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How much sodium per hour do you actually need?

How much sodium per hour? It depends on two numbers that are personal to you: how much you sweat and how salty that sweat is. Put those together and one athlete can lose a few hundred milligrams an hour while another loses well over 1,500mg doing the same session, which is why any single "500mg an hour" rule is a guess dressed up as advice.

Here is my honest take. Sodium is the most overhyped and the most under-thought part of endurance fuelling at the same time. Plenty of people swallow salt tabs on a 90-minute run because a forum told them to, and plenty of others go into a hot Ironman with plain water and no plan at all. Neither is a strategy. Your number is.

Where the sodium goes

You lose sodium through sweat. The amount you lose in a session is simply:

sodium lost = sweat rate (litres per hour) x sweat sodium concentration (mg per litre) x hours

Both of those inputs vary a lot between people.

  • Sweat rate. In a large dataset of 506 athletes across team and endurance sports, whole-body sweat rate averaged 1.21 litres an hour, with a range from 0.26 to 5.73 litres an hour. A later update of the same database with 1,303 athletes put endurance athletes at an average of 1.28 litres an hour.

  • Sweat sodium. The same 2016 study estimated whole-body sweat sodium at an average of 35.9 mmol per litre, ranging from 18.2 to 70.8. Sodium weighs 23mg per mmol, so that is roughly 830mg per litre on average, from about 420mg up to about 1,630mg per litre.

That spread is the whole story. The saltiest sweaters lose nearly four times as much sodium per litre as the least salty, before you even account for how much they sweat.

What that looks like per hour

These are worked examples from the ranges above, not targets. They show how wide the gap between two athletes can be.

AthleteSweat rateSweat sodiumSodium lost per hourLight sweater, not salty0.5 L/h420 mg/Labout 210mgAverage endurance athlete1.28 L/h830 mg/Labout 1,060mgHeavy sweater, average saltiness2.0 L/h830 mg/Labout 1,660mgHeavy, salty sweater2.0 L/h1,630 mg/Labout 3,260mg

A swimmer doing an hour of steady laps in a cool pool, a runner at a summer half marathon and a triathlete on the bike in the heat of a long-course race are not playing the same game. Sport, temperature, intensity and how heat-adapted you are all move your sweat rate around, so even your own number shifts from session to session.

You do not have to replace all of it

This is where most advice goes wrong. Losing sodium is normal. Trying to put back every milligram in real time is not necessary, and chasing it with too much fluid is how people get into real trouble.

The best-known example is the 2002 Boston Marathon. Researchers took blood from 488 finishers and found 13 percent had hyponatremia, meaning blood sodium too low. The strongest predictors were gaining weight during the race, which means drinking more than you sweat, and finishing slower than four hours. Whether runners drank water or an electrolyte drink made no difference. It was the amount of fluid that mattered.

Ultra running tells a similar story. In a 161km race where the air reached 39°C, runners' total sodium intake ranged from about 2,500mg to over 38,000mg. Sodium intake was no different between runners who cramped and those who did not, or between those who developed hyponatremia and those who did not. It is a small study, 20 runners with full food records, but it is a useful reality check on the idea that salt tabs prevent cramps.

So the order of priority is clear. Do not overdrink. Then think about sodium.

When sodium does seem to help

None of this means sodium is useless. In a randomised trial in a half-ironman, 26 experienced triathletes took either salt capsules or a placebo across the race, three doses at the two transitions and mid-bike, adding up to 2,580mg of sodium for the salt group. The salt group finished faster overall and had higher blood sodium at the finish. Sweat rate and sweat saltiness were the same in both groups. The authors put the benefit down mostly to the salt group drinking more of their own accord and holding their electrolytes better.

That fits what I see. Sodium earns its place in long sessions, in the heat, and for heavy or salty sweaters. On a short, cool session it barely matters. I don't take anything extra on a one-hour run, and I wouldn't tell you to either.

How to work out your own number

You can get a decent estimate for free.

  1. Measure your sweat rate. Weigh yourself with no clothes on before a session, train for an hour at a steady effort, towel off and weigh again. Add any fluid you drank. Each kilogram lost is roughly a litre of sweat. Our sweat rate guide walks through it step by step.

  2. Judge how salty you are. A lab or patch test gives you a real concentration. Without one, the signs of a salty sweater are white, gritty marks on your kit and hat, and sweat that stings your eyes. If that is you, assume you sit towards the top of the range.

  3. Do the maths. Sweat rate times your estimated concentration gives you a rough loss per hour.

  4. Test it in training. Try your plan on long sessions that look like your race, in similar heat. Running, cycling and the run off the bike in a triathlon all feel different, so test in the sport you will race in.

  5. Repeat in different conditions. Your sweat rate in January is not your sweat rate in July. Test again when the weather changes.

How to take it on

Sodium comes in three common forms, and most people end up mixing them.

SourceGood forWatch out forElectrolyte drink mixGetting fluid and sodium in togetherSodium per bottle varies a lot between brands, so read the labelSalt capsulesTopping up in long or hot events without drinking moreEasy to take too many if you are also drinking a salty mixSalty foodLong rides and ultras where you are eating anywayHard to dose precisely

On the bike it is easy to carry bottles and sip often. Running makes big volumes harder, so capsules or a stronger mix in smaller sips can work better. Swimmers mostly deal with it before and after, or at feed stations in long open-water events. Triathletes usually do most of the work on the bike, where the stomach copes best.

FAQ

How much sodium per hour for a marathon? Use the same maths. A runner sweating 1.2 litres an hour with average sweat sodium loses roughly 1,000mg an hour. Most of the risk in a marathon comes from drinking too much, not from too little sodium, so start with a sensible fluid plan.

Do I need sodium for a one-hour session? For most people in normal conditions, no. Eat and drink normally afterwards and you will replace it.

Do salt tabs stop cramps? The evidence says probably not on their own. In the ultramarathon study above, sodium intake was the same in runners who cramped and those who did not.

Can I take too much sodium? Yes. Stacking a salty drink, salt capsules and salty food without counting can leave you bloated and thirsty. Know what is in each thing you take.

Is sodium per hour the same in cycling and running? The maths is the same, but your sweat rate can differ between sports and conditions. Measure in the sport you are fuelling for.

MOVE has a built-in sweat test that estimates your own sweat rate and sodium loss per hour, so your hydration numbers come from your sweat, not a population average.

Sources

Put this into practice

MOVE applies these principles automatically, personalised to your sport, weight and training load.

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