Hydration for Ultra-Long Events: Why Replacing Electrolytes Is Your Biggest Challenge

Whether you’re preparing for Qatar’s Samla race that can stretch close to 40 hours, tackling the 90 km East-to-West crossing, or racing a 15-hour T100 triathlon, there’s one universal challenge that decides performance, safety, and survival in long-duration endurance events: maintaining hydration and replacing the electrolytes you lose through sweat.

Hydration in ultra events is not just about drinking more water. In fact, drinking too much plain water can be just as dangerous—sometimes more dangerous—than drinking too little. The real battle is managing fluid + sodium together in the correct proportions over many hours.

This article explains why hydration becomes so difficult beyond 10–15 hours, how sweat losses add up, the danger of both under- and over-hydration, and the practical strategies athletes can use to stay safe and perform at their potential.

Why Ultra-Long Events Are a Hydration Minefield

During normal training sessions of 1–2 hours, you may lose anywhere from 0.5 to 2.5 liters of sweat per hour depending on heat, pace, size, and genetics. But once your event stretches past 10, 15, or 30 hours, these numbers compound to shocking totals.

By the time you reach:

• 10 hours, you may have lost 5–20 liters of sweat

• 15 hours, 7–30 liters or more

• 30–40 hours, as in Samla, 15–50+ liters

Now add the Middle East heat, humidity, sand, sun reflection, and heavy gear—and your sweat rate can spike even higher.

This is why hydration for ultras is not a simple “drink when thirsty” situation. The longer you go and the hotter it gets, the more you risk falling into one of two dangerous traps:

1. Hyponatremia (too much water, too little sodium)

2. Hypernatremia / severe dehydration (too little water, too much sodium)

Both can ruin your race, force medical withdrawal, or in extreme cases be life-threatening.

Sweat: More Than Just Water

Sweat is salty. Sodium is the key electrolyte that your body loses in the highest quantities.

But here’s the twist:Sweat sodium concentration is highly individual.Some athletes lose 400–600 mg/L, while salty sweaters can lose 1500–2000+ mg/L.

Two athletes running side by side in the same race can have completely different sodium needs, even if their sweat rate is similar.

Over a long race, the numbers become huge:

Example:

If you lose:

• 1 liter of sweat per hour

• At 1200 mg/L of sodium

Then over:

• 10 hours → 12,000 mg sodium lost

• 20 hours → 24,000 mg lost

• 35 hours → 42,000 mg lost

This is why beginners often struggle more: not because they’re slower, but because being on the course longer multiplies their total sweat and sodium losses.

The Real Challenges Beyond 10 Hours

1. Your gut slows down

The longer you exercise, the harder it becomes to absorb fluid and electrolytes efficiently.Heat, stress, dehydration, caffeine, and fatigue all reduce stomach emptying.

This means:

• You drink more slowly

• You absorb less

• You fall behind on hydration

2. You become less accurate at sensing thirst

Your brain becomes stressed, your judgement weakens, and you may drink too little or too much.

3. Sodium depletion affects brain and muscle function

Low sodium can lead to:

• Confusion

• Poor decision-making

• Headache

• Nausea

• Muscle cramps

• Loss of power

• Swelling in hands and feet

• Hyponatremia (in severe cases)

4. Water-only drinking dilutes your blood

Many ultra athletes panic when they feel thirsty and keep drinking plain water.But without sodium, water dilutes your blood sodium, leading to dangerous fluid shifts in the brain.

This is one of the biggest risks in races like Samla and East-to-West where aid stations are far apart and temperatures swing drastically.

5. Drinking too much sodium is also a problem

Some athletes overcompensate and drink extremely salty mixes.This can cause:

• Gut distress

• Diarrhea

• Vomiting

• Excess thirst

• Difficulty absorbing fluids

• High blood sodium (hypernatremia)

Both extremes are bad. Balance is everything.

Hydration Strategy for 10+ Hour Events

There’s no single formula for everyone, because sweat rate and sweat sodium vary massively. But these are the universal principles.

1. Know Your Sweat Rate

This is the biggest factor for long events.

If your sweat rate is:

• 0.5 L/h → manageable

• 1.0–1.5 L/h → challenging

• 2.0+ L/h → extremely difficult in long races

Multiply your sweat rate by the expected race duration to estimate your total fluid loss.

2. Know Your Sweat Sodium Concentration

Low-sodium sweaters (~500 mg/L) can get away with low electrolyte intake.

High-sodium sweaters (1400–2000 mg/L) need 2–4× more sodium per hour.

This explains why:

• One athlete can drink mostly water and feel fine

• Another must drink strong mixes (PH1500, 1000mg tabs, etc.)

Without knowing your sweat sodium, you are guessing in the race.

3. Replace enough—not 100%—of what you lose

Trying to replace every drop of sweat is impossible.

Aim instead for:

• 50–70% of your fluid loss

• 40–60% of your sodium loss

This keeps the system stable without overloading the gut.

4. Use the right sodium concentration in your drink

A typical target for ultra events is:

• 800–1500 mg sodium per liter depending on your sweat test

• In hotter conditions, aim higher

• In cooler nights, lower is fine

For salty sweaters, even 1500 mg/L may be necessary.

5. Combine fluids + sodium + food

You don’t need to take all sodium through drink mixes.

You can spread it between:

• Electrolyte drink

• Salt capsules

• Salty snacks

• Energy gels with electrolytes

• Broths at aid stations

This reduces gut fatigue.

6. Night vs. day: adjust as conditions change

Long races like Samla involve huge temperature swings.

Day:

• Higher sweat rate

• Stronger mixes

• More frequent drinking

Night:

• Lower sweat rate

• Easier on the stomach

• Lighter mixes

This is where many beginners fall apart—they keep drinking at daytime rates when the temperature drops.

7. Practice your plan in training

Your gut is trainable.The more you practice drinking 500–800 ml per hour with electrolytes, the better you'll handle it on race day.

8. Look for early warning signs

Low sodium / overhydration:

• Headache

• Nausea

• Foggy thinking

• Sloshing stomach

• Puffy hands

• Weight gain

Dehydration:

• Very dark urine (if you can see it)

• Dry mouth

• Unusual thirst

• Sharp drop in power/pace

• Dizziness

• Heat intolerance

Correct early—don’t wait for collapse.

Conclusion: Replace What You Lose—Safely

Hydration for ultra-endurance events is not guesswork. It is a physiological problem that requires data, planning, testing, and execution.

When your event goes beyond 10, 15, or 30 hours:

• Sweat loss becomes enormous

• Sodium loss becomes a critical risk

• The gut becomes a limiting factor

• Both under- and over-hydration can destroy your race

• Personalized hydration becomes the difference between finishing strong—or not finishing at all

The simplest rule is this:Replace the fluids and sodium you lose, in a balanced and personalized way.