Most runners do not run out of strength on race day. They run out of fuel because their gut refuses to cooperate. Nausea, cramping, bloating, and the frantic search for a portable toilet are not random bad luck. They are the predictable result of asking an untrained digestive system to do a trained athlete's job.
The good news is that your gut adapts. Just like your cardiovascular system improves with structured training, your intestines respond to repeated stimulus. Research in exercise physiology shows that consistent carbohydrate exposure during training drives measurable adaptation in intestinal transport capacity, a process that is earned through deliberate practice, not determined by genetics. This post covers exactly how that process works and how to build it systematically.
Why Does Carbohydrate Absorption Matter for Runners?
At race pace, carbohydrates are the dominant fuel source because fat oxidation cannot keep up with energy demand at high intensities. The documented absorption ceiling for most untrained guts sits around 60 grams per hour. Studies on trained endurance athletes consistently show absorption rates reaching 90 grams per hour when gut training has been applied systematically over time.
That gap is physiological, not genetic. It is the result of deliberate gut training over time.
What Actually Happens in Your Gut When You Train It?
Your small intestine uses specialised transport proteins to pull carbohydrates into the bloodstream. SGLT1 handles glucose, and GLUT5 handles fructose. When the gut is consistently exposed to carbohydrates during exercise, research indicates that the population of these transporters increases. A greater transporter density is associated with higher absorption capacity in the small intestine and a reduction in the volume of undigested carbohydrate that reaches the colon, which is where fermentation and associated GI symptoms tend to originate.
Gastric emptying rate also responds to repeated training stimulus. A regularly fueled gut has been shown to empty faster and more consistently, which reduces the sloshing, heavy sensation that commonly affects runners who fuel only occasionally. The mechanism is straightforward: consistent stimulus drives physiological adaptation. You cannot gut train during race week.
How Do You Actually Build Gut Tolerance for Running?
The framework is progressive, deliberate, and non-negotiable if you want results before your target race.
Start by establishing your current baseline. Run a standard long effort with your existing fueling strategy, record exactly how many grams of carbohydrates you consumed per hour, and rate your GI comfort on a scale of one to ten. That number is your starting point, not your ceiling.
Next, start fueling on runs shorter than you normally would. Most runners only take carbohydrates on efforts over 90 minutes. Begin practising on runs as short as 45 to 60 minutes. Even 20 to 30 grams per hour on a moderate effort run is enough to stimulate SGLT1 and GLUT5 upregulation. The exposure matters more than the volume at this stage.
Increase carbohydrate volume by no more than 10 to 15 grams per hour each week. Jumping from 40 grams to 90 grams overnight overloads the system before the transporters have had time to adapt. A controlled weekly escalation, for example 50 grams in week one, 60 grams in week two, 70 grams in week three, allows the gut to keep pace with demand. If symptoms spike, hold the current dose for an additional week before progressing.
Critically, practise fueling at race intensity, not just easy pace. GI distress is significantly more common at high intensities because blood is redirected away from the digestive system during hard efforts. If you only fuel during easy runs, you are not training the system under the conditions that matter. Include carbohydrate intake during tempo runs, threshold intervals, and race-pace workouts.
Finally, combine carbohydrate sources. Glucose and fructose use different transport proteins, so blending them in a 2:1 ratio allows a higher total carbohydrate volume to be absorbed per hour compared to glucose alone. This dual-source approach is the physiological reason why absorption rates of 90 grams per hour and above become achievable in a trained gut. Look for products that pair maltodextrin or glucose with fructose explicitly.
Frequently Asked Questions
How long does gut training take before a race?
Gut training requires a minimum of 8 to 12 weeks of consistent practise to produce meaningful adaptation. Attempting to build tolerance in the final two weeks before a race is ineffective and increases the risk of GI problems on race day.
Can I gut train without specific sports nutrition products?
You can begin with real food sources such as bananas, dates, or rice-based options during easier runs. For higher-intensity sessions and race simulation, purpose-formulated products with a defined 2:1 glucose-to-fructose ratio give you far more precise control over carbohydrate volume and type.
Why do I get stomach cramps during races but not in training?
Race-day cramps are often the result of consuming higher carbohydrate volumes than your gut has been trained to handle, combined with the physiological stress of race intensity. The solution is to ensure your training fueling matches your race-day plan in both product, volume, and intensity.
How much water should I drink with each gel?
Aim for 150 to 250 millilitres of water with each carbohydrate intake event. Carbohydrate absorption is coupled to fluid intake, and taking concentrated carbohydrate sources without adequate hydration slows gastric emptying and significantly increases GI distress risk.
What Should You Actually Do?
Start fueling on your next moderate effort run, even if it is only 50 minutes. Use 20 to 30 grams of carbohydrates per hour, pair it with water, and note how your gut responds. Increase the dose by 10 to 15 grams per hour each week. Include fueling in your hard sessions, not just your long runs. In the four to six weeks before your target race, run at least two to three full dress rehearsals using your exact race-day products and volumes.
The friction of the fueling act itself matters in the early phases of gut training, when the system is still adapting. Research on gastric emptying consistently shows that product viscosity and palatability influence whether athletes fuel consistently during training, which is the variable that actually drives adaptation.
Science over hype. Every time.
With that in mind: SUPPLME Energy Gel is a liquid supplement formulated with a 2:1 glucose-to-fructose ratio, delivering 32 grams of carbohydrates per 40ml serving. Its low-viscosity liquid format is designed to reduce the practical friction of consistent carbohydrate intake during training. As with any supplement, individual responses vary, and athletes are encouraged to trial any product in training before race-day use.
Start low. Progress slowly. Practise relentlessly.
References
Jeukendrup, A.E. (2010). Carbohydrate and exercise performance: the role of multiple transportable carbohydrates. Current Opinion in Clinical Nutrition and Metabolic Care, 13(4), 452-457. Independent research. Not conducted in association with SUPPLME or its products.
Jeukendrup, A.E. (2014). A step towards personalised sports nutrition: carbohydrate intake during exercise. Sports Medicine, 44(Suppl 1), 25-33. Independent research. Not conducted in association with SUPPLME or its products.
