If nutrition is the “gasoline” for our engine, hydration is the “cooling system.” You can fill up with the finest premium fuel, but if your coolant has leaked out, the engine will overheat and seize within a couple of kilometers. Dehydration is a treacherous and merciless enemy of the ultramarathoner, responsible for an enormous number of DNFs.

8.1. How Much, Exactly? Your Personal Norm

“Drink more!” everyone advises. But how much is “more”?

In everyday life: a sensible guideline is roughly 30–35 ml of fluid per kilogram of body weight per day (for a 70 kg athlete that's 2.1–2.5 liters). And all fluid counts: tea, coffee and soup are included too — the myth that coffee “dehydrates” was debunked long ago; its diuretic effect in habitual consumers is negligible. On top of that, add whatever you lost in training.
How do you know how much you lose? Run a simple test. Weigh yourself without clothes before a one-hour workout in your typical conditions. After the workout, towel off and weigh yourself again. The difference in weight (plus the volume of fluid you drank) is your sweat rate per hour. You'll be surprised how large the number is — often 1–1.5 liters per hour. You can calculate it without doing the arithmetic by hand in the fluid loss calculator.

8.2. By Schedule or by Thirst?

Here we need to be honest. For many years runners were scared with: “If you feel thirsty, you're already dehydrated — drink ahead of time and drink plenty!” Modern sports science sees it differently: the international consensus on hyponatremia (Hew-Butler et al., 2015) explicitly recommends using thirst as the primary signal — it is excessive “preventive” drinking, not moderate thirst, that causes the most dangerous conditions on ultra distances.

The practical compromise for an ultramarathoner looks like this:

Guideline: roughly 400–800 ml of fluid per hour. In the heat and at a fast pace — closer to the upper limit; in the cold — closer to the lower one. Moderate thirst is a normal working signal, not a catastrophe.
Drink little but often: there's no need to gulp down a liter of water at an aid station in one go. Your stomach won't appreciate it. Better to take a few sips every 15–20 minutes. Your hydration pack or soft flasks are your best friends.
The danger of overdrinking: if you drink a great deal of plain water, you can “dilute” the sodium concentration in your blood to critically low levels. This condition is called hyponatremia, and judging by the statistics of fatalities at ultramarathons it is more dangerous than dehydration — up to and including brain edema and death. That's why you should drink not just water, but...

8.3. Salt, Salt and More Salt!

We already know that sodium (salt) is needed for carbohydrate absorption. But its main role is to maintain blood plasma volume. When you sweat, you lose not only water but also salt. If you drink only water, your blood becomes more and more “diluted,” and the body, to save itself from hyponatremia, paradoxically... increases urination, worsening the dehydration even further!

The golden rule: for every liter of fluid you drink, you should be getting on the order of 1 gram of sodium. 1 gram of sodium is roughly 2.5 grams of ordinary table salt (NaCl).
Where do you get sodium?
- Sports drinks: read the label carefully! Many commercial sports drinks contain criminally little sodium (200–300 mg per liter). You need a “proper” drink with a sodium content close to 1000 mg/L.
- Salt tablets: the simplest and most reliable way to stay in control. One tablet usually contains 250–500 mg of sodium. Just wash them down with water on schedule.
- Food: salty crackers and salted potatoes at aid stations are excellent sources.

8.4. What Happens When the “Coolant Runs Out”

Losing just 2% of body mass through sweat (that's 1.4 kg for a 70 kg athlete) already causes a noticeable drop in performance. And it only gets worse from there:

The blood thickens, and the heart has to work overtime to push it through the vessels. Your heart rate shoots sky-high.
Thermoregulation deteriorates. You feel hot, and chills may set in.
Muscle spasms and cramps begin.
Cognitive function declines: you start thinking poorly and can get lost on a marked course.

A loss of 3–4% is already a zone of serious health risk. Don't let it get that far.

8.5. And What About Potassium and Magnesium?

Supplement sellers love to scare us with potassium and magnesium deficiencies, promising deliverance from cramps. Yes, these minerals matter for muscle contractions. But we lose far less of them in sweat than sodium — roughly 4–6 times less potassium, and even less magnesium.

On distances up to 10–12 hours you can stop worrying about them altogether — what you get from regular food and gels is quite enough. On longer races (a full day and beyond) it makes sense to add them to your intake, but without fanaticism. The reigning king of electrolytes at an ultramarathon is sodium. Give him full honors, and he will carry you to the finish.

In the next chapter we'll put everything together and talk about tactics, gear selection and how to recover properly after the race.