Hello Runner,
Welcome back to your weekly moment of endurance nutrition!
Happy Weekend!
How are you?
I’m doing great!
This week was all about training, work, and not much studying.
I’m taking full advantage of the wonderful weather (early spring has perfect weather for running outdoors) and steadily improving my pace.
I took the opportunity to slow down at the end of the week and rest over the holiday. I love Holy Week here in Barcelona because the city empties and the noise level drops significantly. And those who don’t travel take the chance to rest and also spend more quality time with friends and family.
Not long ago, I wrote about Irritable Bowel Syndrome (IBS) here in the newsletter. I explained what it is, how it affects running, common signs and symptoms, and what runners should do if they suspect they have it.
Today’s topic is irregular bowel movements and how they affect carbohydrate absorption during training.
First, to be clear, IBS is not the same thing as irregular bowel movements.
Many runners don’t have IBS, but sometimes experience irregular bowel movements. I myself am a prime example.
Unfortunately, when runners experience irregular bowel movements during training (especially longer sessions) and races, the gel or other supplement used to keep blood sugar stable won’t be properly absorbed. And the chances of the runner hitting the wall are quite high.
Learn more so you don’t end up in that situation.
Why irregular bowel movements impair gel absorption
Modern exercise physiology and sports nutrition literature agree that gastrointestinal (GI) function during running is already compromised, and any baseline irregularity (constipation, diarrhea, IBS-like patterns) reduces the efficiency of carbohydrate absorption from energy gels.
Basic mechanisms repeatedly emerge:
1) Exercise already reduces absorption capacity
During moderate–high intensity running, blood is diverted away from the gut toward muscles. This reduces the intestine’s ability to absorb nutrients.
Even in healthy runners, absorption is “fragile” during races.
2) Irregular bowel patterns = dysfunctional motility
Irregular bowel movements usually reflect abnormal gut transit time:
Diarrhea-prone runners (fast transit)
Nutrients move too quickly → insufficient contact time for absorption
Constipation-prone runners (slow/erratic transit)
Delayed gastric emptying or inconsistent small intestine flow → poor timing of delivery + discomfort
Key insight: Absorption depends on precise timing, and irregular guts lose that timing.
3) Carbs malabsorption is common during exercise
Energy gels are concentrated carbohydrates (glucose, fructose, maltodextrin). These require water and active transporters.
When absorption fails:
Carbs stay in the gut
They pull water in (osmotic effect)
Causing bloating, cramps, diarrhea
This creates a double problem: less energy and more symptoms.
4) Gut damage and permeability (“leaky gut” effect)
Hard running can result in epithelial (the single-layered, protective, and absorptive lining of the digestive tract) damage.
If the gut lining is compromised, even well-formulated gels won’t be absorbed efficiently.
ACSM guidelines acknowledge the relationship between high training stress and gut permeability.
Over time, repeated gut stress without adequate recovery nutrition can make the barrier less effective, reducing the amount of carbohydrate that actually reaches your muscles.
Does this apply to other in-run supplements?
Yes, broadly applies to most.
The same physiological constraints affect:
Supplement type | Effect on irregular GI |
Sports drinks | Slightly better (more dilute), but still affected |
Chews/solids | Often worse |
Fructose-heavy fuels | Higher malabsorption risk |
Electrolytes | Less affected (passive absorption) |
Caffeine | Can worsen motility instability |
STRATEGIES
Best strategy for runners with irregular bowel movements
→ Fix baseline gut regularity first
Before optimizing race fueling:
Normalize bowel habits (diet, fiber timing, proper hydration)
Screen for IBS, food intolerances (e.g., FODMAP sensitivity)
If the baseline is unstable, race fueling will always be unreliable.
→ Train the gut
After the baseline/foundation is stable, it’s time to train the gut.
Repeatedly consuming carbs during long training:
Increases transporter capacity
Improves tolerance and absorption
Protocol:
Start with ~30 g carbs/hour after 60-70 minutes of running
Gradually increase to 60–90 g/hour over weeks
There are other strategies for training the gut. I’ve written about this before in this newsletter. To read it, click here.
→ Choose more absorbable formulations
There are a few better options available to the runner, including:
Glucose + fructose blends (2:1 ratio) → dual transporters
Always take gels with water
Isotonic gels/drinks → less osmotic stress
Hydrogels → encapsulate the carbohydrates for efficient transport to the gut for absorption.
Avoid:
High-FODMAP ingredients (if you like real food during training)
Very concentrated gels without water
→ Reduce pre-race gut stress
Here are some evidence-based approaches to try:
Low-FODMAP* diet 24–48h pre-race
Avoid excess fiber, fat, spicy food, and protein pre-run
Avoid trying new foods 24h pre-race
Avoid dehydration by staying hydrated
*FODMAP is a group of fermentable carbohydrates that includes wheat and other grains, many fruits and vegetables, pulses and legumes, dairy products, and sugar-free sweeteners like sorbitol, mannitol, and xylitol.
As I mentioned at the beginning of this email, I sometimes suffer from irregular bowel movements.
I’ve done plenty of food elimination tests, increased my fluid intake, incorporated fermented foods during the week, and turned to probiotics.
24 hours before a long run or race, I significantly cut back on fiber and fat, increase my fluid intake, and choose leaner proteins. This makes a huge difference for me. But what works for me might not work for you.
What works for everyone is taking care of gut health daily, because that will improve your nutrient absorption.
To help, I’ve created a free guide to improve your gut health and, as a result, boost your running performance.
Before officially launching it on my social media, I’m making it available here for all the runner readers of my newsletter! I hope you enjoy it
Note: Please let me know if you liked the guide. You can reply to this email or reach me on social media.
Product of the Week
This week's Product of the Week is the Vitafor Simfort Ultra 30.
I mentioned in my email that what helped me regulate my bowel movements was adding probiotics to my daily routine.
I chose the one from Vitafor. I was already familiar with it from when I lived in Brazil, and now it’s available in Europe.
I chose the fiber-free version because I prefer to get my fiber from my diet.
Since I’m a partner with Vitafor, they gave me a discount coupon to share with my clients and readers. Coupon: AP002695VITAFOR
Track of the Week 🎧️
This week's Track of the Week is one of my favorites from the pioneers of house music in the '90s. The perfect song for a spring running playlist!
I’ve chosen Digital Love by Daft Punk
Any questions or something you would like to share, drop me an email
Wishing you a great weekend and week ahead!
Here’s to health and good runs⚡️
Ana Paula Alonso