Electrolytes vs. Water : Why is water not enough on long runs?

It’s a classic scenario that many cyclists, trail runners, or long-distance runners have surely already experienced or observed. You head out for a long session, the sun is beating down. Being conscientious, you regularly drink from your bottle of plain water. Yet, after three hours, headaches set in, your legs become heavy as lead, and cramps start to tickle your calves. Even worse, you feel like the water is "sloshing" in your stomach, creating gastric discomfort (the famous "glug-glug") without ever truly quenching your thirst.

The culprit? It isn’t a lack of water, but paradoxically the quality of what you are drinking.

In the world of endurance, pure water has a sacred status, but as soon as the effort exceeds 2 hours, it can become insufficient, or even counterproductive. To understand why your cycling or running hydration must evolve with the duration of the effort, you have to dive into the heart of human physiology: osmosis, sodium, and the little-known risk of hyponatremia.

Decoding a fragile balance and a practical guide to mastering your electrolyte drinks.

Sweat: it’s not just water

To understand why pure water isn't enough, you first have to analyze what you are losing. When you are pedaling up an Alpine pass or running on a technical trail, your body temperature rises. To cool itself down, it activates its natural air conditioning system: perspiration.

But sweat isn't simply water that evaporates. It is a complex chemical cocktail. By sweating, you discharge electrolytes—those minerals carrying an electrical charge that are essential to your body's functioning.

The main electrolytes lost are:

• Sodium ($Na^+$): The king of electrolytes. It is crucial for retaining water in the body and for the transmission of nerve impulses.
• Potassium ($K^+$): It works in tandem with sodium for muscle contraction.
• Magnesium and Calcium: They play a key role in muscle relaxation and bone strength.

Imagine your body like a racing car. If you are losing oil and fuel, but you only put back fuel (water), the engine (your muscles and your brain) will eventually break down. Replacing water without replacing mineral salts creates an immediate imbalance.

The Mechanism of Osmosis: Why Water Needs a "Taxi"

This is where science becomes fascinating. Simply drinking water isn’t enough—it must cross the intestinal barrier to reach your blood plasma and hydrate your cells. This process is governed by a physical phenomenon: osmosis.

Osmosis is the movement of water through a membrane (your intestine) from a less concentrated environment to a more concentrated one, in order to balance both sides.

The Problem with Pure Water (Hypotonic)

Pure water contains almost no solutes, meaning no sugars or minerals. When you drink large amounts during exercise, you create an immediate imbalance in your body. Your blood’s sodium concentration is physiologically stable, naturally around 140 mmol/L.

When pure water reaches the intestine, it cannot instantly cross the intestinal barrier. For active absorption, water absolutely needs sodium. If your drink lacks sodium, your body is forced to draw from its own reserves or wait for the slow passive osmotic process—far too slow to meet the demands of intense effort.

This is why pure water tends to "sit in the stomach," causing the well-known gastric discomfort among runners. Without sodium acting as a "taxi" to transport water molecules through the intestinal wall, hydration is inefficient. Worse, the sudden liquid volume stimulates urine production to eliminate the excess: you drink, you urinate, but your cells remain paradoxically dehydrated.

The Isotonic Solution

This is where isotonic electrolyte drinks come into play. They have the same particle concentration (water + sodium + carbohydrates) as blood. The result? Instant exchange. Water quickly enters the bloodstream, hydration balance is maintained, and digestion is eased.

In short, isotonic drinks ensure that water doesn’t just pass through you—it actually hydrates you efficiently, keeping your performance at its peak.

The Invisible Danger: Exercise-Associated Hyponatremia

While ineffective hydration can harm performance, there’s a far more serious risk during ultra-endurance events (Ironman, Ultra-Trail, Cyclosportive): dilutional hyponatremia.

This is a race doctor’s worst nightmare. Often, amateur athletes, fearing dehydration, drink excessive amounts of plain water at aid stations.

What Happens in the Body?

By drinking water without salt and sweating (losing salt in the process), the sodium concentration in the blood drops dramatically. The blood becomes too diluted. To restore osmotic balance, water leaves the bloodstream and enters the cells, causing them to swell like sponges.

• If this happens in the muscles, it leads to cramps.
• If it happens in the brain (cerebral edema), the consequences can be life-threatening.

Symptoms of Exercise-Associated Hyponatremia

• Nausea and vomiting (often mistaken for indigestion).
• Severe headaches.
• Confusion and disorientation.
• Swelling of the hands and feet (if your ring feels tight or your GPS watch leaves a deep mark, be cautious!).

Paradoxically, these symptoms mimic dehydration. The fatal mistake is drinking even more water at this point.

The Solution?

• Stop drinking plain water immediately.
• Consume sodium right away (electrolyte drinks, salty snacks, or salt tablets).

Hyponatremia is a silent but serious threat. Listen to your body, monitor your hydration strategy, and always include electrolytes during long efforts. Your performance—and your health—depend on it.

Tablets or Powder: How to Choose Your Electrolytes?

Now that we’ve established that sodium intake is non-negotiable for efforts lasting over 2 hours or in high heat, which solution should you choose?

Electrolyte Tablets (Tabs): Pure Hydration

Effervescent tablets have become a favorite among cyclists and trail runners for their "all-terrain" convenience.

Composition:

• Ultra-concentrated in minerals (sodium, magnesium, potassium, calcium).
• Almost zero calories—these are "zero-calorie" drinks.

For Whom?

• Ideal for those who want to separate hydration from nutrition.
• If you prefer getting your energy from solid foods (energy bars, fruit pastes, gels), tablets provide the necessary salt without adding sugar to your bottle.
• Perfect for low-intensity rides or "lipolysis" sessions, where the goal is to train your body to burn fat rather than sugar.

Practical Advantage:

• The tube format is unbeatable. If you need to refill your bottles at a fountain or aid station, just drop a tablet into plain water, and your drink is ready in 30 seconds.
• They often come in light, fruity flavors that prevent sugar fatigue over long durations.

Isotonic Energy Powders: The Complete Fuel

Here, we’re not just talking about hydration—we’re talking about performance support. This is the "all-in-one" solution.

Composition:

• Combines electrolytes with a high carbohydrate load (often a mix of maltodextrin, glucose, and fructose).

For Whom?

• For competitions, high-intensity sessions, or ultra-endurance events where chewing solid food becomes difficult.
• This solution kills two birds with one stone: it maintains hydration balance and continuously fuels your muscles with glycogen to avoid hitting the wall.

Importance of Dosage:

• Unlike tablets, which are flexible to use, powders require precision.
• To remain "isotonic" (and thus perfectly absorbed by the intestines), you must strictly follow the water volume indicated on the packaging.
  • A too diluted drink (hypotonic) will be less effective.
  • A too concentrated drink (hypertonic) can cause stomach cramps, as it will "pull" water from your body into your digestive tract to dilute itself.

Which One Should You Choose?

• For hydration without extra calories: Go for tablets.
• For combined hydration and energy: Opt for isotonic powders.

Test both options during training to see what works best for your body and your race strategy!

Hydration strategy: the practical guide

The "Salty Sweater" test Check your gear after a long ride. If you see white marks (salt deposits) on your jersey or shorts, you're a "heavy salt sweater". In this case, don't hesitate to slightly overdose on your tablets or use Salt Caps (salt capsules) in addition to your usual drink.

Water is the basis of life, but in the context of endurance sports performance, it is chemically incomplete. Understanding that hydration is not just a question of fluid volume, but of water and mineral balance, is often the trigger that enables athletes to reach new heights.

No more post-ride headaches and unexplained cramps. For your next adventure lasting more than two hours, remember: your water bottle needs salt just as much as your legs need training.