Drink when you're thirsty, in small sips, and make sure your water is within reach without stopping. For most runners, that works out to roughly 0.4 to 0.8 liters per hour (14 to 27 oz), with two lines not to cross: don't lose more than about 2% of your body weight during the run, and above all, never finish a run heavier than when you started. The rest is just logistics.
Before you head out: drink early, not a lot
The joint recommendation from the American College of Sports Medicine and US and Canadian dietitians is to drink 5 to 10 ml per kilo of body weight in the 2 to 4 hours before exercise. For a 70 kg (154 lb) runner, that's 350 to 700 ml (12 to 24 oz), spread over the morning.
The same paper includes two details that change how you do it:
- The goal is pale yellow urine, not clear. You don't need to "tank up."
- The time window is there to get rid of the excess. Chugging a big glass right before the start doesn't hydrate you any better; it just sends you looking for a bathroom in the first few miles.
In practice: a big glass when you wake up, another with breakfast, then a few sips until the start. If you run early in the morning, that's often enough.
During: drink to thirst or follow a plan?
That's the real question. For years, the advice was to "drink before you're thirsty." Experts on exercise-associated hyponatremia have turned that advice on its head.
What the medical consensus says
The International Exercise-Associated Hyponatremia Consensus Development Conference (Carlsbad, 2015) states that the safest individualized hydration strategy before, during and immediately after exercise is to drink palatable fluids when thirsty. The reasoning is simple: thirst is a regulating mechanism that limits the risk of drinking too much.
The ACSM paper points the same way, with a useful nuance: thirst is generally a good indicator of the need to drink, without meaning you're already dehydrated. Feeling thirsty on a run is normal. It's not an alarm bell, it's an instruction.
What ultrarunners actually do
In a 161 km (100 mi) race in serious heat, a study by Hoffman and Stuempfle found that drinking to thirst was the most common strategy (67% of runners for at least part of the race), and that weight loss didn't differ with the strategy chosen. Among the top ten finishers, half had lost more than 2% of their starting weight by kilometer 90 (mile 56). The authors conclude that drinking beyond thirst isn't necessary to stay hydrated, even in hot conditions.
The main thing to remember: the 2% threshold is a caution marker, not a cliff. Going slightly past it on a long run isn't a disaster; going well past it, or starting out already dehydrated, is more of a problem.
When a plan still makes sense
Drinking to thirst assumes something people forget: being able to drink the moment thirst kicks in. That's where the difference plays out on the trail. The ACSM paper actually specifies that the 0.4 to 0.8 L/h rate should be adjusted to the runner's tolerance, experience and opportunities to drink.
So a light plan is still useful in three cases:
- When water is scarce. Between two water sources that are far apart, you need to ration what you carry. How to calculate the volume to carry per section is covered in how much water to carry trail running, by distance.
- When you're focused on the terrain. On a technical descent or in a race, you can go long minutes without thinking about it. A fixed cue helps: a few sips on every flat stretch, at the top of every climb or at every map check.
- When drinking is a hassle. If every sip means pulling out a bottle, opening it and slowing down, you'll drink less often than your thirst asks for.
How to actually drink while running
Technique matters as much as quantity. A few principles that work whatever your gear:
- Small sips, often. A few regular sips go down better than a big gulp once your thirst gets strong.
- Drink on the easy sections. Power-hiked climbs and gentle grades are the best moments: your breathing is steadier and your eyes aren't glued to your footing.
- Keep your hands free. Running with a bottle in your hand means one arm stops swinging, and a bottle you eventually stop opening.
- Refill before you run dry. At a water stop, top off even if your bladder is half full: it's the next section that decides.
With a hydration bladder, two adjustments prevent most annoyances: purge the air after filling (squeeze the bladder, valve pointing up) to cut down on sloshing, and run the hose along the shoulder strap so the bite valve stays within reach of your mouth. That's the whole point of a vest like the Sentia hydration vest, which comes with its 2L (68 oz) bladder: water on your back, bite valve under your chin, phone and gels in the front pockets, and you drink without breaking stride. If you're torn between the two systems, compare them in hydration bladder vs. soft flasks, and for the rest of your gear, go back over the criteria for choosing the right hydration vest.
Water, salt, sugar: what to put in it
Plain water is enough for short runs. What you need changes as the effort gets longer or when you sweat a lot.
Sodium
The ACSM recommends taking in sodium during exercise for heavy sweaters (1.2 L/h or more), "salty" sweat, or efforts longer than 2 hours. Salty sweat is easy to spot: white streaks on your shirt or cap.
Watch out for the reverse logic. Salt doesn't protect you from overdrinking: the 2015 consensus points out that sports drinks, whose sodium concentration (10 to 38 mmol/L) is far lower than that of blood, don't prevent hyponatremia in runners who drink too much.
Carbs
For endurance efforts, the ACSM guidelines are 30 to 60 g of carbohydrate per hour between 1 and 2:30 hours, and up to 90 g per hour beyond 2:30 to 3 hours. Some of it can come from your drink, the rest from gels and solid food.
A common setup: plain water in the hydration bladder, sports drink in a front flask. You separate thirst from energy, and you avoid leaving sugar sitting in the bladder. If you do put a sugary drink in it, rinse it when you get back: the method is in how to clean a hydration bladder.
Overdrinking: the mistake you don't see coming
Exercise-associated hyponatremia means blood sodium that's too low, below 135 mmol/L for most laboratories. Its main cause is drinking more than you lose. The 2015 consensus lists these among the risk factors:
- drinking too much water, sports drink or other hypotonic fluids;
- gaining weight during exercise;
- exercise lasting more than 4 hours;
- lack of experience or training;
- a slow pace.
The last three describe the trail runner training for a first long-distance race pretty well. Mild signs are lightheadedness, nausea, swelling and weight gain; severe forms combine vomiting, headache, confusion and even seizures. If a runner becomes confused after drinking a lot, it's not just fatigue: it's an emergency, and you need to call for help rather than give them more to drink.
The simplest safeguard: if your rings feel tight or the scale reads higher after the run than before, you drank too much. Cut back next time.
After: replace what you lost
Weigh yourself before and after a long run. The ACSM recommends drinking 1.25 to 1.5 liters for every kilo lost (about 19 to 23 oz per pound), because some of what you drink is excreted. Spread that volume over the following hours, not all at once.
Weighing yourself has a second benefit: it gives you your own sweat rate. Repeated in cool and hot weather, it beats any generic chart.
The recap
| When | Guideline | Key takeaway |
|---|---|---|
| 2 to 4 hrs before | 5 to 10 ml per kg | Pale yellow urine, no big glass right before the start |
| During | To thirst, often 0.4 to 0.8 L/h (14 to 27 oz) | Small sips, water within reach without stopping |
| Over 2 hrs or heavy sweating | Add sodium | Salt doesn't make up for overdrinking |
| 1 to 2:30 hrs | 30 to 60 g of carbs per hour | Sports drink, gels or solid food |
| Beyond 2:30 to 3 hrs | Up to 90 g of carbs per hour | Test it in training, never on race day |
| Upper limit | Never gain weight | Tight rings, nausea: you're drinking too much |
| Lower limit | About 2% of body weight lost | A caution marker, not an absolute threshold |
| After | 1.25 to 1.5 L per kg lost | Spread over several hours |
Sources
- Thomas, Erdman, Burke — Nutrition and Athletic Performance, joint position statement of the ACSM, Academy of Nutrition and Dietetics and Dietitians of Canada (2016)
- Hew-Butler et al. — Statement of the Third International Exercise-Associated Hyponatremia Consensus Development Conference, Carlsbad (2015)
- Hoffman and Stuempfle — Hydration Strategies, Weight Change and Performance in a 161 km Ultramarathon (2014)