When preparing for a goal marathon, runners are often hyper-focused on nailing down their nutrition in terms of carb-loading the night before, eating a solid breakfast that won’t cause gastrointestinal issues, and figuring out what gels or chews will work to keep them from bonking before they reach the finish line. But something that’s equally important is nailing down your marathon hydration plan. This goes beyond just making sure to drink the on-course water and electrolyte solution offered, as hydration needs vary greatly by individual.

Figuring out a race weekend hydration strategy should be something you pinpoint well in advance — if you’re training for a spring marathon like the Sanlam Cape Town Marathon, you should already be testing your hydration plan — and it starts with determining your general hydration needs.

As a minimum baseline, the Institute of Medicine (IOM) advises that adequate total daily water intake (from fluids and water-containing foods) for men and women ages 19 to 50 years is approximately 3.7 litres and 2.7 litres per day, respectively.

In terms of what you need once you add regular intense exercise to the mix, the American College of Sports Medicine recommends including about 1 to 2 millilitres per kilogram of body weight in the two to four hours before exercise. For a 68-kilogram athlete, a 600ml- to 1-litre bottle of water and/or sports drink would suffice.

“If you’re not sure if you’re drinking enough, two hours prior to exercise, check your urine and if it is dark or you don’t produce much urine, drink another 3 to 5 millilitres per 1-kilogram body weight,” says Dr Lindsay Bakera, a hydration researcher.

Drinking the recommended amount of fluid in those two to four hours before exercise helps allow urine colour to reach a pale yellow and allows time for you to excrete any extra fluids before you start running.

“When we think of hydration, we want to have our daily hydration needs met, but then we also want to look at it from that performance standpoint as well,” adds Meghann Featherstun, sports dietitian and owner of Featherstone Nutrition. “Many runners end up going into races dehydrated because they aren’t aware of how much fluid and electrolytes they need to be replacing, and then end up being unable to recoup that, no matter how much they drink while they’re out there.”

Why Hydration Is So Important for Marathons
According to Starla Garcia, a registered dietitian and owner of The Healthy Shine, dialling in your specific fluid needs for a marathon is important because inadequate hydration can negatively affect your performance by increasing fatigue, decreasing cognitive function and potentially leading to muscle cramping. It’s especially crucial if you’re racing in a warmer climate (or one that is experiencing warmer-than-usual conditions on race day).

“Even a 1% loss of body weight during training makes your heart work harder by beating three to five times faster per minute,” Garcia explains. And even mild dehydration of 2 to 3% bodyweight loss can significantly impair performance.

How to Hydrate the Day Before a Marathon
How much fluid you need for a long endurance event like a marathon also depends on how much you tend to sweat, which can vary greatly by individual, depending on factors such as weather conditions, heat acclimation status, running speed and your bodyweight and genetics, Baker says.

To figure out your sweat rate, Featherstun recommends weighing yourself before and after a run. For every 500g you’ve lost, you’ve lost about 400g of sweat and you should aim to add that to your overall hydration goals for the day.

Consuming sodium will also help you retain fluid, which is where sports drinks and electrolyte solutions come into play. Garcia recommends electrolyte solutions with 300 to 500 milligrams of sodium mixed with water.  Start some trial and error early on in your training cycle to figure out what sits well in your stomach and meets your flavour preferences.

Hydrating the day before a race really shouldn’t differ much from any other day. Baker recommends aiming for that minimum baseline need (about 3.5L for men and 2.5L for women), plus how much you tend to lose by sweating, especially if you’re exercising or doing a pre-race shakeout run, as many marathoners tend to do. If you’re going to be on your feet while exploring a race expo or other events, you’ll want to make sure you’re on top of your fluid needs as well.

If a race is going to be slightly warmer than what you’re used to, you can implement a hyperhydration strategy, in which you consume an electrolyte solution with higher sodium content. This will help keep your body hydrated to the point that it can hang on to extra fluid and sodium to better tolerate those conditions, Featherstun says.

How to Hydrate the Morning of the Marathon
Basically, drinking is the first thing you should do upon waking up on marathon morning. According to Featherstun, you should start your morning by drinking about 500ml of your favourite sports drink with whatever you’re eating two hours before the race.

If your race starts on the later side, as World Marathon Majors like Boston and New York City tend to, you’ll want to start with 500ml four hours before race start, and then again with two hours to go, for 1L in total.

If you read 1L and instantly fear you’ll be beelining for the bathroom shortly after you cross the starting line, you’re not alone. However, making sure you’re sipping and not chugging your fluids should help with that, Featherstun says, along with making sure you’re finished and cut yourself off of fluids with about two hours to go before the start.

“Hopefully you won’t have to stop during the race, but that two hours before is usually pretty key [in terms of overall preparation],” she says.

How to Hydrate During Your Race
Another aspect of figuring out your mid-race hydration plan is determining if you’re going to go with the on-course offerings of water and sports drink, or if you’ll carry what you trained with in a handheld bottle or other hydration device. To figure that out, research exactly what sports drink will be on the course and try that out early on in your training cycle to see if it agrees with you.

If you know you don’t want to rely on the on-course offerings, Garcia recommends choosing a product that has at least 500 to 700 milligrams of sodium per serving.  If you have another preferred brand that doesn’t fall within that range for one serving, you can double up to meet those needs, she says. Just practice this in training to ensure it doesn’t upset your stomach and isn’t overly sweet for your liking.

Additionally, while electrolytes help replenish what’s lost via sweat, carbohydrates are the primary source of fuel for exercising muscles, Baker adds. This is why it’s important to also take in fuel in the form of energy gels during your races to avoid the dreaded crash or “bonk” that can make your race fall apart.

However, if you find you generally can’t tolerate gels or chews, or have experienced dehydration symptoms, such as nausea, that affect your ability to get them down, you can opt for a drink or drink mix with a higher carbohydrate content,  killing two birds with one stone, Garcia says.

“Typically, when you go for longer periods of time, your bloodstream is no longer going to your gut but rather to your muscles, so that’s where a lot of people start to get GI issues,” Garcia explains. Consuming carbs in liquid form may help with this.

If you’re a heavy sweater, you may need to increase fluid intake and sodium concentration beyond what the Institute of Medicine suggests — which is about 400 to 800 millilitres per hour and 450 to 700 milligrams of sodium per litre — during running. Also, consider hyperhydrating before the event with products with higher sodium content.

Hydration needs during a marathon are based on each runner’s sweat rate and weather, Featherstun says. “How much fluid you consume [during the race] can vary from person to person, and you definitely don’t want to overdo it in terms of volume,” she says.

In general, you want to take some fluid with your fuel on course, as well as a cup as needed at aid stations, Featherstun says. A good rule thumb is to take at least a few sips at each aid station, or about 120ml to 180ml per hour.

"We can really only tolerate about 700ml of fluid an hour when running, so we would never aim for more than that,” Featherstun says. “Some people can take sips of fluid throughout [a run] and be totally fine if they’re not super sweaty, while others might need to try to get 500 ml an hour and refill a handheld or stop at every station and take two cups.”

If you have a high sweat rate, you may find it difficult to replace close to 100% of your sweat losses by drinking fluids, but don’t panic. The key: As long as you’re drinking enough to avoid dehydration (and that 2- to 3% bodyweight loss during the run), you should be able to maintain performance.

Finally, if you don’t want to carry your own fluids and aren’t confident you’ll mesh well with the sports drink on the course, it is possible to hydrate sufficiently with water and salt tablets, along with taking gels that are higher in sodium and other electrolytes, Featherstun says. That means skipping the sports drinks and just drinking the plain water on the course, plus carrying your own carb fuel in the form of gels or gummies. And remember: If you do have gels and gummies, make sure to sip water with them to help with digestion.