You work out, you sweat, you drink. Simple, right? Well, on the surface, yes. (Ha, see what I did there? Sweat – surface…) The act of sweating triggers a chain reaction that affects your entire system, and it requires conscious maintenance.

About 60% of the human body is water. While water is found in tissues from your brain to your toes, the majority of the body’s water is contained in muscle tissue and blood volume. That percentage of water varies based on life stage (babies are born at more than 75%) and amount of fat mass. Fat tissue contains only about 10% water, whereas muscle tissue can be up to 75% water. A well-trained athlete’s body can be upward of 70% water!

The basic functions of breathing, digestion, and waste elimination all cause water loss, and we require constant rehydration. When we work out, water becomes even more important. The burning of calories by muscle tissue generates heat, and the body has to cool itself down. Water molecules have a high heat capacity, meaning they can carry large amounts of heat, making them ideal for cooling us down. The circulatory system allows them to carry heat to just below the surface of the skin where it is released through sweat. The sweat evaporates, and the blood just below the surface of the skin is cooled. Voila, your body has regulated its temperature! Magic.

But what happens when there isn’t an adequate amount of water available to keep the body cool? When you’re dehydrated, blood volume drops. Your heart can’t move as much blood with each beat, which means less oxygen gets to your muscles. Waste products build up, and your core temperature rises. To keep up with the workload, the heart has to beat faster. Your heart rate can increase by as much as 7 beats per minute for each 1% of water weight lost. This is not good news for endurance stamina. Dehydration can be the main limiting factor in training and race success, well before fuel becomes the issue.

Dehydration doesn’t discriminate, either. It doesn’t matter how well trained an athlete is. Studies have shown that regardless of fitness level, athletes slow down by about 2% for each 1% loss in body weight due to dehydration. To put that into perspective, a 165-pound athlete sweating off just 3.3 pounds (2% of his body weight) would slow down by 4% during a heavy training session. At an 8-minute per mile pace, this translates into a full 20 extra seconds per mile.

Ok, so what do you do?

Don’t get dehydrated. Easy, right? Wrong. Most people walk around constantly under-hydrated. Athletes are no exception – and many athletes are actually the worst culprits. I regularly cross paths with the same runners on my long runs, and I’ll see them putting in 10, 15 miles without fluids (even in the summer, here in the South). It’s just a very bad idea.

But let’s back up for a minute and address every day needs. The Adequate Intake (AI) daily fluid recommendation for adults is 25-35ml per kg of body weight. To calculate your needs, follow these steps:

1. Take your body weight in pounds and divide it by 2.2. This gives you your weight in kilograms. Here’s an example using the 165-pound male mentioned above: 165lb/2.2 = 75kg
2. Multiply your weight in kg by 25 and 35. Example: 75kg x 25ml = 1875ml and 75kg x 35ml = 2625ml.
3. A US fluid ounce contains approximately 30 milliliters. Divide your milliliter results by 30 to get ounces. Example: 1875ml/30 = 62.5oz and 2625ml/30 = 87.5oz.

In our example of the 165-pound athlete, his daily fluid needs are between 62.5 and 87.5oz. Keep in mind that this is his daily need, without considering training. (While food intake contributes to fluid needs, it is important to remember that most of this volume will not be met without conscious fluid intake.)

Now let’s consider training.

Assuming you’re meeting your normal daily needs, let’s consider what you need to add to meet your training needs. (I’m sticking to the basics here, because things can get complicated and quite detailed when it comes to fluid needs.)

Pre-Training
Most athletes think about fueling before an event or training session, but pre-hydrating is just as important. A general guideline is to drink 17-20oz of fluid 2 to 3 hours before training, and an additional 7-10oz 10 to 20 minutes before training. Obviously if you’re a morning athlete, this can become a problem. If getting up earlier isn’t an option, make sure you are well-hydrated before going to bed. Your urine should be very light in color, and you should have no thirst.

During Training
Plain water is fine for training or events lasting up to an hour, but anything longer than that (or high intensity sessions under an hour) requires electrolyte replacement. Sodium, chloride, and potassium are the three we need to worry about with exercise. They’re responsible for muscle contraction (including your heart beat), proper nerve impulses, fluid balance, and more.

• There are plenty of electrolyte tablets on the market, so try a few to find one you like. I personally am a fan of Nuun™, specifically Nuun™ Energy which also contains caffeine (check out my post on caffeine and endurance performance).
• Sports drinks like Gatorade™ and Powerade™ are also an option for electrolyte replenishment, and they provide the added benefit of carbohydrate energy. Carbohydrate isn’t necessary for all training sessions or events (there’ll be another blog post soon on fueling), nor does everyone tolerate sports drinks well during activity. Again, experiment.
• How much you should drink during activity depends on your individual need. Instead of worrying about ounces, think of gulps. You should be taking in 4-8 gulps of liquid every 15 minutes of activity. Chances are that’s significantly more than you’re doing, which is true for most of us. You’ll know if you’re taking in enough if your pre-activity weight and post-activity weight (both naked) don’t vary by more than a few pounds.

Post-Training
It’s important to continue to hydrate well for the rest of the day after a training session or a race. A general rule of thumb is to replace each pound of body weight lost (again compare naked pre- and post-activity weight) with 16-24oz of fluids.

Put some strategies into place for getting it right.

Ideally we’d all drink plenty of fluid every day, naturally, on our own. Unfortunately most of us don’t. So I suggest you put some strategies into place around hydration, just like you do for the rest of your training:

• Drink all day, every day, and don’t wait for thirst.
• Alcohol and coffee don’t count. (I know, you saw “drink all day!” and got all excited.) Both are diuretics and will dehydrate, not hydrate.
• Take a look at your pee. If it’s not very light yellow or has a strong odor, you’re not getting enough fluids.
• Train your body for hydration. You drink a lot of fluids, you have to pee a lot. But just like you can train your body for speed or endurance or tolerating fuel during activity, you can train it to tolerate proper hydration before and during events.
• Experiment with your hydration strategy from the start of your training cycle. Don’t wait until a week before your race to figure out when/how you’ll get your fluids in.
• If you don’t like plain water, flavor it. There are dozens of flavor options on the market.
• Sugary drinks, juices, and sodas are a bad idea during training and events. Gastric emptying gets wonky, carbonation can lead to cramping, and you can end up with more than you bargained for (think nausea, diarrhea…). If you are comfortable with sports drinks during endurance events, go ahead, but make sure you know your carbohydrate intake if you’re also using gels, chews, or fruit/pretzels during your event. (Again, beyond the scope of this particular post.)

You’re likely putting in a lot of early and/or late hours of training. You’re probably (hopefully!) paying some attention to what you’re eating to support all of that training. Don’t let what you’re drinking – or not drinking – be the weak link on race day. Bottoms up!

 
References

1. Board FA, Medicine IO. Dietary Reference Intakes:, The Essential Guide to Nutrient Requirements. National Academies Press; 2006.
2. Eberle SG. Endurance Sports Nutrition, 3E. Human Kinetics; 2013.
3. Skolnik H, Chernus A. Nutrient Timing for Peak Performance. Human Kinetics Publishers; 2010.
4. https://water.usgs.gov