By Chris Newport

Signing up for any endurance event is a huge commitment requiring a lot of time, energy, and money. Most athletes think about finding the right gear, carving out time to train, or even hiring a coach. But they often leave fueling and hydration planning to the very last minute. Race day should be spent celebrating your athletic accomplishments not heaving on the side of the road or hanging out in the port-a-john. To avoid your gut going on strike, give it a little training and attention; avoid these common mistakes to perform your best.

Not knowing how much fluid to drink

Sweating is your body’s natural way of cooling itself. Some athletes can wring out their clothes after a workout while others barely glisten. Knowing how much to drink is important to avoid a sloshy stomach, dehydration and even more serious, hyponatremia. It is ok to naturally lose some fluid, however, performance suffers after losing about 2% of your body weight in sweat. On the other hand, it’s dangerous to drink more fluid than you lose, which can send you to the hospital.

How do you know how much to drink? Do your own fluid test by weighing before and after a workout. Be sure to empty your bladder, strip down, and weigh yourself. Note the temperature and humidity then perform a 30-minute workout at race pace without fluids. Afterwards, dry off, strip down and weigh yourself again. Subtract your final weight from your starting weight and multiply by 16 to get the number of ounces lost. Multiply that by 2 and you’ll get your projected fluid loss per hour. Do not drink more than that amount per hour. Keep in mind the gut can only absorb about 32-43oz per hour (less in the heat). So if you’ve lost more than that, then you’ll have to practice during training with cooling methods (ice in your hat, white clothing, etc.) while drinking enough fluid.

Getting the electrolytes wrong

The main electrolyte in your sweat is sodium, but you also lose potassium, magnesium, calcium, and chloride, along with other trace minerals. In fact, your sweat is as unique as your fingerprint. If you’re someone who looks like a salty ghost attacked you during your workout, you should consider sweat testing. In a painless procedure, we can measure the amount of electrolytes lost in your sweat.

For example, we’ve had people lose well over 4000mg of sodium per hour while others only lose 300mg. There’s no telling how much you lose unless you get tested. If you’re guessing at the electrolytes you need (and guessing wrong), you can experience nausea, fatigue, cramping, dizziness, dehydration, or hyponatremia. When using an electrolyte replacement product, be sure it contains multiple electrolytes.

Trying new stuff on race day

“Never do anything different on race day.” This phrase is worth repeating. From the pre-race dinner, race morning breakfast, to on-course drinks and snacks, if you’ve not tested it in training, don’t take a chance. If you’re traveling to a race, find or bring familiar foods with you in the days before and on race day. Figure out your fueling and hydration logistics beforehand and practice in training. Scope out the aid stations and know what each race provides for on-course nutrition. For example, if you trained with a hydration belt, use it on race day. If you never tried a chocolate gel, don’t start on race day. Introducing your gut to unfamiliar foods can lead to disaster; stick to what you know!


While many athletes live by the mantra “eat to train and train to eat,” racing is not the time to stuff your face. The faster you go the less digesting your body does. So if you’re eating sandwiches, pretzels, gels, chews and everything else under the sun, chances are it will sit in your stomach instead of going to working muscles. The only way to speed digestion is to slow down or stop. Plus, if you’re slamming gels and sports drinks, your stomach has another reason to revolt. Your body does much better when you feed it small amounts of carbohydrates over time rather than larges amounts all at once. For shorter events (less than 2 hours), go for liquids. Longer races may warrant solids, depending on your pace. And remember, practice eating and drinking those foods and/or drinks in training at the same paces that you’ll be racing.

Create your fueling and hydration strategy by experimenting with training your gut while you train your fitness. If you run into trouble, consult with a sports dietitian and you’ll be ready to rock your race day!

Chris Newport, MS, RDN/LDN, EP-C, CISSN is a Registered Dietitian, Certified Sports Nutritionist, USAT Coach, and exercise physiologist. She founded The Endurance Edge, an integrative coaching center in Cary, NC offering endurance coaching, metabolic testing, sweat testing, nutrition, massage, physical therapy, sport psychology, and more. Get your free hydration guide online or book your sweat test at Find her on Twitter @CoachChris_RD, or YouTube, Instagram or Facebook @TheEnduranceEdge.