Gut Health and Athletic Performance

Ask any endurance member about their worst race, and there is a good chance the story involves their stomach. GI distress — nausea, cramping, bloating, diarrhea — affects 30-70% of endurance members during competition and is one of the most common reasons for DNF (did not finish) in marathons, triathlons, and ultra-endurance events.

The gut is not just a passive tube that absorbs nutrients. It is a dynamic organ that responds to exercise intensity, blood flow redistribution, mechanical jostling, and the foods consumed before and during activity.

Why Exercise Causes GI Problems

During high-intensity exercise, blood flow is redirected from the splanchnic (gut) circulation to working muscles, skin (for cooling), and the heart. At intensities above 70% VO2max, gut blood flow can decrease by 80%. This ischemia-reperfusion cycle damages the intestinal lining, increasing permeability — what researchers call “leaky gut.”

The consequences:

Increased intestinal permeability allows endotoxins (lipopolysaccharides) to enter the bloodstream, triggering systemic inflammation
Impaired absorption of carbohydrate and fluid during exercise
Mechanical stress from running (vertical oscillation) exacerbates symptoms — which is why runners experience more GI issues than cyclists or swimmers

Training the Gut

Just as muscles adapt to training, the gut can be trained to tolerate higher carbohydrate intake during exercise. Studies have shown that members who regularly practice consuming carbohydrate during training sessions increase their intestinal absorption capacity and reduce GI symptoms during competition.

The protocol:

Start low: Begin with 30g carbohydrate per hour during training
Increase gradually: Over four to six weeks, build up to 60-90g/hr (the target for events lasting over 2.5 hours)
Use multiple transportable carbohydrates: Combining glucose and fructose (2:1 ratio) uses different intestinal transporters (SGLT1 and GLUT5), allowing absorption rates up to 90g/hr — versus 60g/hr for glucose alone
Practice with race-day products: Use the same gels, drinks, and foods in training that you plan to use in competition

Dietary Strategies to Reduce GI Risk

Low-FODMAP pre-competition diet (24-48 hours before): Reducing fermentable carbohydrates (onions, garlic, wheat, dairy, certain fruits) can decrease gas production and bloating. This is a temporary strategy for competition, not a long-term dietary approach.
Low fiber before competition: Fiber adds bulk and increases transit time. Switch to refined grains and peeled, cooked vegetables in the 24 hours before a major event.
Avoid NSAIDs: Ibuprofen and other non-steroidal anti-inflammatory drugs, commonly used by members for pain management, significantly increase intestinal permeability during exercise. Avoid them before and during competition.
Hydration: Both dehydration and overhydration can exacerbate GI symptoms. Follow a personalized fluid plan based on sweat rate testing.

Probiotics: What the Evidence Says

The relationship between probiotics and athletic GI health is an active research area. Several strains have shown promise:

Lactobacillus and Bifidobacterium strains may reduce the incidence and duration of upper respiratory tract infections in members
Some evidence suggests probiotics can reduce GI symptom severity during endurance events
The effects are strain-specific — not all probiotics are created equal, and general “probiotic blends” without specified strains and CFU counts should be avoided

The current position: probiotics are not a universal recommendation for members, but may benefit those with recurrent GI issues or frequent illness. Choose products with specific, research-backed strains.

The Dietitian’s Role

GI issues in members are treatable and often preventable. The dietitian should:

Take a detailed GI history during intake, including exercise-associated symptoms, triggers, and current management strategies
Implement a gut training protocol for endurance members who will consume carbohydrate during competition
Design low-residue, low-FODMAP pre-competition meal plans for members with a history of race-day GI distress
Review NSAID use and educate on the gut permeability risks

The gut is a trainable organ. Treat it like one.

Calsanova’s meal planning tools support pre-competition meal templates that dietitians can customize for members with GI sensitivities — including day-type-specific plans for training, travel, and competition days.

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