Dehydration vs Hyponatremia in Ultras: How to Tell Them Apart
In an ultra, dehydration vs hyponatremia is hard to call by how you feel, because both bring nausea, headache, fatigue and wobbly legs. The clues are what you’ve been drinking, whether you’ve gained or lost weight, and how your hands and urine look, but only a blood sodium test confirms it, which is why a runner who is confused or vomiting should go to the medical team, not be handed another cup of water.
The second thing most runners don’t know: the two aren’t opposites. You can be dehydrated and hyponatraemic at the same time, and in ultras it happens often.
What’s the actual difference?
Dehydration is a water deficit. You’ve sweated out more fluid than you’ve taken in, and your body weight drops. The American College of Sports Medicine’s position stand uses a loss of more than 2% of body weight as the line where performance can start to suffer (Sawka et al., 2007).
Exercise-associated hyponatremia (hyponatraemia in British spelling) is a low concentration of sodium in the blood, below 135 mmol per litre, during or up to 24 hours after prolonged exercise. The international consensus statement names drinking more than you lose as the main cause, made worse when the hormone that tells your kidneys to hold on to water stays switched on during exercise (Hew-Butler et al., 2015). Mild cases may cause no symptoms. Severe ones cause brain swelling, and that can kill.
One is about how much water you have. The other is about how diluted your sodium is. That’s why they can overlap.
Can you be dehydrated and hyponatraemic at once?
Yes, and in long ultras it’s common. Martin Hoffman’s group pooled five years of data from 161 km races in northern California, with post-race blood sodium for 669 finishers. Hyponatraemia hit 15.1% of finishers overall and was more common in hotter years. Of the runners with it, only 23.8% had gained weight. Another 40.6% had lost up to 3% and 35.6% had lost more than 3%, so a third of the hyponatraemic runners were also clearly “dehydrated” by the scale (Hoffman et al., 2013).
The same paper found faster runners tended to lose more weight, and top finishers ranged from about 1% heavier to about 6% lighter. Weight loss above 3% didn’t seem to hurt performance. The authors’ conclusion was blunt: excessive sodium supplementation and drinking beyond thirst are both ill advised.
So “I’ve lost weight, therefore I can’t be low on sodium” is wrong. Weight loss makes overdrinking less likely; it doesn’t rule out a sodium problem.
Does this happen in tropical races?
It does, and the closest published data comes from next door. Singapore-based researchers tracked a 42 km marathon and an 84 km ultra run at an average 29°C and 89% humidity, which is a normal Malaysian morning. Three of the eight runners who came into the medical tent with symptoms had hyponatraemia, including two from the 84 km with blood sodium of 131 and 117 mmol per litre. The 117 is a dangerous level. On the scales, 8% of the 84 km runners and 7% of the marathoners finished heavier than they started (Lee et al., 2011). The authors called these the first reported cases of exercise-associated hyponatraemia in tropical Asia.
That last number matters more than the dramatic one. Roughly one runner in thirteen drank more than they sweated in tropical heat, where most of us assume dehydration is the only risk. The same paper recorded weight losses as large as 8% in the ultra, so both problems were on the same course on the same day.
How do you tell them apart on the trail?
Use the clues together, and treat the table below as a way of deciding what to do next, not as a diagnosis.
| Clue | Leans towards dehydration | Leans towards hyponatremia |
|---|---|---|
| What you’ve been drinking | Less than usual, long gaps, skipped aid stations | A cup at every station “to stay ahead”, whether thirsty or not |
| Thirst | Strong and persistent | Often absent, sometimes the drink feels unwelcome |
| Weight (if your race weighs you) | Down 3% or more | Same or up; but remember a third of cases in the Hoffman data were down more than 3% |
| Hands and feet | Normal | Rings, watch strap or shoes feel tight, puffy fingers |
| Urine | Dark and infrequent | Frequent and clear despite feeling unwell |
| Head | Dizzy on standing, lightheaded | Headache, confusion, unusual behaviour, getting worse after drinking |
Nausea, vomiting and fatigue show up in both, and in heat exhaustion too. The Wilderness Medical Society’s guidelines for managing exercise-associated hyponatraemia stress exactly this overlap, and they’re written for settings like remote trail checkpoints where nobody can test blood sodium on the spot (Bennett et al., 2020).
What should you do if you’re not sure?
If you’ve been drinking a lot, feel worse, and especially if your hands are swelling, stop forcing fluids and get yourself to the medical team at the next checkpoint. Don’t keep drinking plain water to “fix” a headache. If a running mate is confused, vomiting repeatedly, or not making sense, that’s a medical emergency; get help and tell the medics what and how much they’ve been drinking.
The reason for caution runs one way. Giving more fluid to a dehydrated runner helps them. Giving more fluid to a hyponatraemic one can make them dangerously worse. When you can’t tell, the safer field choice is to hold off on large amounts of plain water and let someone with a sodium test make the call.
Why doesn’t extra salt protect you?
Because the problem in most cases is too much water, not too little salt. At the Western States 100, 93.9% of runners used sodium supplements, yet hyponatraemic finishers had taken sodium at the same rate as everyone else. What marked them out was overhydration (Hoffman and Stuempfle, 2015). In a smaller study at a 161 km race that reached 39°C, total sodium intake across the day ranged from about 2,500 mg to over 38,000 mg and still showed no link with hyponatraemia, dehydration or nausea (Hoffman et al., 2015).
The biggest dataset on this, 2,135 weighed athletes across endurance events, found that blood sodium fell in a straight line as body weight rose. Weight gain from drinking too much was the principal cause of low sodium after exercise (Noakes et al., 2005). The Boston Marathon study pointed the same way: 13% of finishers tested had hyponatraemia, and weight gain and finishing slower than four hours were the strongest risk factors (Almond et al., 2005).
Sodium still has a place in a long, hot race for some runners, and we explain how to size it in How Much Sodium Do Runners Need Per Hour? It just isn’t a licence to drink without limit.
Who is most at risk in a Malaysian ultra?
The runner most at risk of hyponatraemia is usually not the one at the front. It’s the steady back-of-pack runner on a 15 to 20 hour day who stops at every checkpoint, accepts every cup a kind volunteer holds out, and walks long climbs where sweat rate drops but drinking habits don’t. Night sections on Fraser’s Hill or in the Crocker Range can be noticeably cooler than a lowland start, and a drinking rate set for the midday lowland sections becomes too much.
The runner most at risk of serious dehydration is the opposite: a long exposed section, a missed water refill, an aid station that ran low, or a stomach that refuses everything. On trail ultras where checkpoints can be two or three hours apart, carrying capacity matters as much as intention.
How do you plan for both?
Know your numbers before race day, and let thirst do the fine-tuning on the day. The 2015 consensus recommends drinking to thirst as the main guide in endurance events, which also fits Hoffman’s finding that modest weight loss didn’t hurt performance.
- Measure your sweat rate on a long run in similar heat with our sweat rate calculator. Plan to drink less than that rate, not more.
- Weigh yourself before and after long training runs. If you regularly finish heavier, you are overdrinking, and that’s the habit to fix first.
- Write down what you drink at each checkpoint in your next long training race. Most people are surprised.
- Take off your ring before the start. A tightening watch strap is a free warning sign; a stuck ring is a problem.
- Tell your crew or running partner the warning signs in the table, so someone who isn’t foggy-headed can make the call.
For the fuel side of a long day, our ultra marathon nutrition guide covers carbohydrate and gut training, and the running hydration and electrolytes guide brings all our fluid articles together. If the heat itself is the worry, start with what changes after two hours in Malaysia’s heat.
References
- Sawka MN, Burke LM, Eichner ER, et al. (2007). American College of Sports Medicine position stand: exercise and fluid replacement. Medicine & Science in Sports & Exercise, 39(2), 377-390. doi:10.1249/mss.0b013e31802ca597
- Hew-Butler T, Rosner MH, Fowkes-Godek S, et al. (2015). Statement of the Third International Exercise-Associated Hyponatremia Consensus Development Conference, Carlsbad, California, 2015. Clinical Journal of Sport Medicine, 25(4), 303-320. doi:10.1097/JSM.0000000000000221
- Hoffman MD, Hew-Butler T, Stuempfle KJ (2013). Exercise-associated hyponatremia and hydration status in 161-km ultramarathoners. Medicine & Science in Sports & Exercise, 45(4), 784-791. doi:10.1249/MSS.0b013e31827985a8
- Lee JKW, Nio AQX, Ang WH, et al. (2011). First reported cases of exercise-associated hyponatremia in Asia. International Journal of Sports Medicine, 32(4), 297-302. doi:10.1055/s-0030-1269929
- Bennett BL, Hew-Butler T, Rosner MH, Myers T, Lipman GS (2020). Wilderness Medical Society clinical practice guidelines for the management of exercise-associated hyponatremia: 2019 update. Wilderness & Environmental Medicine, 31(1), 50-62. doi:10.1016/j.wem.2019.11.003
- Hoffman MD, Stuempfle KJ (2015). Sodium supplementation and exercise-associated hyponatremia during prolonged exercise. Medicine & Science in Sports & Exercise, 47(9), 1781-1787. doi:10.1249/MSS.0000000000000599
- Hoffman MD, Stuempfle KJ, Valentino T (2015). Sodium intake during an ultramarathon does not prevent muscle cramping, dehydration, hyponatremia, or nausea. Sports Medicine – Open, 1(1), 39. doi:10.1186/s40798-015-0040-x
- Noakes TD, Sharwood K, Speedy D, et al. (2005). Three independent biological mechanisms cause exercise-associated hyponatremia: evidence from 2,135 weighed competitive athletic performances. Proceedings of the National Academy of Sciences, 102(51), 18550-18555. doi:10.1073/pnas.0509096102
- Almond CSD, Shin AY, Fortescue EB, et al. (2005). Hyponatremia among runners in the Boston Marathon. New England Journal of Medicine, 352(15), 1550-1556. doi:10.1056/NEJMoa043901
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