Water loss changes
the work.
Running produces heat, and sweating is one of the body's main ways of losing that heat. If sweat losses outpace fluid intake, body water falls. That can increase cardiovascular and thermal strain, particularly in longer, harder or hotter sessions.1
Sports medicine position statements commonly use a body-mass loss of around 2% or more as a point where performance and physiological strain can become more important, but that is not a command to replace every drop of sweat in real time. Sweat rates vary enormously between runners and across weather, pace, clothing and acclimatisation.2
Start in a normal hydration state, avoid large preventable fluid deficits, and do not drink so much that you gain body mass during the run.
Do not start
behind.
For most recreational runners, the best pre-run hydration strategy is simple: drink normally across the day, include fluids with meals, and arrive at the session without marked thirst or obvious signs that you are under-hydrated. The goal is euhydration — a normal body-water state — not deliberate overhydration.1
If a known fluid deficit needs correcting, the ACSM position stand recommends slowly drinking about 5–7 mL/kg of body mass at least four hours before exercise. If you still produce little urine or it remains very dark and concentrated, an additional 3–5 mL/kg around two hours before may be appropriate. These numbers are best treated as a correction strategy, not a requirement for every ordinary run.2
Drink normally.
Normal meals and usual fluid intake are often enough when you have plenty of recovery time between sessions.
Correct a known deficit.
If you are clearly under-hydrated, 5–7 mL/kg is the evidence-based ACSM starting point rather than chugging a large volume immediately before running.
Recheck your cues.
If urine remains unusually dark or absent, an additional 3–5 mL/kg may be used. If you are already well hydrated, there is no benefit in forcing extra fluid.
Comfort matters.
Begin hydrated but not with a stomach full of fluid. A plan that causes sloshing, repeated toilet stops or discomfort is not a good plan.
There is no single
magic number.
During exercise, fluid needs should be individualised. NATA recommends using personal sweat rate together with environment, acclimatisation, body size, exercise duration, intensity, fluid access and tolerance rather than prescribing the same fluid rate to everyone.1
A practical performance target is to keep exercise-related body-mass loss below roughly 2% where possible while avoiding weight gain from excess drinking. If you do not know your sweat rate, drinking according to thirst is a sensible safety strategy and helps reduce the risk of overconsumption.13
For many sessions under an hour, water is usually all that is required. Longer runs, intense intervals or exercise in demanding heat may justify a drink that also contributes carbohydrate or electrolytes depending on the rest of the nutrition plan.4
The goal is not necessarily 100% replacement every minute. The useful target is to keep the deficit manageable without exceeding your losses.
Measure your
own run.
Sweat rate is one of the most useful ways to move from generic hydration advice to a plan that reflects your body. Weigh yourself before and after a representative run, track the fluid you drink, and account for any urine produced during the session. Repeat the test in different conditions because the number can change substantially with heat, humidity and intensity.1
Body mass + fluid in − fluid out.
Sweat rate (L/h) = sweat loss ÷ exercise duration (h)
Water is not
always the whole story.
Sweat contains electrolytes, particularly sodium, but sodium losses vary just as much as fluid losses. AIS guidance notes that electrolyte replacement can be useful when sweat losses are high, exercise is prolonged, the athlete is a particularly salty sweater, or rapid restoration of a moderate-to-large fluid deficit is required.5
There is no universal sodium-per-hour target that suits every runner. The AIS notes that guidance for sodium during endurance and ultra-endurance exercise remains uncertain, and higher or more targeted replacement should be based on real losses and individual circumstances rather than “more is better.”5
Sports drinks combine fluid, carbohydrate and electrolytes and can be useful when you want to address hydration and fuelling together. Electrolyte drinks can make more sense when the priority is fluid and sodium without substantial carbohydrate.6
Finish the run.
Restore the balance.
Post-run rehydration depends on how much fluid you lost and how quickly you need to recover. If you have many hours before the next session, normal meals, fluids and thirst can often restore balance without a rigid protocol. When recovery time is short and the deficit is meaningful, a more deliberate plan can help.1
Current AIS electrolyte guidance recommends around 1.2–1.5 times the estimated fluid deficit over 2–4 hours when a moderate-to-large deficit needs rapid correction. Sodium from food, salted meals, sports drinks or electrolyte products helps retain more of that fluid rather than simply increasing urine output.7
If a runner finishes 1.0 kg lighter after accounting for what they drank and produced no urine, that is roughly a 1.0 L fluid deficit. A rapid recovery target may therefore be about 1.2–1.5 L over the next 2–4 hours, alongside sodium and normal recovery food.
The same runner.
A different day.
Heat, humidity, pace, clothing and heat acclimatisation can all change how much you sweat. A hydration plan built on a cool winter long run may not work for a humid summer race. That is why sweat-rate measurements are most useful when repeated in conditions that resemble the event you are preparing for.1
On hot days, practical changes matter: start the run well hydrated, carry or plan access to fluid, choose cooler drinks when available, adjust pace when conditions demand it, and be prepared to change the plan if your stomach, thirst or heat strain changes.
More fluid is not
always safer.
Exercise-associated hyponatremia is a potentially dangerous dilution of blood sodium that is strongly associated with drinking more fluid than the body is losing. The simplest warning sign is this: you generally should not gain body mass during a run from fluid intake.3
Electrolytes do not make unlimited drinking safe. The primary prevention strategy is still to avoid overconsumption. If you do not have a measured hydration plan, using thirst as a real-time guide is supported by the exercise-associated hyponatremia consensus as a safe approach for most runners.8
Confusion, severe headache, repeated vomiting, seizures, collapse or altered mental state during or after prolonged exercise can be signs of serious heat illness, hyponatremia or another medical emergency. Do not try to solve severe symptoms by simply forcing more water.
