Last month we focused on nutrition for training and racing. This month, we dive deeper into hydration as you prepare for the Chicago Marathon. Proper hydration is not only important for year-round training, but especially during the hot summer months when it is easy to lose fluids through perspiration as the body works to cool itself.

A question I am often asked, especially when an athlete struggles with fatigue and cramping during runs, is: “How much fluid should I be taking in?” Hydration, just like caloric intake, is specific to the individual. Below is a general list of guidelines to follow as you train this summer and into the fall leading up to the marathon.

Hydrate Before, During, and After a Workout

Seems simple enough, right? Amazingly, many runners will head out for runs lasting over an hour without drinking anything beforehand or during because they don’t want to carry extra “weight.”

However, think of your body like a nice, juicy steak (apologies to the vegetarians for this visual). A well-hydrated steak looks supple and full of potential, while a dried-out steak appears overdone, shriveled, and worn out. Our “steaks” — the muscles in our arms, legs, and even heart — need to be well-hydrated to function to their full potential.

A good rule of thumb: for runs longer than 90 minutes, you should feel the need to pee at least once. If you make it through the run without stopping, you should need to go as soon as you finish. Observe your urine color post-run.

• Light yellow: Yay! You’ve been hydrating properly.
• Dark yellow: A sign of dehydration.
• Tea or brown-colored urine: This may indicate rhabdomyolysis (rhabdo), a condition where muscle breaks down from intense exercise and releases myoglobin into the bloodstream. Seek medical attention immediately, as the kidneys can be affected.

Determine Your Personal Sweat Rate

Each person is different in how much fluid they need to replace what’s lost during exercise. You can calculate your sweat rate at home using the following steps:

1. Pee, then weigh yourself (preferably without clothes). Convert pounds to kilograms (divide pounds by 2.2).
2. Perform your exercise session and record exactly how much you drank. If you use bottles, weigh them before and after. Subtract to find the difference. (1 gram = 1 milliliter).
3. After exercise, towel dry and weigh yourself again (without clothes).
   • Weight lost = weight pre-exercise – weight post-exercise
4. Weigh your hydration bottle again to find the volume consumed.
   • Volume consumed = bottle weight pre-exercise – bottle weight post-exercise
5. Calculate sweat rate:
   • Sweat rate = (weight lost + volume consumed) / 60 (for a 60-minute session).

Repeat this test a couple of times and keep a log, as sweat rate can vary based on temperature, humidity, clothing, exercise intensity, fitness level, and heat acclimatization.

Sweat Salt Concentration

Some people are heavy salt sweaters, while others are not. Heavy salt loss is often visible as white, salty stains on shirts and hats after exercise. (Note: in drier climates, salt is more visible than in humid environments.)

Most people lose about 1 gram of sodium per hour of sweat-inducing activity. When sodium levels drop and aren’t replaced, athletes may experience brain fog, headaches, muscle weakness, and cramps. Some endurance athletes, especially those with a history of dizziness or cramping, seek medical testing to determine how much salt they lose and how much they need to supplement during activity. Others determine their needs through trial and error during training.

The more you practice, the better prepared you will be on race day — no matter the conditions.

Panic Hydrating vs. Pre-Hydrating

Some athletes wait until the day before the race to “load up” on water. This is a recipe for disaster, as it can flush out valuable minerals, salts, and electrolytes the body needs to function properly.

On the flip side, some athletes are so busy with pre-race activities, work, and travel that they forget to hydrate the day before. The key is to practice in training exactly how you will hydrate before racing.

For long training runs, pay attention to what you drink the day before. Personally, I carry two bottles with me the day prior:

• one with plain water
• one with an electrolyte drink similar to what I’ll use on race day.

I alternate sipping both throughout the day. If it’s extremely warm or humid, I may add an extra bottle of electrolyte mix or a container of coconut water. Coconut water is high in potassium, helps lower blood pressure, supports hydration, and aids muscle function. It also contains magnesium, calcium, and phosphorus, which are important electrolytes lost in sweat.

Note: Coconut water does not contain sodium, so do not rely on it alone for pre-hydration — especially if you’ll be running in humid conditions.

By understanding your hydration needs and practicing your plan in training, you’ll be fully prepared to tackle the Chicago Marathon strong and safe!