Masters Athletes and Gut Issues

Aging doesn't have to mean slowing down — but it does mean paying closer attention to your gut. Gastrointestinal (GI) distress is one of the most common performance limiters in endurance sport, and masters athletes (40+) often notice their digestion isn't as forgiving as it once was. Here's what the science says and what you can do about it.

Does Age Affect Carb Absorption?

Good news: the small intestine's ability to absorb carbohydrates stays largely intact as you age. The two key transporters - SGLT1 (for glucose) and GLUT5 (for fructose) - remain functional in healthy older adults. The real concern is a decline in GLUT4, the transporter that moves glucose from blood into muscle cells, which can lead to insulin resistance. The fix? Keep training. Regular endurance exercise helps maintain and even restore GLUT4 levels.

Takeaway: Masters athletes can and should target 60–90g of carbohydrates per hour in events over 2 hours, using glucose-fructose blends. Age alone isn't the limiting factor, gut training is.

What Causes GI Distress?

Three main culprits:

Physiological: Exercise diverts blood away from the gut to working muscles and skin. At 70% VO2max, gut blood flow can drop by nearly 80%, causing nausea, cramping, and diarrhea.
Mechanical: Running physically jolts the GI tract, making runners more prone to lower GI issues than cyclists.
Nutritional: High-fat, high-fiber, or high-calorie foods eaten too close to training, plus concentrated drinks (juice, soda, smoothies), slow stomach emptying and invite distress.

Takeaway: Support your gut daily by eating every few hours, prioritizing whole foods, and including fermented foods (yogurt, kefir, kimchi). Before workouts, stay hydrated, keep snacks low in fat and fiber, and give meals time to digest.

Gut Training for Masters Athletes

Gut training means progressively practicing carbohydrate and fluid intake during workouts so your GI system can handle race-day demands. Many elite endurance athletes now successfully consume 120–150g of carbs per hour, but that takes time and consistency to build. Masters athletes can absolutely gut train, but may need a longer adaptation window, especially if they've been eating lower-carb or training infrequently.

Takeaway: Start at 30–40g carbs/hour during sessions longer than 90 minutes using a glucose/fructose product. Increase by 10–15g every 1–2 weeks as tolerated, aiming for at least 60g/hour. Practice at least once a week, ideally during high volume workouts and in conditions that mimic race day. Allow up to 10 weeks to fully adapt.

Exercising in the Heat

Heat compounds every gut challenge. Blood shifts to the skin for cooling (away from the gut), and gastric emptying slows as core temperature rises. Masters athletes face added hurdles: reduced cardiovascular cooling capacity and decreased sweating efficiency, both natural effects of aging.

Takeaways for hot conditions:

Drop carb intake by 10–20g/hour until heat-acclimated
Choose liquid calories (sport drinks, diluted gels) over solid foods
Eat small amounts every 10–15 minutes rather than large boluses
Start hydrated and cool
Don't rely on thirst - you may already be dehydrated when it kicks in
Include sodium in everything you drink; plain water isn't enough
Monitor urine color (pale yellow = good) and track pre/post weight to catch dehydration early
Watch for hyponatremia - drinking too much plain water can dangerously dilute blood sodium

The Bottom Line

Your gut is trainable at any age. With smart, progressive fueling, consistent practice, and heat-specific adjustments, masters athletes can perform strongly well into their later years. Age is context, not a ceiling.

References

Skeletal muscle GLUT4 protein concentration and aging in humans - PubMed
Training the Gut for Athletes - PMC
Aging and Gastrointestinal Neuromuscular Function: Insights from Within and Outside the Gut - PMC
Multiple Transportable Carbohydrates and Their Benefits - Gatorade Sports Science Institute
Exploring the Nutrition Strategies Employed by Ultra-Endurance Athletes to Alleviate Exercise-Induced Gastrointestinal Symptoms—A Systematic Review - PMC
Is There an Exercise-Intensity Threshold Capable of Avoiding the Leaky Gut? - PMC
Gastrointestinal symptoms among recreational long distance runners in China: prevalence, severity, and contributing factors - PMC
Aging and Thermoregulatory Control: The Clinical Implications of Exercising under Heat Stress in Older Individuals - PMC
3 Ways Your Hydration Status Changes As You Age | TrainingPeaks