As you gear up for summer training and racing, don’t let the heat get you down! Running in the heat comes with its own challenges – it feels harder, you’re sweating more, and hydration becomes all the more important. Read on to learn more about the risks of dehydration and overhydration (spoiler: neither is good), find out what the heck an electrolyte even is, and understand how to make your own hydration plan so you can stop guessing and start running smarter.
When you run, your body heats up and produces sweat. As sweat evaporates from your skin, it cools you down and helps to regulate your core temperature. When it’s hot outside, your body has to sweat even more to stay cool. This loss of sweat can lead to dehydration… and on particularly hot days, this can happen very quickly. If you don’t replenish the fluids and electrolytes you’ve lost, your body struggles to function properly and can start to shut down.
If you’ve ever been dehydrated on a run before, you know that it is no fun. Dehydration can lead to muscle cramps, fatigue, increased heart rate and rate of perceived exertion (RPE), and slower run times. And these changes can happen after only a 2% loss of body weight through sweat – which can take just 60-90 minutes.1 Severe dehydration can result in kidney failure, seizures, and even death, and can contribute to rhabdomyolysis. On the other hand, proper hydration improves endurance, strength, and recovery – and it feels way better to run if you’re well-hydrated.
Hydration for runners comes down to two main ingredients:
💦 Fluids: Water, juice, tea, and coffee – plus hydrating fruits and veggies like watermelon – all contribute to your fluid needs. Just be sure to limit caffeine intake to about 400 mg per day (the amount found in 3-4 cups of coffee).2
⚡️ Electrolytes: Scientifically, electrolytes are substances that conduct electricity. Practically, they are minerals that perform important roles in the body, such as supporting the regulation of fluid balance, muscle contractions, and nerve transmission. They also help your body hold onto fluids – so if you’ve only had plain water (without electrolytes) before a morning run, you might find yourself making more pit stops than usual.
Sodium is the primary electrolyte lost in sweat, which is why it’s usually found in higher doses in supplements than are other electrolytes. (It’s also why you’ll see so many sodium-related recommendations in this article.) In order from highest to lowest concentration in sweat, the electrolytes are:
Sodium
Chloride
Potassium
Calcium
Magnesium
How Should I Hydrate?
Hydration needs are highly individual. We all know that when it’s hot, we sweat more – but sweat rate (and therefore hydration needs) is also impacted by exercise intensity and duration, genetics, and, to a certain extent, diet.3 The following recommendations are meant to provide a starting point for you in determining your own hydration needs.
Here are some general guidelines to keep you hydrated and feeling fresh for your next run:
Before a run: Don’t start your run as a raisin. Drink 16 oz of fluid about 30 minutes before heading out for a run.1 If you’re a morning runner, you may need even more. And remember that point about electrolytes helping your body retain water? A little sodium in your drink (about 250-500 mg5), especially first thing in the morning, will help you hold onto those fluids and avoid mid-run bathroom breaks. You can thank me later.
During a run: The primary goal of hydrating during a run is to avoid losing more than 2% of your body weight from sweat (the point where performance starts to decline). Most athletes lose between ½ and 2 liters (16 and 64 oz) of sweat per hour of exercise.4,5 This adds up quickly, especially over an ultra distance. The American College of Sports Medicine recommends drinking about 400-800 mL (13-27 oz) of fluid per hour.5 Pace yourself and drink a little bit every 15-20 minutes.
You can calculate your own sweat rate by weighing yourself before and after a run. Every pound you lose equates to 16 ounces lost in sweat. (If you eat or drink anything or urinate during your run, these numbers need to be accounted for.) I recommend repeating this exercise multiple times, in different weather conditions and at different intensities, to better estimate your needs on race day.
What about electrolytes? One liter (34 oz) of sweat typically contains about 250-1500 mg of sodium. (For comparison, the same liter of sweat may contain only 10-15 mg of magnesium.)5 You probably don’t need to take in enough electrolytes to make up for all that you lose in sweat. A good starting point is to consume about 300-600 mg of sodium per hour.1,6 Newer research suggests that unless you are a particularly salty or heavy sweater, replacing just 50-80% of your sodium losses may be enough7… though more is still needed to fully understand this.
After a run: Along with protein and carbs, hydration should be part of your recovery strategy. Drink 16-24 ounces of fluid for every pound lost (the extra water is to account for increased urine output when drinking a lot at once).5 The American College of Sports Medicine recommends including 1000-2000 mg of sodium per liter of water to replenish losses after a run.8 Don’t neglect the carbs here either! In addition to supporting muscle recovery, carbs help your body absorb both water and sodium.6
Hydration matters when you’re not running, too. You may have heard that you need 8 cups of water per day to stay hydrated, but this recommendation actually underestimates needs for most people. The Institute of Medicine (IOM) recommends about 15.5 cups for men and 11.5 cups for women.9 If that sounds like a lot, don’t forget that water found in fruits and vegetables contributes toward this total.
The IOM also recommends 2300 mg per day of sodium for men and 1500 mg per day for women,9 though these recommendations may undershoot actual needs. Some research suggests that low intakes of sodium may be linked to higher rates of mortality, with the lowest mortality rates found among people consuming about 4000-5000 mg per day.10 Athletes especially need more sodium on a day-to-day basis compared with the general population to make up for sodium losses from sweat and should use salt liberally. As always, talk with your doctor about any health concerns before making any dietary changes.
At the end of the day, the best way to figure out how hydrated you are is by checking the color of your urine. Pale yellow urine means you’re good to go. Darker shades signal dehydration and the need for more fluids and electrolytes. Clear urine, on the other hand, could mean you’re over-hydrated and should back off on fluid intake.
Is It Possible to Over-Hydrate?
The reality is, drinking enough water is harder than it sounds. But while dehydration is more common, overhydration can happen too. Symptoms of overhydration include nausea and vomiting, muscle cramps, and headaches – all things you may have experienced in an ultramarathon! This can make it tricky to pinpoint the true cause. Is your headache from over-hydrating, under-fueling, or something else entirely?
But finding the answer is important because overhydration can be really serious. One of the most dangerous potential outcomes is hyponatremia, a condition that occurs when your kidneys are overwhelmed by water and can’t excrete the excess. This leads to a dilution of your blood which can be deadly. The primary cause of hyponatremia is simply drinking more water than your body needs… but some evidence suggests that taking in adequate sodium can help reduce the risk.11
Notably, hyponatremia is more common in ultramarathons than marathons.11 The longer you’re running, the more opportunity you have to drink more water than your kidneys can process. However, by paying attention to your fluid and electrolyte intake – and following a hydration strategy rather than just winging it – you can reduce your risk of hyponatremia and keep your body in balance.
The Bottom Line
There is no one-size-fits-all when it comes to hydration. Take the time to understand your body’s unique needs – by determining your sweat rate and experimenting with fluids and electrolytes to find what works for you. Use your data and experience to create a hydration plan… and then actually follow it. Whether you’re toeing the line for your first half marathon or your tenth 100 miler, hydrating properly before, during, and after your run can make all the difference. Practice your fluid and electrolyte plan on and off the trails, so when the time comes, you’re ready to run at your full potential.
Kerksick CM, Wilborn CD, Roberts MD, et al. ISSN exercise & sports nutrition review update: research & recommendations. J Int Soc Sports Nutr. 2018;15:38.
Wikoff D, Welsh BT, Henderson R, et al. Systematic review of the potential adverse effects of caffeine consumption in healthy adults, pregnant women, adolescents, and children. Food Chem Toxicol. 2017;109(1):585-648.
McCubbin AJ, Lopez MB, Cox GR, Odgers JNC, Costa RJS. Impact of 3-day high and low dietary sodium intake on sodium status in response to exertional-heat stress: a double-blind randomized control trial. Eur J Appl Physiol. 2019;119(9):2105-2118.
Maughan RJ, Noakes TD. Fluid replacement and exercise stress. A brief review of studies on fluid replacement and some guidelines for the athlete. Sports Med. 1991;12(1):16-31.
Sawka MN, Burke LM, Eichner ER, Maughan RJ, Montain SJ, Stachenfeld NS. American college of sports medicine position stand. Exercise and fluid replacement. Med Sci Sports Exerc. 2007;39(2):377-390.
Rehrer NJ. Fluid and electrolyte balance in ultra-endurance sport. Sports Med. 2012;31:701-715.
McCubbin AJ. Modelling sodium requirements of athletes across a variety of exercise scenarios – identifying when to test and target, or season to taste. Eur J Sports Sci. 2023;23(6):992-1000.
Casa DJ, Clarkson PM, Roberts WO. American college of sports medicine roundtable on hydration and physical activity: consensus statements. Curr Sports Med Rep. 2005;4(3):115-127.
Institute of Medicine. 2004. Dietary reference intakes for water, potassium, sodium, chloride, and sulfate. Washington, DC: National Academies Press.
Graudal N, Jürgens G, Baslund B, Alderman MH. Compared with usual sodium intake, low- and excessive-sodium diets are associated with increased mortality: a meta-analysis. Am J Hypertens. 2014;27(9):1129-37.
Veniamakiis E, Kaplanis G, Voulgaris P, Nikolaidis PT. Effects of sodium intake on health and performance in endurance and ultra-endurance sports. Int J Environ Res Public Health. 2022;19(6):3651.
