⏵ Nutrition & fueling

Hydration and Hyponatremia for Ultra Runners

Here is the whole thing in one breath: drink to thirst, get your sodium roughly right, and do not over-drink. Most runners are fine on 16 to 32 oz of fluid and 300 to 1000 mg of sodium an hour, adjusted up for heat and a salty sweat. The dangerous mistake in ultras is not dehydration, it is the opposite, drinking so much plain water that you dilute your blood sodium and tip into hyponatremia. And no, salt tabs alone do not save you from that. I will walk you through the real numbers, how to read your own sweat, how to tell hyponatremia from plain dehydration, and exactly what to do if you feel it coming on mid-race.

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The over-drinking trap

For years the advice was drink early, drink often, get ahead of thirst, never let yourself get dehydrated. And a lot of runners took that to heart and drank on a schedule no matter what. The problem is that the most dangerous hydration mistake in a long race runs the other direction. It is drinking too much.

Hyponatremia comes from too much water, not too little salt

When you drink more fluid than you are sweating out, hour after hour, you dilute the sodium in your blood. Drop it below about 135 mmol/L and that is hyponatremia. The big international consensus statements on this are clear that the primary cause is overconsumption of fluid, usually made worse by your body holding onto water when it is stressed during a long effort. So the runner who is religiously sipping every ten minutes whether thirsty or not, on a cool day, going slow, is actually the one walking into trouble.

And here is the part people get backwards: it is often the careful, well-prepared runner who goes down with it, not the careless one. Slower finishers are at higher risk simply because they have more hours on course to over-drink. It feels responsible to keep drinking. It is not. Mild hyponatremia just makes you feel puffy and off, but a severe case swells the brain and becomes a real emergency. This is the one fueling error that lands people in the medical tent or worse, and it is almost entirely preventable by not drinking water you do not need.

Your hydration numbers per hour

Start with these ranges, then make them yours. They are wide on purpose, because sweat rate and how salty your sweat runs vary a huge amount from person to person, and no honest chart can hand you a single number. These are starting points to test in training, not guarantees.

Per hour	Typical range	How to think about it
Fluid	16 to 32 oz/hr (about 0.5 to 1.0 L)	Drink to thirst and to the day. Cool day sits near the low end, hot and sweaty pushes the high end. The fluid ceiling before water retention starts is high (you can clear roughly 1.0 to 1.5 L/hr), but most runners overshoot need long before they hit it.
Sodium	300 to 1000 mg/hr	Roughly 500 to 1000 mg per liter of fluid you take in. Salty sweaters and hot days go higher, cool days and light sweaters lower. Sodium follows your fluid, so the more you drink, the more you salt.
Sweat rate	About 0.5 to 2.0+ L/hr	This is what sets your fluid target, and it swings hard with heat, pace, and body size. The only way to know yours is to weigh in and out of a run. Guessing here is how people get it wrong in both directions.
Sweat sodium	About 20 to 80 mmol/L (roughly 460 to 1840 mg/L)	How salty your sweat runs. Salty sweaters (white shirt crust, stinging eyes, gritty skin) lose far more salt per liter than light sweaters, so two runners side by side can need very different amounts.

Two of these are personal numbers you can actually measure. Our free sweat rate calculator turns a before-and-after weigh-in into your real fluid loss per hour, and the ultra fueling calculator folds fluid and sodium into a full hour-by-hour plan.

Drink to thirst, done right

The single best guardrail against hyponatremia is also the simplest: drink when you are thirsty, and do not force fluid down on a clock just because it has been fifteen minutes. Thirst is a surprisingly good gauge over a long day. Studies comparing runners who drank to thirst against runners following a fixed schedule found the schedule-drinkers ended up with lower blood sodium and just drank more for no benefit.

Thirst is the gauge, not the clock

Your body is honestly good at this. Thirst ramps up before you are in any real trouble, and it backs off when you have had enough. So let it lead. On a hot, hard climb you will want to drink a lot, listen to that. On a cool, easy cruise you might barely want anything for a while, and that is fine too, forcing water in just because the watch buzzed is exactly the schedule-drinking that gets people in trouble.

This does not mean ignore your fluid entirely and white-knuckle it dry. Drinking to thirst still has you taking in real fluid all day, and you should absolutely carry enough and use the aid stations. It just means the trigger is your body, not a stopwatch. The one quick gut-check during a long race: if your stomach is sloshing, your hands are puffy, and you are not thirsty, you have been drinking too much. Back off the fluid and get some salt.

Get your sodium right (but know its job)

Sodium matters. It helps you hold onto and actually use the fluid you take in, it keeps the drive to drink honest, and getting enough of it blunts how far your blood sodium falls over a long day. Aim for roughly 300 to 1000 mg an hour, scaled to how much you are drinking and how salty your sweat is. A useful rule of thumb is about 500 to 1000 mg of sodium per liter of fluid.

Salt is the second lever, not a free pass to over-drink

Be clear-eyed about what sodium can and cannot do. The research consensus is that taking sodium during a race softens the drop in your blood sodium, but it does NOT prevent hyponatremia if you are over-drinking. You cannot salt-tab your way out of pounding too much water. The fluid is the thing that dilutes your blood, so the fluid is the thing you control first. Sodium is the important second lever, not a hall pass to drink as much as you want.

So take your salt: electrolyte drink mix, salt capsules, broth, salty real food like chips and pretzels and potatoes. Lean toward the high end of the range when it is hot, when you sweat salty, and when you are drinking a lot. But build the plan around getting the fluid right, then layer sodium on top of it. Get those in the right order and you stay out of trouble on both sides.

Want the deep dive on the sodium side specifically? Read how much sodium per hour for ultra running.

Read your own sweat

All these ranges only get useful once you know where YOU land in them, and the good news is the two numbers that matter are easy to find. Your sweat rate sets your fluid target. How salty your sweat runs sets your sodium target. Both vary so much between people that general advice can only get you in the ballpark.

Weigh in and out to get your sweat rate

Do a sweat test on a normal training run. Weigh yourself naked right before, run for an hour at a steady effort while tracking exactly how much you drink, then towel off and weigh yourself naked again right after. The math is simple: body weight lost (converted to fluid) plus what you drank equals what you sweat out that hour. A pound lost is roughly 16 oz of fluid. Do it in a few conditions (cool day, hot day, hard effort) because your rate climbs steeply with heat and pace, anywhere from about half a liter an hour to over two liters.

For the salt side, you mostly read the tells: white crust and salt rings on your hat and shirt, sweat that stings your eyes and tastes really salty, gritty skin, cramping in the heat. Sweat sodium runs from about 20 to 80 mmol/L across runners, and the salty sweaters sit at the top, losing more than double the salt per liter of a light sweater. If that sounds like you, push toward the high end of the sodium range. If you want the exact number instead of the eyeball test, a lab sweat test will measure your sweat sodium directly.

Run the weigh-in math automatically with our sweat rate calculator, then build the whole fueling and hydration plan around it.

Hyponatremia vs dehydration: how to tell them apart

This is the one that scares people, because the early symptoms overlap. Both can hand you a headache, nausea, and a flat, sluggish feeling. But the fixes are opposite, so getting it wrong is dangerous. The clearest tell is what your body weight and your fluid behavior are doing. Here is the side by side.

Sign	Hyponatremia (over-drinking)	Dehydration (under-drinking)
Body weight vs start	UP or unchanged (you drank more than you sweat)	Down more than about 2 to 3% (you sweat more than you drank)
Thirst	Often not very thirsty, may feel sloshy or bloated	Thirsty, dry mouth
Urine	Clear and frequent, or you have stopped peeing despite drinking a lot	Dark, scant, infrequent
Hands and rings	Puffy fingers, tight ring, swollen-feeling face	Normal or shrunken
Head	Headache, nausea, confusion, vomiting (can get serious fast)	Headache, lightheaded, sluggish
What it needs	STOP drinking water, eat salt, get help if confused	Drink fluid with some sodium, slow down, cool off

The danger sign that overrides everything is anything neurological: confusion, repeated vomiting, a headache that keeps getting worse, or your coordination going. That points toward brain swelling from hyponatremia and it is a medical emergency. When you genuinely cannot tell which one it is, the safe move is to stop the plain water and take in salt. This guide is educational and is not medical advice.

What to do if you feel it coming on

Say it is hour 8, your hands are puffy, your stomach is sloshing, you are not thirsty, and your head is pounding. Your instinct is going to be to drink water to fix the headache. Do not. That is the exact wrong move and it is how a mild case becomes a serious one. Here is the response, in order.

Stop the water, take salt, wait to pee

Step one is the counterintuitive one: stop drinking plain water immediately, because more fluid dilutes your blood sodium further. Step two, if you are still thinking clearly and can keep food down, take in concentrated sodium and hold off on fluid. Salty snacks, broth or bouillon, chips, salt capsules, even a small amount of a saltier drink. Step three, wait until you actually start peeing again before you drink much of anything, because that is the signal your body is finally shedding the extra water instead of clinging to it. Do not try to flush it out by drinking more. That makes it worse.

Step four is the line you do not cross. If you get confused, keep vomiting, your headache is getting worse instead of better, or your coordination is going, that is a medical emergency and you need help and a real sodium check right now, not a finish line. Tell your crew or the next aid station exactly what is going on so someone is keeping an eye on you. This is one of the very few situations in ultrarunning where stopping or dropping is unquestionably the right and smart call. No buckle is worth it.

Dial it in for heat (without overcorrecting)

Heat is where this gets interesting, because it pushes you toward both failure modes at once. You sweat way more, so your real fluid and sodium needs climb, but a hot, slow day is also exactly when people over-drink and tip into hyponatremia. So you scale up, but you still let thirst lead.

More fluid and more salt, still anchored to thirst

On a hot day your sweat rate can easily double, so your fluid moves toward the top of the 16 to 32 oz range and your sodium climbs right along with it toward 700 to 1000 mg an hour or more, especially if you sweat salty. Take the salt seriously here, because hot races are where most well-trained runners fade, and it is almost always a fluid and sodium problem, not a fitness one. Drink mix, salt caps, broth at the stations, salty food in your pack.

But scaling up is not the same as drinking on a panic schedule. The trap is hearing it is hot and then forcing down far more water than even a high sweat rate justifies, which dilutes you right back into hyponatremia. So go up, yes, but keep thirst as the anchor and keep the salt going with the fluid. And do your hot races a favor by training in the heat first, so race day is not the first time your body has to manage all of it at once.

Heat also slows you down, which means more hours on course and more total fuel. Plan the pace and the calories around that with how to build an ultramarathon fueling plan and how to pace an ultramarathon by effort.

⏵ Rehearse it, do not guess it

A generic fluid range does not know your sweat rate, how salty you run, or the heat and vert of your course. Summit Line builds a hydration and fueling plan dialed to YOUR numbers and your exact race, then tracks how your gut and legs handle it on every long run, so race day is rehearsed instead of guessed. Drink to thirst, get the sodium right, and have a plan you have actually practiced.

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Hydration and hyponatremia FAQ

What is exercise-associated hyponatremia (EAH)?

Hyponatremia means your blood sodium has dropped too low, below about 135 mmol/L, during or in the day after a long effort. In ultras it almost always comes from drinking more fluid than you are losing, which dilutes the sodium in your blood. Your body also tends to hold onto water when you are stressed and racing for hours, which makes it worse. Mild cases just feel off (headache, nausea, puffy hands), but severe cases can swell the brain and turn into a medical emergency with confusion, seizures, and worse. It is the one fueling mistake that can actually kill you, and the wild part is that it comes from doing what feels responsible: drinking a lot of water.

How much water should I drink per hour during an ultra?

For most runners, somewhere between 16 and 32 ounces an hour (about 0.5 to 1.0 liters), drinking to thirst and to the conditions. A cool day sits near the low end, a hot day with heavy sweat pushes the high end. The honest answer is that there is no single number that fits everyone, because sweat rates swing from about half a liter an hour to over two liters depending on heat, pace, and your size. So drink when you are thirsty, do not force fluid on a schedule just because the clock says to, and figure out your actual sweat rate by weighing yourself before and after a long run. That weigh-in turns the guessing into a real number.

Do salt tablets prevent hyponatremia?

Not on their own, and this trips up a lot of people. The consensus from the research is blunt: taking sodium during a race can soften the drop in your blood sodium, but it will NOT prevent hyponatremia if you are over-drinking. The actual cause is too much fluid diluting your blood, so the actual fix is not drowning yourself in the first place. Salt matters, you should absolutely take it, but it is the second lever, not the first. Think of it this way: sodium helps you hold and use the fluid you take in, but no amount of salt tabs makes it safe to pound water you do not need. Get the fluid right first, then dial in the sodium.

How do I know if I am a salty sweater?

Look for the tells after a hard, hot run. White crust or salt rings on your hat, shirt, and skin, sweat that stings your eyes and tastes really salty, gritty skin, and a tendency to cramp or feel wiped out in the heat. Sweat sodium ranges roughly from 20 to 80 mmol/L across runners, and salty sweaters live at the high end, which means they can lose more than twice the salt per liter that a light sweater does. There is no body type or fitness level that predicts it, so two people on the same run can need very different amounts of sodium. If the salt tells sound like you, lean toward the high end of the sodium range, around 700 to 1000 mg per hour or more on hot days. A sweat test from a lab can give you an exact number if you want to nail it down.

What are the warning signs of hyponatremia versus dehydration?

They overlap at the start (both can give you a headache, nausea, and a flat, sluggish feeling), but a few things separate them. The biggest tell is body weight and fluid behavior: hyponatremia usually comes with weight that is UP or unchanged from your start, puffy hands and a tight ring, a sloshy or bloated stomach, and either clear frequent peeing or no peeing at all despite drinking a lot. Dehydration goes the other way: weight down more than a couple percent, real thirst, a dry mouth, and dark scant urine. The danger sign with hyponatremia is anything neurological, confusion, repeated vomiting, a bad headache that will not quit, or stumbling, because that points toward brain swelling. If you see those, treat it as an emergency. When in doubt, the safe move is to stop drinking plain water and take in salt and salty food.

What should I do if I think I have hyponatremia mid-race?

First, stop drinking plain water right away, because more fluid makes it worse, not better. If you are still thinking clearly and able to keep food down, take in concentrated sodium: salty snacks, broth or bouillon, chips, salt capsules, even a few pinches of salt, and wait until you actually start peeing before you drink much of anything again. Do not try to flush it out with water, that is the exact wrong instinct and it is how mild cases become serious ones. If you are confused, vomiting repeatedly, getting a worsening headache, or feel your coordination going, that is a medical emergency and you need help and a sodium check now, not a finish line. Tell a crew member or aid station what is going on so someone is watching you. This is one situation where dropping or pausing is absolutely the right call.

This guide is for training and educational purposes and reflects current sport-science consensus and reputable coaching practice, including the Wilderness Medical Society and International Exercise-Associated Hyponatremia guidelines. It is not medical advice. Hydration is very individual: your fluid and sodium needs depend on your sweat rate, how salty your sweat runs, the heat, and your pace. Hyponatremia can be a life-threatening emergency. If you suspect it, or you have a medical condition that affects fluid or sodium balance, stop and seek qualified medical care.