Proper hydration is one of the most critical factors in long distance running performance, yet it's also one of the most misunderstood. Both dehydration and overhydration can seriously impair your performance and, in extreme cases, pose genuine health risks. Understanding the science behind hydration and developing a personalised strategy is essential for any runner tackling distances beyond 10 kilometres.
This guide breaks down the fundamentals of running hydration, helping you develop a science-backed approach that works for your body, your training, and your goals.
Understanding Your Hydration Needs
There's no universal answer to "how much should I drink?" because hydration needs vary dramatically between individuals. Factors that influence your fluid requirements include:
- Sweat rate: Some runners lose over 2 litres per hour while others lose less than 500ml in the same conditions
- Body size: Larger runners generally need more fluid
- Running intensity: Harder efforts mean more sweating
- Environmental conditions: Heat, humidity, and altitude all increase fluid loss
- Individual physiology: Genetics play a significant role in sweat composition and rate
- Acclimatisation: Heat-adapted runners sweat more efficiently but may lose more fluid
The goal of hydration isn't to replace every drop of sweat you lose—that's actually impossible during exercise. Instead, aim to prevent excessive dehydration (losing more than 2-3% of your body weight) while avoiding overhydration.
Calculating Your Sweat Rate
Knowing your personal sweat rate is invaluable for planning hydration strategy. Here's how to calculate it:
- Weigh yourself nude immediately before a run
- Run for one hour at your typical training pace in typical conditions
- Track exactly how much you drink during the run
- Weigh yourself nude again immediately after, towelling off any sweat
- Calculate: Pre-run weight - Post-run weight + Fluid consumed = Hourly sweat rate
For example, if you weighed 70kg before, 69kg after, and drank 500ml during the run, your sweat rate is approximately 1.5 litres per hour (1kg weight loss + 0.5L fluid consumed).
Repeat this test in different conditions—hot days, cool days, different intensities—to understand how your sweat rate changes. This information helps you adjust your hydration plan for race day conditions.
How Much to Drink During Runs
Current sports science guidelines suggest aiming to replace 50-80% of fluid losses during exercise. For most runners, this translates to approximately 400-800ml per hour, with the specific amount depending on your sweat rate and conditions.
Rather than following rigid schedules, learn to drink to thirst while using your knowledge of your sweat rate as a guide. If you know you lose 1.2 litres per hour, aim to drink roughly 600-900ml per hour, adjusting based on how you feel.
Hydration Strategies by Distance
Half Marathon (21.1km): For most runners, carrying 500-750ml in a running vest or belt is sufficient. Focus on taking small, regular sips every 15-20 minutes rather than large gulps at aid stations.
Marathon (42.2km): You'll likely need to refill at least once. Carry 500-1000ml capacity and plan your refilling strategy around aid stations. Practice your race-day hydration during training runs.
Ultra Marathon (50km+): Extended events require careful planning. Carry maximum capacity in your vest (typically 1.5-2L) and know where water points are located. Many ultras have mandatory hydration capacity requirements.
The Role of Electrolytes
When you sweat, you lose more than just water—you also lose electrolytes, particularly sodium. These minerals are essential for muscle function, nerve signalling, and maintaining fluid balance in your body.
For runs under 60-90 minutes in moderate conditions, plain water is usually sufficient. However, for longer efforts, adding electrolytes becomes increasingly important:
- Sodium: The primary electrolyte lost in sweat; critical for fluid balance and muscle function
- Potassium: Important for muscle contractions and heart function
- Magnesium: Helps prevent cramping and supports energy production
- Calcium: Essential for muscle function and bone health
Sodium is by far the most critical electrolyte for runners. Sweat sodium concentration varies widely between individuals—some runners are "salty sweaters" who lose significantly more sodium than average. Signs that you might need more sodium include visible salt stains on your clothing, a strong craving for salty foods after running, and frequent muscle cramps.
Choosing Hydration Products
The sports hydration market is crowded with products, from simple electrolyte tablets to complex carbohydrate-electrolyte drinks. Here's how to navigate the options:
Electrolyte Tablets and Powders
Products like Nuun, SiS Hydro, and Precision Hydration tablets dissolve in water to provide electrolytes without significant calories. These are ideal when you want to separate hydration from fuelling—you can drink electrolyte water while getting calories from gels or food.
Sports Drinks
Traditional sports drinks like Gatorade and Powerade combine electrolytes with carbohydrates (typically 6-8% concentration). These provide both hydration and fuel but can cause stomach issues for some runners, particularly at higher concentrations.
Plain Water
Don't underestimate good old water. For shorter runs and in situations where you're getting electrolytes from food or salt capsules, water is perfectly adequate and easy on the stomach.
Never try a new hydration product on race day. Test everything during training to ensure it agrees with your stomach. What works perfectly for one runner may cause significant GI distress for another.
Pre-Run Hydration
Starting your run well-hydrated sets you up for success. In the 2-4 hours before a long run or race, aim to drink 400-600ml of fluid. This allows time for your body to absorb the fluid and for excess to pass through as urine before you start.
A simple indicator of adequate pre-run hydration is urine colour—aim for pale yellow. Very dark urine suggests dehydration, while completely clear urine might indicate you've drunk too much.
Avoid chugging large amounts of fluid immediately before starting. This doesn't help hydration (the fluid hasn't been absorbed) and often leads to stomach discomfort or urgent bathroom needs during your run.
Recognising Dehydration and Overhydration
Learning to recognise the signs of both dehydration and overhydration helps you adjust your strategy in real-time during runs.
Signs of Dehydration
- Thirst (though this can lag behind actual dehydration)
- Dark-coloured urine
- Decreased performance and increased perceived effort
- Dizziness or lightheadedness
- Headache
- Muscle cramps
- Nausea
Signs of Overhydration (Hyponatremia)
Overhydration—drinking too much fluid—dilutes sodium levels in your blood and can be dangerous. Watch for:
- Bloating or puffiness
- Nausea and vomiting
- Headache
- Confusion
- Weight gain during the run
Hyponatremia (low blood sodium from overdrinking) is a medical emergency. If you or a fellow runner shows signs of confusion, seizures, or loses consciousness after drinking large amounts of fluid, seek immediate medical help.
Post-Run Rehydration
Rehydrating after your run supports recovery and prepares your body for future training. The general guideline is to drink 1.5 times the fluid you lost during your run, spread over several hours.
If you weighed 1kg less after your run than before (accounting for fluid consumed), aim to drink about 1.5 litres over the next 2-4 hours. Including sodium in your post-run fluids helps your body retain the fluid rather than passing it straight through as urine.
Pairing rehydration with a post-run meal that includes salt naturally helps restore electrolyte balance. Many runners find that their body naturally craves salty foods after long runs—this is your body's way of guiding recovery.
Developing an effective hydration strategy takes experimentation and practice, but the investment pays dividends in better performance, improved recovery, and safer running. Start by calculating your sweat rate, practice your strategy during training, and fine-tune based on how your body responds.