Last updated: 21 September 2026
Drink to thirst on short runs and check your own numbers on long ones. Athletes sweat anywhere from 0.5 to 2 litres an hour, so no single target fits everyone. In heat, sodium matters once a session passes an hour, and overdrinking plain water carries its own risk.
How much do you actually sweat when you run in Malaysia?
More than most people guess, and the gap between runners is huge. Athletes in general sweat between about 0.5 and 2 litres an hour, and a few pass 3 litres (Baker, 2017). The KL Marathon’s own weather page puts Kuala Lumpur at 29 to 35°C in the day with humidity around 80%. In air that wet, sweat evaporates badly, so a lot of it drips off without cooling you. You lose the water and get less of the benefit.
The arithmetic gets uncomfortable fast. A 70 kg runner sweating 1.5 litres an hour loses 3 litres in two hours. That is 4.3% of body mass if none of it is replaced. The ACSM’s 2007 position stand sets the line at about 2%, which is 1.4 kg for the same runner.
Should you drink to thirst or follow a plan?
For most recreational runners, drink to thirst first, then check yourself with a weigh-in. Expert guidance splits on this. The ACSM’s 2007 position stand says the aim is to keep weight loss under 2% and to tailor the plan to the person. The 2015 international consensus on exercise-associated hyponatraemia goes further and says to use thirst as your guide, because drinking beyond thirst has been linked to deaths in athletes, marathon runners among them.
Our view: on easy and moderate runs, thirst is good enough. A plan starts to earn its place on long, hot, hard efforts, and only after you’ve measured your own sweat.
How do you measure your own sweat rate?
Weigh yourself before and after a one-hour run, and count everything you drink. The ACSM lists this as the way to estimate individual sweat rates.
- Weigh yourself in minimal dry clothing before the run, after using the toilet.
- Run 60 minutes at the effort you’d race at. Note how much you drink.
- Towel off and weigh again in the same clothing.
- Sweat loss in litres = kilograms lost + litres drunk (1 kg is about 1 litre).
Example, for illustration only: 70.0 kg before, 68.9 kg after, 0.5 litres drunk. That is 1.1 + 0.5 = 1.6 litres an hour.
If you lose more than about 2% of your starting weight, you drank too little for that run. If your weight goes up, you drank more than you sweated, and that is the direction to worry about.
One test is one snapshot. Sweat rate moves with pace, humidity and heat acclimatisation (Baker, 2017), so a test in an air-conditioned gym tells you little about race morning. Repeat it in the conditions you’ll race in.
Is plain water enough in the heat?
For runs under an hour, yes. Past an hour you are also losing sodium, and how much varies enormously. Baker’s review puts sweat sodium at roughly 10 to 90 mmol per litre. One mmol of sodium is 23 mg, so that works out to about 230 to 2,070 mg in every litre of sweat. A runner losing 1 litre an hour at a middling 40 mmol/L gives up about 920 mg an hour.
The ACSM’s 1996 position stand suggests 0.5 to 0.7 g of sodium per litre of fluid for exercise longer than an hour. Drink 500 ml an hour at that strength and you take in 250 to 350 mg. Against a 920 mg loss, that leaves a wide gap on a long, hot run.
This is where a sodium tablet earns a place. Elec Stamina is a swallow tablet made in China. Each tablet has 220 mg of sodium, and the label suggests 1 to 2 tablets every 10 km, up to 10 a day. At a 6 min/km pace that is 220 to 440 mg an hour, so it covers part of the 920 mg example above, not all of it. It has no immediate effect, so treat it as a schedule item: take it with water at set points in a long session, not as a rescue when you feel bad. It doesn’t replace drinking, and it isn’t a licence to drink more.
For a 45-minute easy run you don’t need it. Water and a normal meal are fine.
What should you drink before a hot run?
About 500 ml roughly two hours before, then let thirst take over. That figure is from the ACSM’s 1996 stand, which also says cool fluid (15 to 22°C) gets drunk more readily. In Malaysia a bottle in your bag won’t stay cool for long. Freeze half of it overnight and top up with water in the morning.
How do you know you’re getting it wrong?
Too little shows up as a weigh-in loss of more than 2%. Too much shows up as weight gain during a long run, puffy fingers, nausea and light-headedness, which are the mild symptoms listed in the 2015 hyponatraemia consensus. These overlap with heat illness. If someone is confused, vomiting or has a bad headache, stop and get medical help.
FAQ
How much water should I drink when running in hot weather?
Drink to thirst on short runs. On long ones, measure your sweat rate and aim to finish within about 2% of your starting weight.
Do I need electrolytes for a 10K?
Usually not. Most recreational 10Ks last under an hour, and the guidance on sodium in fluids starts at exercise longer than an hour.
Can I just drink plain water on a long run?
You can, but it leaves the sodium gap open. Drinking more than thirst asks for is the bigger risk.
Will drinking more stop cramps?
Probably not. The best evidence links cramps to fatigue and pace. See Why Runners Cramp in Malaysian Heat.
Related: Why Runners Cramp in Malaysian Heat · Elec Stamina
Written by Stamina Sports Malaysia, distributor of Katsuo Stamina in Malaysia since 2015.
Sources
- Baker LB. Sweat Testing Methodology in the Field: Challenges and Best Practices. Sports Science Exchange 161, 2017. Link
- ACSM Position Stand, Exercise and Fluid Replacement. Med Sci Sports Exerc 2007;39(2):377-90. Link
- ACSM Position Stand, Exercise and Fluid Replacement. Med Sci Sports Exerc 1996;28(1):i-vii. Link
- Hew-Butler T et al. Statement of the Third International Exercise-Associated Hyponatremia Consensus Development Conference. Clin J Sport Med 2015;25(4):303-20. Link
- KL Marathon, Weather. Link