Gut Training: Why You Should Rehearse Your Nutrition During Training

Why Does the Gut Need Training?

The human gut can learn to absorb high-sugar drinks during exercise. Research shows:

• Untrained gut: Absorbs a maximum of 30–45 g of carbs per hour; exceeding this causes nausea and diarrhea
• After 6 weeks of training: Can absorb 60–75 g/hr
• After 12 weeks of training: Can absorb 90–120 g/hr

The key enzyme is SGLT1 (sodium-glucose transport protein), which is upregulated with training.

The Two Goals of Gut Training

1. Increase absorption capacity: More glucose transport proteins in the intestinal wall
2. Improve tolerance: Reduce the likelihood of nausea, cramping, and diarrhea during exercise

12-Week Training Plan

Week 1–2: Foundation Phase (30 g/hr)

• Force yourself to consume 30 g/hr of carbs during every long session (>90 min)
• Source: 1 energy gel + 200 ml water, every 45 minutes
• Focus: Get used to “eating while exercising”

Week 3–4: Adaptation Phase (45 g/hr)

• Increase to 45 g/hr
• Example: Energy gel + half an energy bar + 200 ml sports drink
• Start trying solid foods (banana, white toast)

Week 5–6: Breakthrough Phase (60 g/hr)

• Reach 60 g/hr (the generally recommended minimum)
• Introduce dual-source carbs: glucose + fructose at a 2:1 ratio
• Example: 500 ml sports drink (glucose + sucrose formula)

Week 7–8: High-Volume Phase (75 g/hr)

• Increase to 75 g/hr
• Start simulating race intensity (Z3–Z4)
• Many people experience gastrointestinal discomfort at this point—this is normal

Week 9–10: Race Phase (90 g/hr)

• Reach 90 g/hr (IM-level fueling)
• Practice eating immediately after T2
• Rehearse various combinations of fuel products

Week 11–12: Rehearsal Phase (80–90 g/hr)

• Maintain 80–90 g/hr
• Use only the brands you will use on race day
• Fully simulate race-day conditions

The Importance of Dual-Source Carbs

Glucose is absorbed via SGLT1, while fructose is absorbed via GLUT5. Opening both pathways simultaneously can:

• Increase total absorption by 50–75%
• Lower intestinal osmotic pressure, reducing cramping
• Speed up gastric emptying

The Golden Ratio 2:1 (Glucose:Fructose)

Example: 60 g glucose + 30 g fructose = 90 g/hr

Common fuel product ratios:

Product	Carb Source	Ratio
Maurten 320	Maltodextrin + fructose	0.8:1
SiS Beta Fuel	Maltodextrin + fructose	1:0.8
Tailwind	Glucose + sucrose	~2:1
GU Roctane	Maltodextrin + fructose	~2:1

Gut Monitoring During Training

After every long session, record:

1. What you ate and when
2. Whether you experienced nausea, cramping, excessive gas, or diarrhea
3. Bowel movements within 2 hours after training

If you experience a heavy stomach or nausea: reduce concentration, dilute with more water
If you experience cramping or excessive gas: lower the fructose ratio
If you experience diarrhea: reduce the amount, check fuel ingredients (FODMAP)

Race-Day Fueling Rehearsal Checklist

• [ ] The fuel products you’ll use on race day have been tested in 5+ long training sessions
• [ ] The sports drink brand provided at the event has been tested (IM typically uses Gatorade Endurance or Maurten)
• [ ] 90 g/hr has been achieved at least 3 times in training
• [ ] Rehearsed the scenario of eating immediately after T2
• [ ] Rehearsed fueling in rain/high heat

Common Gut Training Mistakes

Mistake 1: Only Training at Easy Pace

At race intensity (Z3–Z4), gut blood flow decreases by 50%. You must rehearse at high intensity for it to be meaningful.

Mistake 2: Training with A, Racing with B

Brand, formula, and flavor all affect tolerance. Train with what you race with.

Mistake 3: Starting Only a Month Before the Race

Gut adaptation takes 8–12 weeks. Start Gut Training at the same time as your triathlon base phase.

Mistake 4: Ignoring Liquid Concentration

If carb concentration (osmolality) is too high (>10%), water is drawn into the gut, causing diarrhea. 6–8% is the optimal range for sports drinks.

Dietary Support for Gut Training

Daily nutrition can support gut health:

• Probiotics: Yogurt, miso, kimchi (1 serving per day)
• Prebiotics: Onion, garlic, banana, oats (feed the good bacteria)
• Avoid antibiotics: Unless necessary, as they disrupt the gut microbiome
• Increase fiber gradually: A sudden jump in fiber can backfire

Conclusion

Gut Training is the hidden level of triathlon preparation. With the same training volume and the same fuel products, an athlete with a well-trained gut can easily absorb 90 g/hr, while someone who hasn’t trained it will fall apart at 45 g/hr. Start 12 weeks early, and you’ll be more than 30 minutes ahead on race day.

Related Reading

• Gut Training: An Evidence-Based Method to Expand Carb Absorption Capacity in Two Weeks
• Gut Training for Triathlon Fueling: Treat Your Gut Like a Muscle
• Gut Training: Treat Your Gut as a Trainable Organ to Keep Eating in the Second Half
• The Complete Guide to Gut Training: Help Your Gut Adapt to High Carb Intake