Marathon Hydration Guide: Timing, Amounts, and Drink Types

Dehydration is one of the most common — and most preventable — performance killers in marathon racing. Yet many runners still aren’t sure how much to drink, or which fluids actually work best. From an exercise physiology perspective, this article lays out the evidence-based answers.

I’ve experienced this firsthand. In a half marathon, I developed near-dehydration symptoms in the latter half of the race and couldn’t deliver the performance I was capable of. That experience pushed me to dig into the science.

Here, I’ll walk you through proper hydration before, during, and after exercise — and explain the consequences of getting it wrong.

Hydration Timing and Amounts for Marathon Racing

Here are evidence-based recommendations for hydration timing and fluid amounts during a full marathon.

Before the Race

The goal of pre-race hydration is to normalize your body’s fluid status before you start. Here’s how to do it:

Your urine color is one of the most practical hydration indicators. Aim for pale yellow — darker means you need more fluid, while clear may indicate over-hydration.

To maximize fluid retention, pair your drinks with slightly salty foods or beverages. Research also shows that flavored drinks — such as tea — are retained better in the body than plain water.

During the Race

The goal of mid-race hydration is to prevent body weight loss of more than 2% from fluid loss. Here are the recommended guidelines:

That said, avoid over-drinking. Taking in too much fluid dilutes sodium in the bloodstream and can lead to exercise-associated hyponatremia (EAH) — a potentially dangerous condition that causes nausea, confusion, and in severe cases, seizures.

Drinking to thirst is also a valid, evidence-backed approach※1. If you’re unsure how much to drink, listening to your body’s thirst signals is a safe default.

Fluids with low carbohydrate and sodium content are commonly called hypotonic drinks. These are optimized for rapid fluid absorption. However, during a race you typically need both hydration and carbohydrate fueling, so you don’t have to rely exclusively on hypotonic drinks.

For example, energy gels have a very high carbohydrate density. Consuming them without water can cause them to sit in the stomach. Pairing a gel with water helps dilute the carbohydrate content and speeds up absorption of both fluid and fuel.

After the Race

Post-race hydration focuses on restoring electrolytes and muscle glycogen. The priority isn’t just drinking as fast as possible — it’s making sure you get adequate sodium and carbohydrates alongside the fluid.

Post-exercise, you’ll tend to drink a large amount quickly, and much of it will pass through as urine. This is why the target is 1.5× your body weight loss — drinking an equal amount won’t fully restore fluid balance.

This was demonstrated by Shirreffs et al. (1996)※5, who showed that replacing only a 1:1 ratio of lost fluid was insufficient to fully restore body fluids. The same study confirmed that sodium in the drink significantly improved fluid retention.

Drinks recommended for post-race recovery are commonly called isotonic sports drinks — beverages with carbohydrate and electrolyte levels roughly matching the body’s own fluids.

The sections below provide a deeper look at how hydration works and what affects absorption speed.

Why Hydration Matters: Three Key Functions

When you lose more than 2% of your body weight through fluid loss, you’re dehydrated — and endurance performance can start to decline. The main cause of fluid loss during exercise is sweating.

For a 60 kg runner, 2% body weight equals 1.2 kg. It’s not unusual to lose that much or more on a regular training run — so during a marathon, the stakes are even higher.

In endurance events like the marathon, the hydration goal is clear: keep fluid loss below 2% of body weight. Along with water, sweat also contains minerals such as sodium, potassium, and magnesium — collectively called electrolytes — which need to be replaced too.

Why does it matter so much? Maintaining fluid and electrolyte balance during exercise serves three critical functions:

Thermoregulation

As you run, your body temperature rises because working muscles generate heat as a byproduct of energy production. To keep going, your muscles need to keep contracting — and that means heat keeps building up.

Your blood carries this heat back to the heart and distributes it throughout the body, releasing it at the skin surface. Sweating is the primary mechanism that cools the skin: as sweat evaporates, it draws heat away from the skin, which in turn cools the blood flowing near the surface.

When you’re dehydrated, sweating decreases — and so does your ability to regulate body temperature. On top of that, reduced blood volume means less blood available to transport heat away from working muscles, compounding the problem.

Muscle Function

Muscle contraction depends not just on carbohydrates and fats for fuel, but also on electrolytes — particularly sodium and magnesium. When electrolyte levels drop, muscles can’t contract as smoothly, and performance suffers.

Circulatory Function

The circulatory system delivers oxygen and nutrients to working muscles via the bloodstream. When fluid levels drop and blood volume decreases, the heart has less blood to pump — which reduces its capacity to transport oxygen and energy substrates.

A drop in circulatory function directly limits the muscles’ supply of oxygen and fuel, which is why dehydration leads to a measurable decline in endurance performance.

How Much Fluid Do You Lose During a Marathon?

The main driver of fluid loss during running is sweating. Sweat rate varies enormously depending on external conditions like temperature and humidity — and equally, on individual physiology.

Table 1 summarizes sweat rates across various sports.

Sport	Median [L / hour]	Range [L / hour]
Water polo (match)	0.79	0.69–0.88
Swimming	0.37
Basketball (match)	1.6	1.23–1.97
Soccer (summer training)	1.46	0.99–1.93
American football (summer training)	2.14	1.1–3.18
Tennis (men, summer match)	1.6	0.62–2.58
Half marathon (winter race)	1.49	0.75–2.23
Cross country (summer training)	1.77	0.99–2.55

The half marathon row tells a clear story: runners lose a median of 1.49 L of sweat per hour. The wide range also confirms just how much individual variation exists — but even at the lower end, the fluid losses are substantial.

At that sweat rate, going just one hour without fluid replacement would push most runners past the 2% body weight threshold. In a full marathon, regular, consistent hydration isn’t optional — it’s essential.

Carbohydrate Concentration and Fluid Absorption Rate

During a marathon, you’re drinking on the move — and you need to replace both fluid and carbohydrates. Elite runners use custom special drinks; recreational runners typically pair gels with water at aid stations. But here’s what many runners don’t realize: the carbohydrate concentration of your drink directly affects how quickly your body can absorb it.

Here’s what the research tells us about the optimal fluid intake volume and carbohydrate concentration for rapid absorption.

The following experiments investigated this:

Reference studies: Costill DL & Saltin B (1974)※2, and Neufer PD et al. (1986)※3

The key findings:

Of these four variables, the only one you can control as a runner is the carbohydrate concentration of your drink. The threshold of 139 mmol/L is equivalent to 25 g/L of carbohydrate.

To put that in concrete terms: Pocari Sweat contains 62 g/L of carbohydrate and Aquarius contains 47 g/L. Both exceed the 25 g/L threshold, meaning drinking them straight during exercise may slow gastric emptying.

The graph below shows how much fluid remained in the stomach 15 minutes after consuming 400 ml of a salted drink, as a function of carbohydrate concentration. It illustrates how sharply gastric emptying slows once carbohydrate concentration climbs above the threshold.

Here you can clearly see that above roughly 100–200 mmol/L, the volume of fluid remaining in the stomach increases sharply.

In practice, however, few recreational marathon runners actively limit their carbohydrate concentration to below 25 g/L during a race. Elite runners, for example, use Maurten drink mix, which contains 160 g/L of carbohydrate — far above that threshold.

Maurten works for elite athletes partly because of its dual-transporter carbohydrate design: it combines glucose and fructose. Glucose is absorbed via SGLT1 and fructose via GLUT5 — two separate intestinal transport systems. Using both simultaneously allows for high carbohydrate intake without the bottleneck that slows absorption with glucose alone.

Currell & Jeukendrup (2008)※4 reported that a glucose + fructose combination improved endurance performance compared to glucose alone.

The practical takeaway: if rapid fluid absorption is your priority, drink water separately rather than pairing it with a running gel. But if you’re using a drink for carbohydrate fueling, diluting a gel with water rather than consuming it neat will speed up absorption of both.

Isotonic vs. Hypotonic Drinks: Which Should You Choose?

Sports drinks broadly fall into two categories: isotonic sports drinks and hypotonic drinks.

Isotonic sports drinks — such as Aquarius or Pocari Sweat — contain higher levels of carbohydrates and electrolytes, formulated to roughly match the osmolality of your body’s own fluids. Because of their higher carbohydrate content, they’re absorbed more slowly than hypotonic drinks.

Hypotonic drinks have lower carbohydrate and electrolyte concentrations, with an osmolality set below that of your body fluids. Their lower concentration means faster absorption, making them well-suited to mid-race hydration.

That said, as noted above, you’ll usually need to fuel with carbohydrates during a race as well. Taking into account the carbohydrate density of any gels or other foods you’re consuming alongside your drink will help you choose the right fluid for each stage of the race.

Individual Variation in Fluid and Carbohydrate Absorption

Fluid and carbohydrate absorption rates vary significantly between individuals. The same hydration and fueling strategy that works perfectly for one runner can cause GI distress in another.

My recommendation: practice your race-day hydration and fueling strategy in training. If your stomach handles it well during a workout, you can be reasonably confident it won’t cause problems on race day.

That said, racing is different. When you’re running at your absolute limit, GI function can decrease — meaning fluids and carbs that you tolerated in training may cause stomach trouble during the race itself. Practicing at race-level effort, not just easy running, will give you a more accurate picture of what your body can handle.

Summary

References

※1 Noakes TD (2005) “Hydration in the marathon: using thirst to gauge safe fluid replacement” Sports Medicine

※2 Costill DL, Saltin B (1974) “Factors limiting gastric emptying during rest and exercise” Journal of Applied Physiology

※3 Neufer PD, Costill DL et al. (1986) “Effects of exercise and carbohydrate composition on gastric emptying” Medicine & Science in Sports & Exercise

※4 Currell K, Jeukendrup AE (2008) “Superior endurance performance with ingestion of multiple transportable carbohydrates” Medicine & Science in Sports & Exercise

※5 Shirreffs SM et al. (1996) “Post-exercise rehydration in man: effects of volume consumed and drink sodium content” Medicine & Science in Sports & Exercise