lasting more than 8 hours, requirements can push to 1,000–1,500 mg per hour.
These numbers come from sweat rate and sweat concentration data, not from what feels like it should work. If you have had cramping, swelling in the hands or feet during long events, or bloating from drinking large volumes of plain water, your sodium intake likely needs adjustment.
Hyponatremia: the dangerous failure mode
Hyponatremia — low blood sodium — is the opposite of dehydration, and it is more dangerous. It occurs when athletes drink large volumes of plain water without replacing the sodium they are losing in sweat, diluting blood sodium below safe levels.
Symptoms overlap with dehydration: nausea, headache, confusion, swelling. Athletes and race medics sometimes respond by giving more fluids, which worsens the condition. In severe cases, hyponatremia causes seizures and death. It is more common in slower athletes (who spend more time on course and have more time to over-drink) and in women, due to lower body mass and hormonal factors affecting fluid regulation.
The prevention is simple: do not drink to a schedule that ignores sodium. Drink to thirst and replace sodium proportionally to sweat loss.
Cramping: is it sodium or not?
The relationship between sodium, hydration, and cramping is more complex than the salt tablet manufacturers want you to believe. Research shows cramping has multiple causes — neuromuscular fatigue, pacing too hard too early, and electrolyte imbalance. Sodium deficiency alone does not cause cramping in most athletes.
That said, in salty sweaters who have not replaced sodium across a long event, there is a real correlation. If you cramp repeatedly in races but rarely in training, and if your training sessions are shorter than your race events, sodium depletion is a plausible contributor worth addressing.
Sodium sources and timing
Sports drinks are the most practical sodium delivery mechanism because they combine fluid and sodium in a measured dose. Most isotonic sports drinks provide 400–600 mg of sodium per 500 ml serving.
Gels vary considerably — some have as little as 40 mg per serving, others provide 200 mg. Salt tablets (SaltStick, Precision Hydration, etc.) allow you to add sodium independently of fluid intake, which is useful when you are drinking to thirst rather than a schedule.
For events over 4 hours, sodium-rich food at aid stations — broth, pretzels, boiled potatoes with salt — provides a meaningful contribution. Many ultra runners deliberately seek out savoury aid station options in the second half of the race when sweet palatability fades.
How to personalise your sodium intake
The best proxy for your sodium needs is your sweat rate and sweat saltiness. You can estimate sweat rate with a simple towel-dry weigh-in protocol: weigh before and after a training session of known duration, accounting for fluids consumed. Each kilogram of body mass lost approximates one litre of sweat.
Sweat saltiness is harder to measure without lab testing, but salty sweat residue on skin, white marks on kit, and a history of cramping or hyponatremia are useful indicators. If any of these apply, add 200–400 mg per hour above the baseline recommendation.
A personalized fueling plan that factors in your measured or estimated sweat rate, your race distance, and your temperature conditions gives you a specific sodium target — not a range of 500–1,500 mg that still leaves you guessing. The athletes who never cramp or bloat in races have this dialled. They are not lucky; they are prepared.