Research · 4 min read · 2026-10-12
How much should you drink when you run?
13 percent of Boston Marathon runners had too little sodium in the blood, mostly from drinking too much. Why thirst is a good guide.
"Drink before you get thirsty" has been common advice for runners. Research shows that drinking too much in a marathon can be more dangerous than drinking too little.
What the researchers did
At the 2002 Boston Marathon, researchers weighed runners before and after the race, took a blood sample at the finish and asked how much they had drunk. 488 runners provided a usable blood sample.
In a smaller study from 2019, nine trained endurance athletes did three one-hour cycling time trials in 30 °C: without drinking, drinking when thirsty, and drinking enough to replace almost all the sweat they lost.
What they found
In Boston, 13 percent of the runners had too little sodium in the blood, a condition called hyponatremia, and 0.6 percent had a critically low level. It was linked to gaining weight during the race, which happens when you drink more than you lose. Runners who took more than four hours were at much higher risk than those under 3:30. Drinking more than three litres and drinking at every mile were also linked to it. Sports drink instead of water did not protect against it.
In the cycling study, the three ways of drinking gave the same performance over an hour.
What it means for you
Drink when you are thirsty. You don't need to drink at every station, and slower runners who are out for a long time should be especially careful not to drink too much.
A simple check is to weigh yourself before and after a long run. If you are heavier afterwards, you drank more than you needed. Headache, nausea, swelling or confusion during or after a long race can be signs of hyponatremia. Get medical help, and don't keep drinking.
What the studies can't tell you
The marathon study shows a link, not that each litre caused the problem. The cycling study was small and lasted an hour, which is much shorter than a marathon. In hot conditions and long races, you may need to drink more than in cool weather.
How we use it
Your long runs in PX7 Training are a good time to find out how much you actually drink and need, so you have a plan for race day.
Sources
- Almond et al. (2005), Hyponatremia among runners in the Boston Marathon. New England Journal of Medicine.
- Perreault-Briere et al. (2019), Effect of thirst-driven fluid intake on 1 h cycling time-trial performance in trained endurance athletes. Sports.
Written by Jørgen Larsen for PX7 Training.