In one large sweat-testing dataset, endurance athletes lost an average of 51.7 mmol of sodium per hour. That works out to roughly 1.2 g, close to half a teaspoon of salt, soaked into your singlet before most of Kuala Lumpur has had breakfast. Stretch that over a three-hour Sunday long run and you are near 9 g of salt.
Most runners hear that and reach for salt tablets. For many of them, that is the wrong reaction. The loss is real, but whether you need to replace it during the run comes down to two things you can actually measure.
Where the 1.2 g comes from
The number comes from the Gatorade Sports Science Institute, which pooled data from 1,303 athletes tested between 2000 and 2017. Endurance athletes in that group sweated about 1.28 litres an hour on average. One mmol of sodium is 23 mg, so their average hourly loss comes to about 1,190 mg.
The average hides a big spread. The standard deviation on that sodium figure was 27.8 mmol an hour, so one standard deviation around the mean spans roughly 550 mg to 1,830 mg an hour. That is more than a threefold difference between runners who would both count as normal.
Endurance athletes were among the biggest losers in the dataset. Only American football players lost more sodium per hour, at 55.9 mmol. Footballers, basketball and baseball players all lost less, mainly because they sweat less per hour and stop sooner.
What Malaysian humidity changes
Heat makes you sweat. Humidity makes that sweat less useful. Sweat cools you by evaporating, and thick, wet air slows evaporation down. Researchers at Universiti Sains Malaysia in Penang described it in a 2019 study: in humid air, sweat drips off the skin instead of evaporating, so much more of the fluid you lose does nothing to cool you.
The same study tested nine well-trained runners who were not heat-adapted, in warm-humid and warm-dry conditions. After an hour of running, they lasted 9.1 minutes in the final all-out test in humid air, against 12.7 minutes in dry air. Drinking water during the run did not reduce their thermal strain in either condition.
Those runners were not adapted to heat, and that matters, because adaptation works in your favour. In one lab study of eight men exercising in the heat for ten days, sweat sodium concentration fell from about 54 to 36 mmol per litre, a drop of roughly a third. The catch is that heat-adapted bodies tend to sweat more, so the total sodium lost can still be large. Training in the Klang Valley does not make you a low-sodium runner. It makes your sweat less salty, but you can still lose a lot overall.
Losing sodium is not the same as needing to replace it
This is where most sodium advice goes wrong. Your body holds a buffer, and blood sodium only drops in particular situations.
Alan McCubbin, an accredited sports dietitian, modelled this directly. He tested sweat sodium from 20 to 80 mmol per litre, sweat rates from 0.5 to 2.5 litres an hour, and fluid replacement from 10% to 90% of losses. The result was blunt. For a football match and an elite marathon, replacing sodium was unnecessary in every realistic scenario. The 160 km ultramarathon was different: runners whose sweat held 40 mmol per litre or more, and who drank back over 80% of their losses, needed to replace at least 47% of their sodium.
The model found that two things drove sodium needs: the share of your sweat loss you drink back, and how salty your sweat is. Duration multiplies both. McCubbin has also written that there is little if any evidence that sodium during exercise improves performance, at least in efforts under four hours.
One honest gap: an elite marathon takes about two hours, while a recreational marathon in Malaysian heat can take five. The race scenarios in the model did not cover that middle ground. The same two drivers still apply, so the longer you are out and the more you drink, the more your sodium plan matters.
The water bottle is the bigger risk
The dangerous form of low blood sodium, exercise-associated hyponatraemia, usually is not caused by sweating out too much salt. An international consensus panel concluded that the main cause is drinking too much dilute fluid, likely combined with hormone signals that make the body hold on to water during exercise. More salt is not the fix. Less fluid is.
The panel’s advice is to drink when thirsty. Early signs include lightheadedness, nausea and gaining weight during an event. If you finish a long run heavier than you started, you drank too much, whatever was in the bottle.
Find your own number
An average is only a starting point. You can measure your own sweat rate with a bathroom scale, which is essentially how the big dataset did it: change in body mass during exercise, corrected for what athletes drank and passed.
- Use the toilet, then weigh yourself in minimal clothing before a 60-minute run.
- Run at a normal training effort and note how much you drink.
- Towel off and weigh again in the same clothing.
- Add the weight you lost to the fluid you drank. That is your hourly sweat loss, with 1 kg being roughly 1 litre.
Do it once on a hot, still morning and once after overnight rain. The gap between the two tells you how much your race-day plan should shift with the weather.
Saltiness is harder to measure at home. White tide marks on your cap or singlet hint that your sweat is on the salty side, but they are not a measurement. If you race ultras and see those marks often, a proper sweat sodium test is worth paying for.
When sodium belongs in your plan
Up to about two hours, drinking to thirst: your normal meals cover your sodium. Salty food or drink is a season-to-taste choice rather than a requirement.
From two hours to around five, such as a long run or a recreational-pace marathon at KLSCM, it depends on your numbers. If you drink back most of what you sweat and your sweat is salty, planned sodium starts to make sense. If you drink to thirst, you likely have more room than most advice suggests.
Ultras are where the modelling says sodium genuinely matters, when long exercise is combined with drinking back more than 80% of losses and sweat sodium of at least 40 mmol per litre. Plan your intake per hour, not per aid station.
What this looks like at KLSCM
The KLSCM full marathon cut-off has run to 7 hours 15 minutes, so a large part of the field is out there for five hours or more. That runner is in a completely different position from the two-hour elite the model said needs nothing.
Say you measured 1.2 litres an hour at race pace during training. Over five hours that is about 6 litres of sweat. At 920 mg of sodium per litre, the illustration figure below, the theoretical loss is around 1,100 mg an hour.
That number is not a target to match. Your drinking rate, your food, your own sweat concentration and the length of your race all change what you actually need. What it does tell you is whether you are the kind of runner the research is talking about: long time on feet, heavy sweating, and drinking back a lot of it.
Turning your number into a plan
Each Elec Stamina caplet contains 220 mg of sodium. On its own that figure means nothing. Next to your own numbers it does the work for you.
The arithmetic runs like this. Take your hourly sweat loss in litres from the scale test, multiply it by your sweat sodium in mg per litre, and you have your hourly loss. Without a sweat test you cannot know your own concentration, so use 40 mmol per litre, about 920 mg per litre, only as an illustration. That figure sits above the average, so it overstates the loss for a lot of runners. It is the level at which McCubbin’s ultramarathon scenario made targeted replacement necessary, not a diagnosis of salty sweat.
Then decide what share you intend to replace. The ultra scenario in the model landed near half for salty sweaters drinking back most of their losses. Divide that figure by 220 and you know how many caplets an hour that is for you, and how many to carry for a race that takes eight hours rather than four.
Whatever the maths says, stay within the directions on the pack, and rehearse the plan on long runs at race effort, in the heat you will race in.
Running KLSCM, an ultra, or another long race in the heat?
Measure your sweat rate on a long training run first. Once you know roughly how much fluid you lose an hour, you can build a sodium schedule around your race duration instead of guessing at an aid station.
Elec Stamina provides 220 mg of sodium per caplet, which makes it simple to carry and to count.
References
- Barnes KA, Anderson ML, Stofan JR, Dalrymple KJ, Reimel AJ, Roberts TJ, Randell RK, Ungaro CT, Baker LB (2019). Normative data for sweating rate, sweat sodium concentration, and sweat sodium loss in athletes: an update and analysis by sport. Journal of Sports Sciences, 37(20), 2356-2366.
- Che Muhamed AM, Yusof HA, Stannard SR, Mündel T, Thompson MW (2019). The efficacy of ingesting water on thermoregulatory responses and running performance in a warm-humid condition. Frontiers in Physiology, 10, 507.
- Chinevere TD, Kenefick RW, Cheuvront SN, Lukaski HC, Sawka MN (2008). Effect of heat acclimation on sweat minerals. Medicine & Science in Sports & Exercise, 40(5), 886-891.
- McCubbin AJ (2023). Modelling sodium requirements of athletes across a variety of exercise scenarios: identifying when to test and target, or season to taste. European Journal of Sport Science, 23(6), 992-1000.
- McCubbin AJ (2022). How much sodium do I need? mysportscience.com
- Hew-Butler T, Rosner MH, Fowkes-Godek S, et al. (2015). Statement of the 3rd International Exercise-Associated Hyponatremia Consensus Development Conference, Carlsbad, California, 2015. British Journal of Sports Medicine, 49(22), 1432-1446.
- Hew-Butler T, Loi V, Pani A, Rosner MH (2017). Exercise-associated hyponatremia: 2017 update. Frontiers in Medicine, 4, 21.