Anyone who’s trained for a marathon through the winter will greet summer running with open arms, but it comes with its own set of challenges—chief among them having to run with water. Figuring out when it’s worth running with water, how much you should take and the best way of carrying it is not as simple as it sounds. Should you run with a bottle in your hand, tuck a soft flask into a running belt, or go all out and wear a hydration vest?
What is straightforward, at least, is that the performance benefits of staying hydrated outweigh the extra weight of water that you have to carry. Research both in labs and during trail races has demonstrated that runners who are better hydrated have faster finishing times, a lower core body temperature and enhanced recovery, and it also helps them to feel less hot and tired.
To answer the trickier questions, we sought the advice of sports scientist Alan Ruddock, senior lecturer in the physiology of sport and exercise at Sheffield Hallam University.
Alan Ruddock is an accredited sport and exercise scientist and a fellow of the British Association of Sport and Exercise Sciences. He’s the laboratory director for the Sport and Physical Activity Research Centre at Sheffield Hallam University in the UK. He has provided physiological support for Olympians, Paralympians and World, Commonwealth, European and British champions in a range of sports and has co-authored over 30 peer-reviewed scientific manuscripts.
How much water should you drink while running?
The point of drinking water while you run is to replace the fluids lost during exercise. “You lose water through sweating and a minimal amount through water vapor when you breathe,” says Ruddock.
Working out your sweat rate will tell you how much water you need to replace. Sweat rate is determined by body temperature, and it’s different for every individual.
“Fitter people will sweat earlier because their threshold, in terms of their core temperature, will be lower. The rate at which we sweat is determined by how many sweat glands we have and how we can activate them."
Typically sweat rates range between one liter and three liters an hour. Women tend to sweat less than men, which can make body temperature regulation more tricky in hot environments. Sweat rates will also vary depending on the temperature, wind strength and how hard you are running.
“There is a simple way to assess the amount of water you need. Weigh yourself before and after exercise and either drink nothing or account for the amount of water you have ingested,” explains Ruddock.
It is easiest to calculate this after a 60-minute run in order to measure your sweat rate per hour. One gram of lost weight equates to around a milliliter of water, so a loss of 1kg will be the equivalent of one liter.
In an ideal world, you would want to drink the amount of liquid that you lose while running, but as Ruddock points out, this isn’t always practical.
“If you sweat three liters an hour how can you possibly drink this amount of water while running? You would need to be drinking constantly,” he says. “And the other problem is that even if you can get it into your gut, maximal gastric emptying rate is somewhere around 600ml per hour, but is highly individual, dependent on the amount of fluid within the stomach, the composition of the fluid and the intensity of exercise.”
Knowing your sweat rate is therefore most useful for recovery because you are restricted by your gastric emptying rate. During a sweaty run you can aim to drink up to one liter an hour, but the rest of the fluid loss should be topped up once you stop.
“We say that for recovery you need to drink 1½ times the amount you have lost,” says Ruddock. “If I have lost two liters of fluid I will need to ingest three liters in recovery.”
Should I also drink electrolytes while running?
The jury is still out on the effectiveness of electrolytes to control your sodium levels. There is also no evidence to support the argument that electrolyte solutions prevent cramping, says Ruddock. This is because the body is so good at self-regulating.
“The body is really good at defending itself against electrolyte losses,” he says. “It’s really important from a blood pressure perspective and to control the amount of fluid that is in our muscles, so the body takes care of it.”
But we do need electrolytes for another purpose—to absorb water via osmosis.
“If you have ever drunk too much water when you are running and you get this sloshing effect, that is due to water going in and out of the gut,” explains Ruddock. “Ingesting electrolytes partly solves this issue. It will help maintain stability in terms of osmotic potential.
“Electrolyte drinks are good for ensuring that we don’t get gastro-intestinal upset and what we call ‘runner’s gut’.”
What is the best way to carry water when running?
“Around the center of your body mass with a backpack is definitely the most efficient way to carry water,” says Ruddock.
In comparison, holding water in a handheld bottle is a far less biomechanically efficient way to carry liquid.
“If you are adding more weight to the lever arm that is going to require more torque around the shoulders and could have implications for shoulder fatigue. It could also cause imbalances on one side,” says Ruddock. “And also how much water can you really carry in your hand? It’s not a significant amount.”
And it's not just about how you carry water, but also how much you carry. Carrying too much can be counterproductive.
“Running economy will be affected by how much fluid you carry,” says Ruddock. “If you carry two kilos of fluid then you will become less economical and you will use more energy. If you are using more energy then you are producing more heat, then you are sweating more.”
Experiment during your training with carrying different loads over varying running durations.
“If you can still run at your target pace with an amount of fluid on your back that keeps your heart rate in the zone or at the perceived exertion that you want it to be, then you will be OK to carry that amount,” says Ruddock.
Failing that, another option is to structure runs in loops so you are able to grab water as you pass your house each time. Alternatively, run to a friend’s house and back again saving the need to carry water. But most importantly, drink plenty of fluid once you stop running.
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Lily Canter has worked as a journalist for more than 20 years and currently specializes in running and fitness. She regularly contributes to Coach as well as Runner’s World, Well+Good, Fit&Well and Live Science. Lily is a UK Athletics running coach, the founder of the Great Bowden Runners club and a participant in multi-day ultra races. Her biggest racing achievement to date is placing second at the Ultra Challenge 100km in the Lake District. She has a BA in English Literature, an MA in Print Journalism and a PhD in Journalism Studies. She is also co-host of the award-winning podcast Freelancing For Journalists and teaches feature writing, podcasting and freelancing to university students.