It’s no fun. It can feel awful. But many of us runners have been there – having GI distress on race day. We work so hard to train and then get to race day and might experience nausea, vomiting, diarrhea and cramping. What gives? There are many reasons why we might experience GI issues on race day, and we will discuss some of the reasons that I see most with clients who come to me looking for help. Take a look below to see what might be causing your race day GI upset and what you can do to avoid it.

The Food You've Consumed in Previous Days

If you know me <The Bagel Queen>, you know that I strongly encourage carb loading before race day. Carb loads can be 1-3 days <or more in some cases>, and the goal of a carb load is to increase your carb intake <check out our carb load calculator> to stock your glycogen stores for race day.

If you are a newbie carb loader, or are unaware of how other ingredients in those foods might affect you, you may need some help choosing your carb-tastic foods.

Early in the carb load you may be able to tolerate more fiber and/or fat in those foods <whole grain bread, veggies, and fruit> you may want to start to choose lower fat & lower fiber foods as you get closer to race day. Simple carbohydrates that are low in fiber and fat <white bread, white bagels, pretzels, juice, etc> will help decrease the risk of GI distress on race day. Although, I do have some athletes who have guts of steel that can tolerate all the different foods pre-race, many of us need to be intentional about what foods we consume in larger quantities.

What you can do: Choose low fiber & low fat foods in the 1-3 days leading up to your race.


One of the biggest issues I see causing my client’s GI distress is dehydration. Dehydration can be caused by going into a run dehydrated, not drinking enough fluid during runs, and/or not consuming adequate sodium. If you are a race-day water-only drinker, you may need to consume more electrolytes. If you are a heavy and/or salty sweater, you may need much more fluid and sodium than you think.

Knowing your sweat rate <amount of sweat per hour> and sweat composition <amount of electrolytes in your sweat> will help you understand what your needs are in order to create a race day hydration plan that works for you. Head over to our Hydration page for more information & a general place to start.

What you can do: Start exercise well-hydrated and look into your sweat rate & sweat composition to help meet your hydration needs <we love the hDrop wearable hydration monitor!>

Sport Drink & Fuel Concentration

Your choice of race-day fuel & sports drinks can contribute to GI distress on race-day if you aren’t taking them properly. Too much sugar in your GI tract without adequate liquid can cause nausea, cramping, and diarrhea. Most gels require 2-4 oz of water or sports drink to be consumed at the same time to aid in digestion and absorption. Some newer gels <Maurten, Never Second> do not require fluid, due to their specific formulations.

You also want to make sure that you mix your drinks carefully to avoid over-concentrating. Some sports drinks are now made with certain carbohydrates that are able to be concentrated in higher amounts without causing GI distress <like Skratch Clear Hydration>.

We used to think that gels had to be taken strictly with water, but newer research has shown that it’s ok to mix some types of gels & sports drinks, depending on carbohydrate concentration and osmolality.

What you can do: Know if you need fluid with your gels & mix sports drinks according to the instructions, trial & error, and practice.

Core Body Temperature

When core body temperature increases, your body will pull blood flow away from your gut & muscles to cool the skin, which decreases gastric emptying and absorption of nutrients. Core body temperature will likely increase as the length of your endurance race increases – so that’s one of the reasons why I recommend to front-load your nutrition <get it in while you can>. Core body temp will also rise with increased temperature and humidity.

What you can do: dress appropriately for the weather, hydrate well, front load nutrition, ice or cool water in your hat, or down your shirt to keep you cool.

Rate of Perceived Exertion 

As your rate of perceived exertion (RPE) increases, gastric emptying and absorption of nutrition decreases. Most times, you will feel this later in a race, but sometimes, depending on other factors <temperature, wind, illness, etc.> this might be an issue from the start line. Know your body and know if you need to slow it down for the race to keep RPE lower. This is another big reason that I suggest front-loading your nutrition & carb loading – fuel when you can so you have stores left at the end to fuel your race if you are unable to take in more gels.

What you can do: be aware of your RPE and adjust pace if needed, front load nutrition, and practice your race day fueling & hydration plan during training.

Sources: PMID: 2287256, 28919842, 25997181, 17465596

If you need help figuring out what is causing your race day GI issues, apply for 1-on-1 nutrition coaching or Last Minute Crunch Time Session with Meghann here.

Disclaimer: The content in our blog articles provides generalized nutrition guidance. The information above may not apply to everyone. For personalized recommendations, please reach out to your sports dietitian. Individuals who may chose to implement nutrition changes agree that Featherstone Nutrition is not responsible for any injury, damage or loss related to those changes or participation.