How much should endurance athletes drink, and what should it actually be?

Why does hydration matter so much for endurance performance?

Your body is roughly 60% water, and it uses that water for multiple things that all matter during exercise: blood volume (which determines how much oxygen your heart can deliver per beat), sweat production (which is how you shed heat), metabolic function, and maintenance of electrolyte balance. When fluid loss outpaces intake — which happens constantly during exercise — each of those jobs gets harder.

The clearest performance signal is that dehydration of roughly 2% of body weight or more reliably degrades endurance performance in most studies. For a 70 kg athlete, that's about 1.4 kg of fluid loss, which in moderate conditions takes around 90 minutes to 2 hours of steady exercise to produce without drinking. Below 2%, performance effects are subtler and more contested. Above 3–4%, performance drops sharply and thermal regulation starts to break down. Above 6–8%, you're entering dangerous territory.

The other side of the coin is drinking too much, which matters for long events but is never discussed as much as dehydration. Over-drinking plain water dilutes blood sodium, and when blood sodium drops far enough, the resulting state — exercise-associated hyponatremia — is both performance-destroying and potentially fatal. Severe hyponatremia has killed endurance athletes in marathon, Ironman, and military contexts. It is almost always caused by athletes drinking more plain water than their body can handle while sweating heavily.

How much should you actually drink during exercise?

A broad, practical range for most athletes in most conditions is 400 to 800 millilitres of fluid per hour of exercise. Below 400 ml per hour you're likely to end up meaningfully dehydrated on long efforts. Above 800 ml per hour in moderate conditions you're approaching the range where hyponatremia risk starts to matter.

That range shifts up in hot conditions (heavy sweaters in heat can easily need 1000–1500 ml per hour), down in cool conditions (400 ml per hour is often plenty on a cool day), and varies enormously by body size, sex, sweat rate, and acclimation status. A fit 85 kg male in 30°C humidity needs substantially more fluid than a 55 kg female in 18°C cool conditions. The 'rule' is a starting point, not a universal prescription.

The most useful thing you can do is measure your own sweat rate once, so you know where your personal number sits instead of guessing from a range. The test takes 20 minutes.

How do you measure your sweat rate?

The sweat-rate test is simple, practical, and the result is the single most useful piece of personalized hydration data you can have. You only need a scale.

• Weigh yourself naked, before exercise.
• Exercise for 60 minutes at a realistic training or race intensity. Drink a known amount of fluid during the session (e.g. 500 ml), and record exactly how much.
• Empty your bladder if needed, towel off, and weigh yourself naked again.
• Your sweat rate per hour (in litres) = (starting weight in kg) − (ending weight in kg) + (fluid drunk in litres). If you urinated during the session, add that volume back too.
• Repeat the test under different conditions (cool day, hot day, hard effort, easy effort) to build a small personal table of what your sweat rate actually is in each scenario.

The first time most athletes do this test, they discover their sweat rate is meaningfully higher than what they've been drinking during long sessions. That alone explains a lot of end-of-race collapses.

Why does sodium matter so much more than other electrolytes?

Sweat is not just water. It contains dissolved electrolytes — primarily sodium, with smaller amounts of chloride, potassium, magnesium, and calcium. Of these, sodium is the one that matters most for endurance performance, by a large margin, for two reasons.

The first reason is volume. Sweat sodium concentration varies from athlete to athlete but is typically 400 to 1500 milligrams per litre. A heavy sweater in a hot race can lose 8 grams or more of sodium across a long event — a genuinely large amount that cannot be replaced by a normal diet alone. The other electrolytes are lost in much smaller quantities and are quickly replaced with normal food intake.

The second reason is the hyponatremia risk. Blood sodium has to stay within a narrow range for the nervous system to function normally. When an athlete drinks large amounts of water without sodium during a long effort, blood sodium drops, neural function starts to impair, and in severe cases the consequences are coma, seizures, or death. Potassium, magnesium, and calcium imbalances from exercise are rarely dangerous; sodium imbalance is the one that kills.

This is why the 'electrolyte blend' marketing on many sports drinks is misleading. A drink with 50 mg of sodium per serving is not actually an electrolyte drink in any meaningful sense — the magnesium and potassium content in it is basically decorative. The sodium number is the one to read on the label.

How much sodium should you take per hour?

Practical sodium targets scale with your sweat rate, the salt content of your sweat, the temperature, and the duration of the session.

• Moderate conditions, normal sweater — 300 to 500 mg of sodium per hour is typically adequate for efforts over an hour.
• Hot conditions, heavy sweater — 700 to 1000 mg per hour is a realistic range. Some athletes genuinely need 1500 mg or more, and they'll usually know because they have visible salt stains on their kit after hot sessions.
• Long-duration events (marathon, Ironman, 100-mile ultras) — err toward the higher end of the range, and include sodium consistently across the whole event rather than front-loading it.
• Short events (under 60 minutes) — sodium is not really necessary during the event itself. A normal meal beforehand provides what you need.

The easiest way to hit these targets is with a sports drink or electrolyte mix that reports its sodium per serving on the label. Plain electrolyte tabs in water bottles (Nuun, LMNT, Saltstick, Precision Hydration) work too. Real food can contribute sodium as well — a small bag of pretzels or salted boiled potatoes is a reasonable source during long rides or ultras.

What should you actually drink: water, sports drink, or pure electrolyte mix?

The choice depends on the session, and there's no one right answer. Each option is suited to different situations.

• Plain water — fine for short sessions (under 60 minutes) in moderate conditions. Not sufficient alone for long or hot sessions because it provides no sodium and no fuel.
• Traditional sports drinks (Gatorade, PowerAde, regular Maurten) — provide carbohydrate for fuel, sodium for electrolyte balance, and fluid all in one package. These are the default for most training sessions and races of 90+ minutes at moderate intensity. A typical sports drink delivers roughly 30–60 grams of carbs and 300–500 mg of sodium per 500 ml.
• High-carb drink mixes (Maurten 320, Precision Fuel 30/60/90, SIS Beta Fuel) — concentrated carbs (60–90+ grams per serving) with sodium. The best choice for long races where you're trying to hit the high end of the carb intake range (90 g/h or more) and don't want to manage the separate logistics of gels plus water.
• Electrolyte-only tabs / mixes (LMNT, Nuun, Saltstick) — water with sodium and minimal or no carbs. Useful alongside solid food or gels, or in Zone 2 sessions where you want hydration and electrolytes without the carbs.
• Hot-weather purpose-built mixes (Precision Hydration PH 1500, SIS Go Hydro) — very high sodium content (1500+ mg per serving) for heavy sweaters in hot conditions.

How should you hydrate before and after exercise?

Pre-session hydration is mostly about not starting dehydrated. Aim to finish a normal glass of water with your pre-session meal, and another 250–500 ml in the 30 to 60 minutes before starting. If your urine is pale straw before you head out, you're well hydrated. If it's dark yellow, you're starting in the hole, and drinking 500 ml will help.

Aggressive pre-hydration ('tanking up' in the hour before) is generally counterproductive — your kidneys can only process so much water so fast, and the excess either comes out as urine in the first hour of exercise or dilutes your blood sodium for no benefit.

Post-session hydration is about replacing both fluid and sodium. A useful rule is to drink 1.5 times the volume you lost during the session (if you know your sweat rate and session length, you can calculate this), and include sodium with the fluid. Plain water alone after a long, sweaty session leaves you behind on sodium and can mask the replacement you actually need. The simplest version: after any hard or long session, drink a glass of sports drink or electrolyte mix along with your recovery meal, not just plain water.

What is exercise-associated hyponatremia and how do you avoid it?

Exercise-associated hyponatremia (EAH) is a medical condition in which blood sodium concentration drops below 135 mmol/L during or immediately after prolonged exercise. It is caused by drinking too much fluid relative to sodium losses over a long period — classically, a slower marathon runner who drinks at every aid station for 4+ hours without consuming sodium alongside.

Early symptoms are easy to miss: bloating, puffy fingers, mild nausea, headache, confusion. Severe EAH causes vomiting, disorientation, seizures, coma, and death, and has killed athletes in marathons, Ironmans, and military training. It is not common but it is not rare — studies of marathon finishers have found 10 to 20% of participants in some races had measurable hyponatremia, most asymptomatic.

Avoiding EAH is simple in principle. First: don't over-drink. Drink to thirst or to a pre-planned rate tied to your sweat rate, not to a 'stay ahead of thirst' instruction. Second: include sodium in what you drink during any long or hot event. Plain water alone in large volumes over long durations is the mechanism of failure. Third: if you gain weight during an endurance race (weighing heavier at the finish than at the start), you almost certainly over-drank; that is a clear warning sign.

What are the most common hydration mistakes?

Five errors show up repeatedly in amateur endurance athletes.

• Starting races dehydrated. Often because of nerves, early mornings, and skipping the pre-race fluid routine. Starting with a 1% deficit means 2% by kilometre 20. Drink the morning of the race.
• Drinking plain water for hours on long hot efforts. The classic hyponatremia setup. Always include sodium on anything over 90 minutes in the heat.
• Under-drinking because 'drink to thirst' was oversimplified. Thirst is a reasonable signal in cool conditions and short-to-moderate efforts. In hot conditions or long efforts, thirst often lags actual need — the advice should be 'drink to thirst, but know your sweat rate and plan for it'.
• Changing drink products on race day. Your gut is accustomed to what you've been training with. New products at higher volumes in race conditions is one of the most common causes of race-day GI disaster.
• Obsessing over non-sodium electrolytes. Most 'advanced electrolyte' products sell you magnesium, potassium, zinc, and assorted other minerals that rarely matter during exercise. Sodium is the one that matters. Read the sodium number on the label and ignore the rest.