The Challenge of Ultra Marathons for Athletes with Diabetes

Running 50, 100, or even more miles in a single event demands extraordinary physical and mental endurance. For athletes with diabetes—whether type 1 or type 2—the challenge multiplies. Every step requires balancing fuel, hydration, and insulin while managing the body’s extreme stress response. Among the most common and disruptive obstacles is gastrointestinal (GI) distress. Nausea, bloating, cramping, diarrhea, and vomiting can derail even the best-laid race plans. Understanding why these issues occur and how to prevent or respond to them is essential for finishing safely and achieving peak performance.

Why Gastrointestinal Issues Are Amplified in Diabetic Ultra Runners

Gastrointestinal problems during ultra marathons are common among all athletes, but diabetes introduces unique physiological factors that raise both the frequency and severity of symptoms. Several mechanisms are at play:

  • Delayed gastric emptying from hyperglycemia. High blood glucose slows the rate at which the stomach empties its contents. This can cause food and fluids to sit in the stomach longer, leading to bloating, nausea, and vomiting. For diabetics whose blood sugar may run high during the early stages of a race due to stress hormones, this is a major risk factor.
  • Altered gut motility and autonomic neuropathy. Long-standing diabetes can damage the autonomic nerves that control intestinal movement, a condition called gastroparesis. Even in athletes with well-managed diabetes, subclinical nerve changes may slow peristalsis and increase the likelihood of constipation or alternating bouts of diarrhea during prolonged exertion.
  • Hypoglycemia-triggered sympathetic responses. A sudden drop in blood sugar activates the fight-or-flight system, diverting blood flow away from the gut toward working muscles. This ischemia can trigger cramping, urgency, and malabsorption of nutrients taken in during the race.
  • Osmotic effects of high-sugar fueling products. Many gels, chews, and sports drinks are hypertonic. For a diabetic runner who may already have elevated blood glucose, these concentrated sugars can pull water into the gut, causing osmotic diarrhea and dehydration.
  • Increased intestinal permeability. Strenuous endurance exercise is known to weaken the tight junctions in the intestinal lining, a state often called “leaky gut.” In diabetic athletes, chronic low-grade inflammation and fluctuating glucose may worsen this breakdown, allowing bacterial byproducts into the bloodstream and triggering systemic inflammatory responses that further impair gut function.

Pre-Event Preparation: Building a Resilient Gut Before the Start Line

Smart preparation begins weeks before race day. The goal is to reduce GI vulnerability by stabilizing blood sugar, testing fueling protocols, and training the gut to handle the load it will face during the event.

Dietary Planning and Gut Training

In the two to three weeks leading up to the race, shift toward a diet rich in easily digestible complex carbohydrates—white rice, well-cooked oats, bananas, and low-fiber vegetables. High-fiber foods, legumes, and cruciferous vegetables are best minimized in the final 48 hours to reduce gas and bloating. Simultaneously, practice your race-day nutrition strategy during long training runs. The gut is trainable; consuming 60–90 grams of carbohydrate per hour during workouts (using the same brands of gels, chews, or drinks you plan to use in the race) helps your intestines adapt to absorbing large amounts of sugar without producing distress. Make notes of which products cause bloating or cramping and eliminate them.

Blood Sugar Optimization Pre-Race

Arriving at the start line with stable blood glucose is critical. For athletes on insulin, this may require a low-dose basal adjustment the night before and a reduced bolus for the pre-race meal. The pre-race meal itself should be eaten three to four hours before the start, be low in fat and fiber, and contain primarily carbohydrate (approximately 1–2 grams per kilogram of body weight). A typical example: white bread with a thin layer of jam and a small banana. Avoid high-fat foods like nut butters or avocados that slow gastric emptying. Monitor blood sugar closely during the final hour before the race; if it’s trending high, a small corrective bolus may be needed, but be cautious—a large correction could lead to hypoglycemia during the first miles when insulin sensitivity is still high.

Hydration Status and Electrolytes

Dehydration worsens GI symptoms, but overhydration can also cause problems. Aim for a hydration plan that maintains body weight within 2% of start weight. In the days before the race, increase sodium intake slightly to help retain water. Avoid excessive plain water intake in the hour before the start; instead, sip an electrolyte beverage with 300–500 mg of sodium per liter. For diabetics, consider products that use sucrose or glucose rather than fructose alone, as some individuals with impaired fructose absorption develop gas and diarrhea from high-fructose drinks.

Medication Adjustments and Consultation

Never make major changes to insulin or oral medications without guidance from your healthcare team. However, many diabetic ultra runners find that adjusting their long-acting insulin dose by 10–20% on race day helps prevent hypoglycemia without predisposing them to prolonged hyperglycemia. For athletes taking metformin, note that it can cause GI side effects (diarrhea, nausea) on its own. Some choose to reduce or skip the dose the day of the race, but this must be weighed against the risk of hyperglycemia. Consult with your endocrinologist and a sports dietitian who understands endurance exercise to create a personalized plan.

During the Race: In-the-Moment Strategies for GI Management

When you’re 20 miles in and nausea begins to creep in, having a clear action plan prevents panic and keeps you moving forward.

Fueling Frequency and Composition

Switch from large, infeed boluses of fuel to smaller, more frequent doses. Aim for 15–20 grams of carbohydrate every 20 minutes rather than 60 grams all at once. This reduces the osmotic load in the stomach and maintains a gentle rise in blood sugar. If liquids are better tolerated than solids, opt for a diluted sports drink or even a homemade mix of maltodextrin and electrolyte powder. For those who prefer real food, options such as small pieces of banana, white rice wrapped in seaweed, or plain potatoes have worked well for many diabetic athletes. Avoid anything high in fat or protein during the high-intensity portions of the race, as these delay gastric emptying.

Managing Nausea and Vomiting

If you feel your stomach turning, stop running and walk. Deep breathing helps activate the parasympathetic nervous system, shifting blood flow back to the gut. Rinse your mouth with cool water or an electrolyte drink, but do not force yourself to drink. If vomiting occurs, rest for 5–10 minutes, then try taking small sips of a very dilute electrolyte solution (half the concentration of your usual drink). Once you can keep that down, introduce a small amount of carbohydrate (10–15 grams). Be mindful of your blood sugar: vomiting can cause hypoglycemia if you don’t reabsorb any fuel, but hyperglycemia if you had taken insulin and then are unable to eat.

Diarrhea and Cramping

Sudden diarrhea often results from osmotic overload or from excessive caffeine intake. If you feel the urgent need to stop, do so in a designated aid station toilet or portable unit. After such an episode, replace lost fluids with an electrolyte solution but avoid high-sugar drinks immediately, as they can worsen diarrhea. Consider carrying loperamide (Imodium) as part of your medical kit, but use it sparingly: it slows gut motility and can increase the risk of constipation or ileus later in the race. For abdominal cramping without diarrhea, check your blood sugar. Hypoglycemia can present as cramping that mimics GI distress. A quick glucose tab or gel may resolve both the blood sugar issue and the cramp.

Blood Sugar Checks and Continuous Glucose Monitors

Frequent blood glucose checks are non-negotiable. A continuous glucose monitor (CGM) is a game changer because it gives trend arrows that help you predict hypoglycemia before symptoms start. However, CGMs can be less accurate during extreme dehydration, so calibrate with a finger stick if the sensor readings seem off. If your CGM shows a rapid downward arrow, take 15–20 grams of fast-acting carbohydrate immediately—even if you are not yet symptomatic. Conversely, if you see a stubborn high plateau, do not attempt to correct aggressively with insulin; the high could be a response to intense effort or dehydration, and correcting it during the race risks a dangerous low later. A small correction (0.5–1 unit of bolus insulin) might be considered only after 30 minutes of confirmed hyperglycemia with stable or upward trending arrow and adequate fluid intake.

Pacing and Body Tuning

Starting too fast raises heart rate and shunts blood away from the gut. Maintain a pace that allows you to speak in full sentences for the first 20% of the race. Listen to your body: if nausea or cramping is severe, walking for a few minutes can restore gut function. Many experienced diabetic ultra runners adopt a run-walk strategy from the beginning—running 5 minutes and walking 1 minute—which keeps the GI system in a better balance than continuous running.

Post-Race Care: Rebooting the Gut and Stabilizing Metabolism

Crossing the finish line is not the end of GI management. The hours and days after an ultra marathon require careful refueling to restore normal gut function and maintain blood sugar stability.

Gradual Rehydration and Electrolyte Correction

Rehydrate slowly to avoid overwhelming the kidneys or triggering diarrhea. Sip an electrolyte solution with sodium, potassium, and magnesium. Avoid chugging plain water, which can dilute blood sodium levels and worsen cramping or nausea. The first 500–750 ml should be taken over 30 minutes, then continue with small amounts every 15 minutes as tolerated.

Resuming a Normal Diet

Start with easily digestible carbohydrates—a few saltine crackers, a piece of toast, or a small bowl of plain white rice. Avoid high-fat or high-protein meals for the first two hours post-race, as the gut remains sensitive. After your stomach settles, include a small amount of protein to support muscle repair (e.g., a scrambled egg or a protein shake made with whey or pea protein). Gradually reintroduce fiber over the next 24–48 hours; starting with cooked vegetables like carrots or zucchini before switching to raw greens.

Blood Sugar Monitoring After the Race

Insulin sensitivity often increases dramatically after an ultra marathon and can remain elevated for 24–48 hours. This means that normal pre-race doses of insulin may be too high. Check blood sugar every 2–4 hours during the recovery period. Many athletes need to reduce their basal insulin by 20–30% for the next day or two. If you use an insulin pump, consider setting a temporary basal rate decrease. Oral medication users (especially those on sulfonylureas or meglitinides) should discuss with their doctor whether to reduce doses post-race. Do not skip meals to avoid correcting low blood sugar; eat regular, balanced meals.

Restoring Gut Microbiome and Reducing Inflammation

Intense exercise disrupts the gut microbiome. After the race, consider incorporating fermented foods (plain yogurt, kefir, miso soup) or a high-quality probiotic containing Lactobacillus and Bifidobacterium strains. Avoid alcohol immediately post-race, as it further impairs intestinal barrier function. Anti-inflammatory foods such as ginger tea, bone broth, and turmeric can help calm the gut lining. Prioritize sleep—sleep deprivation worsens leaky gut and impairs glucose metabolism.

Personalized Plans and Professional Support

Every diabetic athlete responds differently to the demands of ultra marathons. What works for one runner may cause severe distress in another. Building a personalized GI management plan requires collaboration with experts who understand both diabetes and endurance sports.

Working with an Endocrinologist

An endocrinologist can help fine-tune insulin regimens, advise on adjusting long-acting basal doses on race day, and recommend appropriate modifications for oral medications. They can also evaluate for undiagnosed gastroparesis or other diabetic complications that may require special dietary adjustments or medication changes. The Endocrine Society offers resources to find specialists familiar with exercise management.

Consulting a Sports Dietitian

A sports dietitian with experience in endurance events can design a fueling plan that considers your individual glucose patterns, the timing of insulin peaks, and your race nutrition preferences. They can help you calibrate your carbohydrate intake per hour, choose the best electrolyte products, and teach you gut-training protocols. The Academy of Nutrition and Dietetics’ find-an-expert tool can help locate qualified professionals.

Leveraging Technology

Continuous glucose monitors and smart insulin pumps with automated adjustments are transforming how diabetic athletes manage during ultras. Devices like the Dexcom G6 or G7 and the Tandem t:slim with Control-IQ can provide real-time data and micro-adjustments that reduce the cognitive load of self-management. However, ensure you test the technology extensively during training before relying on it in a race. The American Diabetes Association provides guidelines for using CGM during exercise.

Mental Preparation and Accepting Uncertainty

Despite all preparation, GI issues can still arise. Having a mental plan for how you will handle unexpected nausea, diarrhea, or vomiting is as important as the physical strategies. Practice mindfulness techniques that help you stay calm and focus on the next step rather than the full distance. Remind yourself that many elite athletes with diabetes have completed ultras with careful GI management—including former professional cyclist and type 1 diabetic Phil Southerland, co-founder of Team Novo Nordisk, who has spoken about leveraging thorough planning to overcome GI hurdles. Team Novo Nordisk’s resources for athletes with diabetes can offer additional inspiration and practical tips.

Final Considerations

Managing gastrointestinal issues during an ultra marathon as a diabetic athlete is not about eliminating all discomfort—it is about having a robust, flexible system that allows you to adapt when things go wrong. Through careful pre-event preparation, disciplined in-race execution, and thoughtful post-race recovery, it is possible to significantly reduce the severity of GI problems and maintain the energy needed to cross the finish line. The keys are consistent blood sugar monitoring, testing fueling strategies during training, and building a healthcare team that supports your athletic goals. With these tools, the remarkable achievement of completing an ultra marathon becomes a reachable, real-world goal—not just a dream.