Hydration and Sodium for Trail Runners

Hydration advice in endurance sport has swung from one extreme to the other. Twenty years of "drink as much as you can, ahead of thirst" produced a wave of hospitalised runners with dangerously low blood sodium. The correction produced a generation who under-drink on principle.

Both errors cost you a race. Only one of them can kill you.

The two failure modes

Dehydration

Losing fluid faster than you replace it. Consequences scale with severity:

• 1-2% of body mass: essentially no meaningful performance effect. Normal for any long effort.
• 2-4%: measurable performance decline, higher perceived effort, elevated heart rate, impaired thermoregulation.
• 4%+: substantial performance loss, meaningful heat illness risk, cognitive impairment.

Notably, some fluid deficit during a long race is normal and largely unavoidable. Finishing a 100 km race 2% down on body mass is not a failure; it is what happens.

Exercise-associated hyponatremia

Blood sodium concentration falling below normal, almost always because of drinking more fluid than you are losing - diluting the sodium you have rather than losing an unusual amount of it.

Early symptoms - nausea, bloating, headache, confusion, puffy fingers - overlap almost exactly with dehydration, which is what makes it dangerous. A runner feeling unwell drinks more, gets worse, drinks more. In severe cases it progresses to seizures, cerebral oedema, and death.

The tell: if you have been drinking steadily and are gaining weight or your rings and watch strap feel tight, and you feel worse rather than better after drinking, stop drinking. Take sodium and salty food, and get medical attention if symptoms progress.

Risk factors: long duration, slower finishing times, excessive plain-water intake, low body mass, and consuming most fluid as plain water. Research has found female runners take a substantially higher proportion of their fluid as plain water than male runners - around 83% versus 52% in one study - which is part of why the incidence is higher in women.

How much to drink

Guided by thirst, sanity-checked with numbers.

Thirst is a reasonably good regulator for most runners in most conditions, and drinking to thirst is the position taken by most current consensus statements. Its weakness is that it under-responds in extreme heat and in very long events, and it is blunted at high intensity.

So use these as a frame, not a schedule:

If you are drinking noticeably more than the top of your band and still thirsty, you are probably behind on sodium rather than fluid.

Measure your own sweat rate

Individual sweat rates range from about 0.4 L/h to over 2 L/h. Any general recommendation is therefore wrong for most people. Fortunately, measuring your own takes one session.

The protocol:

1. Empty your bladder, then weigh yourself naked. Record it.
2. Run for exactly one hour at a representative effort, in representative conditions.
3. Record exactly how much you drink, in millilitres.
4. Towel off, weigh yourself naked again.

Sweat rate = (weight lost in kg x 1000) + fluid drunk in ml

Example: 0.8 kg lost, 500 ml drunk → 800 + 500 = 1.3 L/h.

Repeat in different conditions - a cool day and a hot day at minimum - because your rate in heat can be double your cool-weather rate. Two or three data points give you a usable personal model.

Sodium concentration varies too, from roughly 200 to over 2,000 mg per litre. Rough field indicators of a salty sweater: visible white residue on your face and clothing after a hot run, a persistently salty taste, and a history of cramping. Lab sweat testing exists and is reasonably accurate if you want the real number.

Sodium: how much and why

Sodium does two things that matter here: it helps retain the fluid you drink rather than passing it straight through, and it maintains the blood sodium concentration that keeps hyponatremia away.

Practical targets:

• 500-800 mg of sodium per litre of fluid in moderate conditions.
• 800-1,200 mg/L in heat, for long durations, or if you are a salty sweater.
• Over very long races, roughly 300-600 mg of sodium per hour total from all sources is a reasonable frame.

Count what you are already getting. This is the part people miss. A typical sports drink serving has 200-500 mg of sodium. Many gels have 50-200 mg. Aid station broth, crisps, and pretzels contribute substantially. Runners who add salt tablets on top of a full sports-nutrition plan sometimes end up well over target, which mostly just makes them thirsty.

Add salt capsules when you are drinking mostly plain water, when it is hot, or when your own testing shows you lose a lot. Do not add them reflexively.

The cramping question

The evidence for cramping being caused by electrolyte loss is weaker than the marketing suggests. Current thinking leans toward neuromuscular fatigue - altered reflex control in an overworked muscle - as the primary driver, with dehydration and sodium loss as possible contributors in some athletes.

Which means the most reliable anti-cramp interventions are:

1. Pacing. Cramps overwhelmingly appear when you exceed what you trained for.
2. Training specificity. Muscles that have done the work do not cramp as readily.
3. Adequate carbohydrate. Underfuelled muscles fatigue faster.
4. Sodium, which does help some athletes, particularly heavy salty sweaters.

If you cramp at the same point in every race, the answer is more likely in your training block than in your bottle.

Heat

Heat is the multiplier on everything above. Sweat rate can double, gut absorption drops, perceived effort rises at any given pace, and your sodium losses increase.

Heat acclimation works and is worth doing. Ten to fourteen consecutive days of 60-90 minutes of exercise in heat produces plasma volume expansion, earlier sweating onset, and reduced sodium concentration in sweat. Most of the adaptation appears within a week and it decays over two to three weeks without exposure. Sauna use after training is a reasonable partial substitute if you cannot train in heat.

On a hot race day: start better hydrated, drink at the top of your band, raise sodium concentration, use external cooling aggressively (ice in a bandana, cold water over your head and forearms at every stream), and accept a slower pace. Fighting heat with effort is a losing trade.

A practical setup

For a warm 8-hour mountain race, a runner with a measured sweat rate of 1.0 L/h:

• Two 500 ml soft flasks. One with drink mix, one with plain water. This decouples fluid intake from carbohydrate intake, which matters when you need more of one than the other.
• Target 600-700 ml/h, drinking to thirst, checking every 20 minutes that you have actually been drinking.
• Sodium roughly 500 mg/h from mix, gels, and aid station food, with two salt capsules in a pocket for the hottest section.
• Weigh in before the start if possible. Even an approximate before-and-after tells you a lot for next time.
• Cool at every water source. Head, neck, forearms.

The mistakes

1. Drinking on a rigid schedule regardless of thirst. The behaviour that caused the hyponatremia problem in the first place.
2. Drinking plain water exclusively over many hours. Fluid without sodium is the classic risk profile.
3. Adding salt tablets without counting what is already in your drink and gels.
4. Using a generic sweat rate. Yours may be half or double the average. Measure it.
5. Treating hyponatremia symptoms with more water. The single most dangerous error in the sport, and an understandable one, since the symptoms mimic dehydration.
6. Ignoring heat acclimation and then being surprised by a hot race.

Hydration is one of the few things in ultrarunning where a single measurement session replaces a great deal of guessing. Do the sweat rate test once this month, and stop reading generic advice - including this article's tables.