For many runners, nutrition is a major source of race day stress. Understanding how the GI(gastrointestinal) system works is the key to preventing race day discomfort. During a long event, your body uses up a lot(if not all) of your glycogen stores to provide your body with energy. As your reserves get low, you will grow tired. This is why you have to take in carbs during a long event. It is important to note that the GI system is trainable. This is one more good reason to practice long runs with the nutrition plan that you will use on race day. If you had one bad experience with a certain food it is not the end all be all, you just have to do some work. GI problems are more likely to occur with a food that is high in fiber or fat. If you are concerned that you are going to have stomach upset, try a variety of carbs like fructose dang glucose together. Each type of carb that you ingest uses a different kind of transporter to oxidase so it will be absorbed more easily than a single type of carb that requires one transporter to work longer. It is also important to take in your carbs before you start to feel symptoms of hypoglycemia (low blood glucose). If you are beginning to feel sudden fatigue, heaviness in the legs, vision changes, dizzy, nauseous or confused, you are probably already hypoglycemic. When people take in carbs at this point the GI system works so hard to absorb the glucose as quickly as possible, this can cause abdominal pain. If you are able to take in the carbs before you get to this point you may be able to avoid the GI upset.
When you are running, or doing any type of exercise, the blood normally directed to the GI system is shunted to the musculoskeletal system and the cardiovascular system to help them meet increased demand. For shorter races when you are just taking in Gatorade or water the stomach can handle it, but once you have to take in carbs this can be a lot of work for the minimal blood supply to the stomach. When you are at a downhill or cruising at a slower pace, your GI system is able to use more blood than if you are working uphill or picking up your pace when your muscles need your blood more. For this reason, you want to take in enough energy to keep from depleting your blood glucose and your glycogen stores but you don’t want to take in more carbs than your stomach can deal with.
Fuel planning can help avoid GI problems. Whether you have a history of GI issues or not, everyone should practice a plan for fueling during a race. When planning out your fueling you need to take into consideration how long you will be racing for. You should plan on taking in 30-60 grams of carb per hour for any event lasting longer than an hour. Use the 1 hour mark as a deadline, you want to take in your calories before you hit each hour mark. There is a 15 minute delay in the time you take in the carbs and the time it is absorbed, take this into account when you are planning out your nutrition breaks.
Coleman, E. Nutrition for the Marathon and Beyond: Optimize your performance by proper fueling. Marathon & Beyond. Sept/Oct 2012. 88-101. Jeukendrup, A. Nutrition for Endurance Sports: Marathon, Triathlon and Road Cycling. Journal of Sports Science. 2011.29.(S1).S91-S99. Stellingwerff, T. Contemporary Nutrition Approaches to Optimize Elite Marathon Performance. International Journal of Sports Physiology and Performance. 2013.8.573-578.
Wardenaar, F et.al. Nutrient Intake and GI Complaints of Ultramarathon Runners. International Journal of Sports Nutrition and Exercise. 2015.
Elizabeth Brewer, SPT & Kathleen Leninger, PT, DPT, NY Custom PT & Performance