H2–Oh, You Got This: Trail Running Hydration Basics

Once trail running feels a bit more comfortable, or maybe you have race on the schedule, you may find yourself wanting to hit the trails for a longer duration of time. More time means carrying your own resources and that means figuring out how to do that. You may be used to stashing water along your road-running route. Or you may sometimes carry a big backpack full snacks, maps, and water bottles when you go hiking. When you trail run, though, you want to feel comfortable and svelte while still having to access to everything you need. This month, we will talk about what you need or at least need to consider in order to stay hydrated for a run up to two hours in duration. This article, like all the articles in this column, is geared toward newer trail runners, whether you come from road running, other outdoor activities, or are new to running altogether.

Basic Principles of Trail Running Hydration

Two hours on the trails is a considerable amount of time—especially if it’s hot and/or humid. When it comes to trail running, it’s long enough that you’ll need water to stay happy and healthy. If you don’t drink water for that length of time, most people will begin to see a decline in their performance and, in some cases, experience thirst and symptoms of dehydration. As a result, for any run that is longer than 60 to 90 minutes, you will want to consider carrying water with you to maintain a healthy hydration level and optimize your performance. Much shorter than this, and you can likely rehydrate in a short period of time after your run is over. How much water do you need to carry and drink? The answer is simple: it depends.

Requirements for hydration vary widely based on the individual. Very generally, people lose about 32 fluid ounces/0.95 liters of sweat/water in volume—which weighs 2.1 pounds/0.95 kilograms—for every hour they exercise. This can and will be affected by a lot of factors, including the temperature, humidity level, how dehydrated you are when you start, your level of fitness, and more. In order to determine your own sweat loss while running, weigh yourself before you run and then again after. Calculate the difference between your pre and post-run weight, add in the weight of any fluid you ingested during your run, and you will know your sweat rate:

Your weight loss + Weight of any fluid intake = Your sweat rate (in water weight)

Of course, you’ll need to convert that weight back to its representative volume of water, an easy conversion—and there are online calculators that can do it for you, too. Simply, 1 fluid ounce of water equals 0.065 pounds of water and 1 liter of water equals 1 kilogram of water.

To determine how much you would need to drink each hour to counteract your fluid loss, divide your sweat rate (now converted to water volume in your favorite unit of measurement) by the number of hours you will be running. The difference gives you a rough idea of how much water it takes each hour to keep you completely hydrated while running:

Your sweat rate in water volume ÷ Number of hours running = Volume of fluid needed each hour

If you track this change every time you run, in a training log perhaps, you will be able to recognize patterns and start to understand what it takes to keep you hydrated in various types of conditions and on various types of runs. Studies show that performance starts to dip when a person has lost 2% or more of their body weight in sweat. For a 120-pound/54.4-kilogram female, losing 2.4 pounds/1.1 kilograms) will hinder performance. Knowing that the average person loses about 2 pounds of water for every hour he or she exercises, it makes sense that an individual can only run about 60 to 90 minutes without hydrating before performance begins to suffer.

Stephanie Violett - 2016 TNF50 Mile - handheld water bottle

Stephanie Howe Violett using a handheld water bottle to carry water during the 2016 The North Face Endurance Challenge 50 Mile Championships. Photo: iRunFar/Meghan Hicks

Although it may seem counterintuitive, it may not be prudent to replace 100% of your fluid loss while running. Drinking too much can cause serious health emergencies and reduce your ability to perform. Replacing all fluid lost may also cause gut discomfort and bloating, especially if you are running at a challenging-for-you effort. The goal is to stay ahead of that 2% loss so that your performance does not suffer and you do not become dehydrated. Once you know your sweat rate, you can determine how much water you need to carry on a run. To demonstrate this, let’s assume a sweat rate of 32 fluid ounces/0.95 liters per hour. If you are planning to go for a two-hour run, the math is pretty easy–multiply your sweat rate by the number of hours you plan to run to calculate the amount of water it would take to replace 100% of your fluid loss. In this case, we would need to ingest 64 ounces/1.9 liters in order to completely replace the fluid lost while running. Here’s the equation:

Sweat rate (in water volume) x Number of run hours = Total fluid replacement needed (in water volume)

Remember that you don’t necessarily have to drink and carry 100% of the fluids you expect your body to lose during your run. Let’s shoot for a 1% fluid loss during our two-hour run, in our example. For a 120-pound/54.4-kilogram female, that is 1.2 pounds/.54 kilograms–or about 18.5 ounces/0.55 liters that we can afford to lose. Subtract that number from your total fluid replacement needed to calculate the amount of fluid you need to carry with you to ensure your performance doesn’t suffer. In this case 64 ounces/1.9 liters – 18.5 ounces/0.55 liters = 45.5 ounces/1.35 liters. In equation form:

(Sweat rate x Number of run hours) – Affordable fluid loss = Fluid volume needed for your planned run

There is a case to be made that you should carry 100% of your total fluid-replacement needs. You may be starting your run a bit dehydrated, or you may become lost, sidetracked, or experience an unplanned change in weather or temperature. Each of these variables would require a potential need for more fluid intake. If you aren’t certain of your route or are concerned about changing variables, you may want to consider carrying the extra fluid. If you are fairly certain of the route and your needs on that route, the planned 1% variation in need should be an adequate buffer.

All of this said, our current scientific understanding of hydration can be boiled down to the basic principle of drinking when you are thirsty and not drinking if you are not–that the thirst mechanisms of the human body do a good job of helping us self-regulate our hydration. The guidelines we describe here are meant to help you understand basic ideas about sweating during endurance running and they should also help you estimate how much fluid you may want to carry with you when you are venturing into longer runs up to about two hours in length. Once you are out there, and pretty much all the time, follow your thirst.

How to Carry Your Fluids

Now that you know how much you will want to consider drinking while you are running, let’s figure out how to carry that with you. There are a variety of ways to carry fluid. Below you will find a very basic analysis of the types of packs on the market. As with any gear, there are advantages and disadvantages to every piece of gear. I will try to explain these based on my experiences on the trail as well as my experience in running retail. We will start with the most minimal solution and progress to the most maximal.

Water Stash

This option is typical of road runners and a great option if you will be crossing the same point multiple times on your run. Prior to beginning your run, stash some water in an inconspicuous place. Each time you pass that place, take a drink of water. You do run the risk of needing water away from your stash and not having it. If the time or distance between passes is short enough, this is a great option as you have an unlimited capacity for both quantity and variety. Pair this with a handheld water bottle for a fool-proof option.

Handheld Water Bottle

You can typically carry about 12 to 20 fluid ounces/0.35 to 0.6 liters in a handheld water bottle. If you have one on both hands, you double your carrying capacity. There is very little storage capacity for other gear so if you are planning to carry much more than a phone and a few gels, this option likely won’t work. These can often feel like running with a hand weight and over the course of several hours can cause fatigue in the arms. If you like to run with minimal gear or you have the option of refilling throughout your run, this is a great option.

Ian Sharman - 2016 Western States 100 - handheld water bottles

Ian Sharman running the 2016 Western States 100 with two handheld water bottles. Photo: iRunFar/Meghan Hicks

Portable Filter/Straw

A somewhat recent introduction into trail running is the filtered straw. This is a wide-mouth straw with a filter on one end that allows you to dip the straw right into a source of water and drink directly from it without running the risk of waterborne illness. If you want to carry the absolute least amount of gear possible, a straw filter is the best option. It is lightweight and the only capacity limitation is the number of times you come across a water source. If you are running in a desert where water is scarce, however, this may not be the best option. This will also add time to your run as you stop to drink versus sipping from a bottle on the go.

Waist Pack

You can typically carry about 20 to 30 fluid ounces/0.6 to 0.9 liters around your waist. This can be carried in one big bottle or several small bottles. If you prefer multiple fluid choices (sports drink, coconut water, and regular water as examples) this may be a great option for you. Some women may dislike this option as the belt must be worn at the waist, not the hips, in order to prevent bouncing. This doesn’t seem to affect men as much. One big advantage of this option is that it doesn’t hold in heat as much as a vest or backpack will, as it only covers a small part of your torso. It also doesn’t hold as much gear as a backpack or a vest. If you are planning to carry a jacket, gloves, or other gear, this may not have enough storage capacity.

Vest

This seems to be the most popular solution out there and, as a trail runner, you will find an abundance of choices in terms of style and carrying capacities in this category. You can carry anywhere from 20 to 100 fluid ounces/0.6 to 3 liters in a vest in just a hydration bladder with options for adding additional bottles to carry a variety of beverages in other pockets. In addition, you will also have more carrying capacity for clothing, trekking poles, and other gear you may want.  A vest is different from a backpack in its construction and fit on the body. A vest is designed to sit close to the body while utilizing the straps and pockets for easy access to and distribution of gear both on the front and the back of the body. It should bounce very little.

If you are starting with shorter runs in hopes of increasing to longer runs, this is a consideration. If you plan to run recreationally with two to three hours as your ceiling, you will want to look for a small vest or look to a less-complex option. If you run hot or have a high sweat rate, bear in mind that this option traps the most heat as it covers much of your front and back torso.

Moab trail running - running vest

An example of a running vest, used to carry water and extra supplies for trail running. Photo: iRunFar/Meghan Hicks

Backpack

A backpack is similar to a vest in its carrying capacity but is different in the construction. Instead of using a vest-like fit that is snug to the body to prevent bounce, a backpack will use straps to support the weight using your torso and/or hips, and most of the gear is carried on your back. It will also be more versatile in its use. If you like to mountain bike or day hike and you only want one piece of gear, this may be the best option for you.

Kinds of Fluids to Hydrate With

Once you determine which type(s) of contraptions you would like to use to carry your hydration, the final step is to determine how best you would like to hydrate. Of course, water is the go-to option. However, it is no secret that there are a variety of poisons out there from which you can pick. Here are a few things to consider adding to your water to make it more potent.

Electrolytes

Electrolytes are forms of salt that are required for your nerves to work properly. Nerve function is vital to muscle function so without these salts, your performance can suffer. There are seven main electrolytes and some of them are more vital than others to runners. Electrolytes leave your body in your sweat, and you may (or may not) need to ingest more of them when you run to maintain the proper balance of water and electrolytes inside your body. This article won’t go into the nitty gritty of electrolytes but it is here to make you aware of the concept and to inform you that adding them to water is a common way to replace the electrolytes that come out in your sweat. It is common to use a powder, tablet, or an additional liquid to achieve the water-electrolyte balance that you need. I recommend finding a nutritional source that you trust, such as a local running store’s experts, an online source you use regularly (these two articles here on iRunFar provide an in-depth look at hydration and electrolyte requirements for long-distance running), or a book (like Where the Road Ends: A Guide to Trail Running by iRunFar’s editors) for advice in this area. Then experiment on your shorter runs, 45 to 90 minutes, to find something that feels right before taking it out on longer runs.

Calories

I know it may come as a shock to you but you burn calories while you run! Your body only stores a short supply of easy-access calories before it starts to turn to fuel sources that can impact your performance. Some people choose to add calories to their water to replenish their quick-access supply of calories as they are running so they can keep up the good work. There are many options for replacing calories and every option will have its reasons for why the formula is perfect. If this sounds interesting to you, you may want to read this article here on iRunFar to learn more. Also, check back here next month because we will talk about the basics of nutrition during long-distance running.

Flavors

If something tastes good, you are more likely to drink it, and that’s why there’s a case for running with flavored water—especially if you are someone who has a hard time drinking while running. If you choose to add just straight flavor, be conscious of what else is going in there with you as some artificial sweeteners may cause gut discomfort. If you are truly going for flavor only, consider adding some fruit to a pitcher of water the night before your run. Before you pour the water into your pack, take out the fruit. This will give you the flavor without any calories or bacteria build-up in your bottle or mouthpiece.

Now that you know how much fluid you will need to drink while running and you are familiar with how to carry it and what you can put in it to help you drink and perform, it’s time to put your newfound knowledge to the test. If you are worried about creating a hydration plan, just remember, H2–Oh, you got this!

Call for Comments (from Meghan)

  • Have you ever tracked your sweat rate for running or in other sports? How has it varied based upon your condition or the conditions through which you are running?
  • Do you already use some sort of gear to carry your water? What do you personally like and what hasn’t worked so well for you?
  • Do you have any other helpful hydration tips that you can share?
Luis Alberto Hernando - 2016 IAU Trail World Championships

Luis Alberto Hernando using a running vest during the 2016 IAU Trail World Championships. Photo: iRunFar/Meghan Hicks

Rhielle Widders

is passionate about introducing her favorite sport to newcomers. She created and directed the Park City Trail Series, a four-race series designed to get people running on dirt, from 2010 to 2014. When she isn’t in Park City, Utah, where she lives, you will find her traveling to try out new dirt. Follow her on Instagram.

There are 18 comments

  1. fabien

    Very interesting article!
    As a new trail runner – less than a year – I am far from running ultras, but many questions are still coming about carrying hydration. Talking about time rather than distances, I usually take an hydration belt (holding two small 150ml flasks) for runs less than 1:30, and I switch to a vest for longer runs.

  2. Eric Coppock

    I’d be careful with this. The premise that fluid intake should replace all weight lost during endurance exercise is simplistic, and can be outright dangerous under the right conditions.

    1. Clinton Zirk

      That was covered:

      “Although it may seem counterintuitive, it may not be prudent to replace 100% of your fluid loss while running. Drinking too much can cause serious health emergencies and reduce your ability to perform. Replacing all fluid lost may also cause gut discomfort and bloating, especially if you are running at a challenging-for-you effort. The goal is to stay ahead of that 2% loss so that your performance does not suffer and you do not become dehydrated. “

  3. Kyle T

    I would include the inline water filter, such a Sawyer Mini and a SteriPen as options for water filtration/purifying. An inline filter allows you to fill a bladder or bottle straight from the source and continue running while drinking clean water without stopping. The SteriPen is similar, but you do have to wait until the UV light does it’s job before moving on. Both are better options to a Lifestraw in my opinion because you aren’t having to figure out a way to get your face close to the water, and they use much less effort. However, the LifeStraw is a great emergency piece and is easy to carry if you aren’t planning to use water sources during a run.

  4. Micah Larson

    I think it is also important for each individual to rely on their own thirst as a gauge of when to drink water. The hypothalamus does a great job of regulating the desire to take in fluids, and can be relied upon during running. In the field of exercise science, there are two schools of thought on the topic. One view is that fluid intake should closely match that of fluid loss. This has been the predominant school of thought for years, but one that has been challenged by some within the last couple of years. The other viewpoint is that the body provides sufficient feedback to the runner when fluids are needed. If you are thirsty while you are running, then drink water. Some of these scientists who hold the viewpoint that thirst is the best indicator of hydration needs, also question the role of electrolytes in performance as well. It is definitely interesting reading material if you are interested in debated topic in exercise science circles. A great resource on the hot button topics related to electrolytes and hydration was done by the sports scientists blog. https://sportsscientists.com/2007/10/fluid-intake-dehydration-and-exercise-part-i-history-of-fluid-intake-and-a-conflict-of-interest/
    I have no affiliation with the blog, but we did discuss it in depth and my advanced exercise physiology class in grad school.

  5. Ecoromeu

    As it is a great variability among individuals in regards to the amount of fluid to ingest during a run, the same can be said in relation the old and weak scientific evidence on the 2% body weight loss impacting in performance. Some individuals can lose way more than that and still performing at their best. Drinking to thirst still the best advice, in my opinion.

  6. Sniff

    I think I read this in Koop’s book, or maybe Steve House’s I don’t recall. I like the idea of keeping calories separate from hydration. In a 50mi race I was using tailwind(which I like) but my stomach turned on me. That left me with no calories or WATER!! I know its all n1, but this has helped me.

    1. Meghan Hicks

      Sniff,

      N=1 is always the case when it comes to endurance hydration, electrolyte balance, and nutrition. I am like you, in that I prefer to keep my hydration and calories as separate ingestibles. I began in ultrarunning years ago ingesting my calories with my liquids, and this worked for a few years until I began exploring faster paces in ultras and long runs in prep for ultras and encountering gut problems with drinking both my water and calories together. I found that, when running with more effort, keeping calories and water separate let me independently control my intake of both and avoid gut issues.

      That said, I know plenty of people who experience various forms of issues trying to ingest gels, bars, and the like, but can get their calories in when consumed in liquid form. And that the combo of water and calories together allows them to run strong and/or race hard.

      Thanks for sharing your N=1!

  7. Andy DuBois

    There is NO research to support the idea that more than 2% dehydration negatively impacts performance ( if allowed to drink to thirst ) . All studies climbing this didnt allow the subjects to drink to thirst and or started the subjects already dehydrated via the use of diuretics and or sitting in a sauna before the test .

    So once you scrap the idea that 2% loss is bad then the whole premise for weighing yourself and basing tour fluid intake on that falls down
    http://mile27.com.au/dehydration-impair-endurance-performance/

  8. JJ

    Water can also be chemically purified. I carry Aquamira in a tiny dropper bottle in my vest on longer summer runs. The only downside is you have to wait 15 minutes for it to do the job, but it’s a great light-weight, low-cost water purification solution.

    As far as how much and how often to drink, listen to your body. Drink when you’re thirsty, and take one or two more swigs than you think necessary.

    Also beware during races the aid stations may not have what your stomach is used to, e.g. Heed vs. EFS. It may also be mixed stronger than you’re used to as well- and this can also cause stomach issues. If you’re sensitive to this, dilute or carry your own electrolyte and mix it yourself with the water at the aid station.

  9. Meghan Hicks

    All,

    Thanks for the comments. I want to kindly remind commenters to this article and this column, Trail Running 101, that this is a beginner trail running column. We aim to create fundamental understandings of how the body works when trail running and what people need to know in order to be healthy and happy trail runners.

    As such, this article isn’t aimed at ultrarunners, or high-level trail runners necessarily. This article is aimed at creating a basic understanding of what liquid the body may need and how to carry it for runs up to about two hours in length. As you can see, we also link readers to more verbose and advanced tacklings of hydration science elsewhere on the website.

    There is a good amount of scientific data showing that body-mass loss of 2 to 5% during several kinds of running leads to decreased performance. So, we used 2% as our knowledge starting point for this article. Indeed, there are also multiple studies showing that highest-level performers in multiple endurance sports had significantly higher levels of body-mass loss from sweat during their incredible performances. But this article isn’t written for the performance outliers–it’s written for folks just dipping into trail running.

    As we put this column together each month, we’ve got on our minds the fact that there’s a fine line between overgeneralizing/oversimplifying and laying out the basics, and that we want to stay on the latter side of that line. I sincerely appreciate the discussion that happens here in the comments section, especially if you think we have missed the mark and have overgeneralized/oversimplified, but I also sincerely hope that some of the super-experienced readers of iRunFar can keep in mind the basic premise/desired scope of Trail Running 101.

    Thanks so much!

  10. MikeInEverwet

    A couple (like 2 or 3) drops of peppermint oil in a 2 liter backpack can offset the new smell and be a nice change from just water. Also helps cut the sweetness of gels without promoting GI issues. If minty isn’t your thing, cinnamon oil might be? I haven’t tried that but similar thought process.

  11. Marty Hoffman, MD

    I feel obligated to comment on the misconception that weight change is equal to change in hydration status. Change in body mass does not reflect an equal change in hydration status largely due to the release of water bound with glycogen, the production of water during substrate metabolism, and the utilization of stored substrate. These factors may be trivial during short bouts of exercise, but they become very important when we’re talking about exercise for multiple hours.

    Our calculations show that one would be overhydrated by 4-6% of body weight if they drank enough to keep a stable weight during a 100-mile ultramarathon (see our abstract on page S1-2 at this site: http://journals.humankinetics.com/doi/pdf/10.1123/ijspp.2017-0167). In other words, one should lose 4-6% body weight to maintain a stable body water balance during a 100-mile ultramarathon. One could then lose some additional fluid from the body water pool beyond that before performance would be seriously impaired.

    Another concern comes from the concept of determining sweat rate from pre-exercise to post-exercise weight measurements. This may sound rational, but recognize that sweat rate varies considerably across conditions. As an example, consider a situation in which you have presumed you need to match all of your weight loss during exercise with fluid intake, and you have calculated that your rate of sweat loss is 1000 ml per hour. But, the conditions for a 4-hour run you do are a bit different than your test conditions and you have overestimated your needs by 250 ml per hour. If you drink that full 1000 ml per hour, then you have consumed a full liter (~1.4% body weight) more than necessary to keep your weight constant. But, when you also consider the effect of release of water bound with glycogen, the production of water during substrate metabolism, and the utilization of stored substrate, you really needed to lose roughly 3% of body weight to keep the body water pool stable. Thus, you have actually overhydrated by over 4% of your body weight during this 4-hour run. Under the right circumstances, that could induce symptomatic exercise-associated hyponatremia, a condition from which people have died.

    If you want to keep it simple, then you can use the sweat rate calculation to estimate the maximum amount of fluid that you might need access to during a run, but don’t use this calculation to tell you how much you should drink. Simply drink when you’re thirsty (for more on this, see: http://www.sciencedirect.com/science/article/pii/S1080603216001101).

    Marty Hoffman, MD

    1. Andrew L

      Good stuff here, and I agree that trying to limit weight loss to 2% can be a bad idea. When it’s not hot, I personally don’t bring water for even runs of 2-3 hours. In the colder to moderate temperature months (less than about 50 F), I am happy to go all an all day ski tour without bringing or wanting water. This is from experience, not a recommendation.

      Maybe there’s something different (or off) with me, but I’m often still needing to take occasional pee stops 2 hours into a run (peeing clearish) without having had a drink during the run. I limit what I drink before the run to what my thirst dictates too, which is very little. Back 20 years ago when I still believed the all the “hydrate or die tm”/drink 8 cups talk, I would often feel the urge to pee every 5 minutes or so for the duration of a 2 hour run. What’s more, I would feel better and better the further into the run I would get, as I peed out excess fluid and went from overhydrated to normal. That’s ridiculous of course, so I stopped drinking so much.

  12. Jackie

    I would encourage new trail runners, and actually all of us, to try new things during training to see what will work best. I started with waist-packs, tried a handheld, and have settled on the vest as my favorite tool for carrying my “stuff”. I have a 2L vest for long runs (2+hours), and a race vest for “shorter” long runs, or races with aid stations. I set the timer on my watch to beep every 30 minutes to remind me to eat, take a salt capsule every hour and I drink gatorade, never water. If it’s not hot I skip the salt.

  13. Andy DuBois

    Any scientific evidence that more than 2% dehydration decreases performance has been done with the subjects not allowed to drink in the test and dehydrated from the start through the use of diuretics and or time in a sauna . As such the conclusions cant be extrapolated to mean that when allowed to drink to thirst and are dehydrated they are negatively affecting performance.

    If you were sat in a sauna for 30 minutes then given a diuretic and then ask to ride to exhaustion without being allowed to drink your performance would suffer , however allow that person to start the test hydrated an drink to thirst and performance wont suffer .

    The argument that studies show performance is negatively affected by more than 2% dehydration does not hold up when people are allowed to drink to thirst.

    When allowed to drink to thirst there is no studies that show dehydration negatively affects performance .

    Whether you are an elite or just dipping into trail running makes no difference – drink to thirst and you’ll be fine . Whether thats for beginners doing 1-2 hour trail runs of elites doing Badwater – same advice applies- drink to thirst

    Keep in mind the back of the pack are usually more hydrated often over hydrated cos they tried to replace 100% fluid losses and as such more of a risk for hyponatremia

    If we want to keep it simple and give a basic understanding then the recommendation that we drink to thirst is about as simple and effective as we can get., The rules don’t change for elites to back of the pack – we are all humans and work the same way .

    the simple 101 of hydration should be – drink to thirst . Much simpler than weight before and after etc ( which as Marty has pointed out is a flawed concept anyway )

  14. Jamie Morton

    I really like that you have at least covered this in a systematic way and have mentioned that vests trap heat. I have read many articles cover hydration options including vests and so far yours is the only one that has commented on that as a downside. I do sweat a lot and I live in a warmer climate and to me its a big downside. I bought a great vest (osprey) which I will still use in the colder months, but it just makes me too hot in the warmer months and it seems to me I sweat a lot more because of it which kind of defeats the purpose.

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