How Much Water Runners Really Need

First, a little biology lesson: When you run, you lose fluid, namely water, through sweating and breathing—significantly more through the former. "Heat is a major by-product of your working muscles", says Monique Ryan, RDN, a sports nutritionist with more than 25 years of experience advising professional and endurance athletes and teams. "The more heat they generate, the more your body temperature rises, and the more you'll sweat".

That sweat is the No. 1 way your body regulates its temp, explains Ryan. The evaporation of the liquid is what cools your skin (kind of gross, but a reason to not wipe dampness away, as much as you may want to). You need to either be stocked up on water or bring in fresh water to keep this process intact. "When fluid losses through sweat are not replaced, your body temperature rises further and exercise becomes harder", says Ryan.

To understand why, nerd out with us for a sec. When you sweat, your muscles get less blood than usual, says Ryan. That's because a significant amount of it gets redirected from the working muscles to the skin to assist the sweating process. And with less blood flow, your muscles just can't go as hard for as long, she says. Plus, your heart has to work harder to pump the blood that's left, which can stress your cardiovascular system and crank up your heart rate. All of that can make your usual easy-peasy 5-miler feel like a complete struggle.

Making matters worse? The weather. As the temperature and humidity rise, you can (and likely will) lose even more fluid via sweat. The temp thing is obvious: The hotter you are, the more your body needs to cool itself. With humidity, even at a mild 40 percent, moisture doesn't evaporate as well from your skin. This interferes with your cooling process and leads to, you guessed it, more sweat, says Ian Klein, a specialist in exercise physiology, cross-training and injury prevention at Ohio University.

So, the no-brainer solution to prevent your runs from feeling awful? Always be hydrated, says Brian St. Pierre, RD, the director of nutrition for Precision Nutrition.

The funny thing about hydration is that when you're doing it right, you don't even notice—running feels normal. Dehydration feels like the opposite of that. Your concentration and focus may take a hit, your heart might beat faster than normal and, as mentioned, your run could feel worse than usual, says Ryan.

The longer you go without replenishing your fluids, the more severe the symptoms can get. If you notice that your breathing feels laboured, your pulse is faster than an EDM song and/or you're dizzy, fatigued or even confused, these may be signs of heat stroke, an emergency situation that can cause permanent damage to your brain, heart and internal organs. So, yeah, reach for the clear stuff and avoid at all costs.

You may think being thirsty on a run or during a race is merely annoying. But it can have a real impact on how fast you move and how good you feel.

Research has shown that when you lose more than 2 percent of your body weight to dehydration, it can lead to a drop in endurance performance. (Think there's no way you'd ever get that parched? If you weigh 68kg, that means losing 1.4kg of water weight, which can easily happen on a hot day and/or during a hard run.) Other research shows that dehydration can result in a higher rate of perceived exertion, and that it's a common event. In fact, 70 percent of runners experienced one or more incidents in which they believed dehydration caused a major downfall in their performance, according to research published in the Journal of Athletic Training.

When the amount of water in your body changes, it can also mess with your body's levels of electrolytes and essential minerals, like sodium, potassium, calcium, chloride and magnesium. "You need these electrolytes for your muscles to function properly", says Ryan. They move nutrients into your cells, move waste out of your cells and help regulate nerve function and muscle function, including your heart. All of which is, well, crucial for performance.

While far too little water can clearly kill your runner's high, you shouldn't have to stress too much about your intake. If you drink whenever you're thirsty, you're at least warding off the threat of dehydration.

That said, if you work best with a specific goal, here are some stats that might help you set some targets: A report found that women who were adequately hydrated consumed 2.7 litres of water from all drinks and foods each day, and men averaged 3.7 litres. About 80 percent of that came from water and other beverages, and the other 20 percent came from water-rich foods (fruits and vegetables are an especially good source).

Of course, these numbers are based on the average American. If you're a get-after-it runner and you often go fast, far or both, you likely sweat more, which means you have more fluids to replace.

The idea is to replace that sweat as you lose it. "If you're training for longer than an hour, you should be drinking anywhere from 710ml to 945ml per hour of your run", says Ryan Maciel, RD, the head performance-nutrition coach at Precision Nutrition. That's about 235ml every 15 to 20 minutes. P.S. You want to sip, not chug. Aside from taking in too much water and coughing it back up (sexy), gulping can also cause gastro-intestinal distress (even sexier), says Maciel.

Make sure you rehydrate when you finish your workout too. One easy way to gauge how much to drink is to weigh yourself before and after you've trained for an hour or more, says Maciel. "If you lose half a kilogram, then you should replace that with about 475ml of fluid", he says. If you lose more, drink more during the hours after your run.

In general, the most straightforward way to monitor your hydration status is the age-old pee test, says Maciel. "If your urine is dark, you need to drink more. If it's pale yellow to clear, you're pretty well hydrated".

Relying on public water fountains to stay hydrated during your workout is at best tough and at worst impossible. Gripping a water bottle throughout your run, especially a hot, sweaty run, can be a total drag too. A few good solutions: handheld bottles that fit into a carrier with a band that slides over your hand; bottle belts, which put the weight of the water on your hips; and vests that let you stash bottles on your chest or have bladders with straws on the back. Or if you're in a city, ask a friend (or two) to meet you with water somewhere along your route.

Keep your form in check by, every now and then, switching the hand that's holding the bottle, the side that the bottle is stashed on in your belt, or if a belt has multiple bottles, taking sips from each one instead of downing one at a time. As for hydration bladders, fill them with only as much water as you need for your workout, not necessarily to the brim.

Running for less than two hours? Hydrate with just water, recommends Maciel. If you're going to be out on the road or trail longer than that, or if it's hot as hell and you're sweating a lot, you need to replace your lost electrolytes along the way, he says.

To get those electrolytes, some runners like to sip sports drinks, while others prefer gels or chews. There isn't one best method, so experiment with a few different products to see what works for you. Just remember that more isn't more when it comes to these supplements.

To get the most out of your training, you should be hydrating well all the time, and especially in the weeks leading up to your event, says Maciel. "Chugging water the night before a race is not going to make up for not hydrating enough all those other days", he says. (Plus, it might land you in a portaloo at every mile marker.) If you trained while slightly dehydrated, you just won't have the fitness you could have had if you'd taken in more fluids.

How much you'll need to drink along the course depends on the length of the race. If you'll be running for less than an hour—say, a 10K or 5K—you likely won't need to hydrate during the run. For anything longer, fall back on your hydration protocol for long runs (aiming to sip 710ml to 945ml each hour), says Maciel.

Races typically have hydration stations along the course so you don't have to stress about carrying fluid with you (many races don't even let you bring your own), and most offer water and a sports drink. Look at the course map ahead of time to see how far apart those stations are and whether the event separates them by miles or kilometres (5 miles apart is a lot further than 5K!), then plan your hydration strategy accordingly. Oh, and do yourself a favour and don't drink anything you didn't try (and feel good about) during your training.

In any event—training run, daily jog or big race—make this your new mantra: A hydrated run is a happy run.