The "Endurance Resilience" System

Hey there!

Welcome to Day 1 of The Endurance Athlete’s Resilience System.

Today we're talking about Mistake #1: The Single-Sugar Failure.

Here’s what we’ll cover:

• Why glucose only gets you so far
• Why relying on a single sugar source tanks training and race performance
• And how to fix it without blowing up your stomach

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The Mistake

Most athletes don’t realize their gut can only absorb about 60 grams of glucose per hour.

This limit exists because the glucose transporter in the small intestine (SGLT1) reaches maximum rate quickly, especially during long endurance efforts. When that happens, fuel sits in the stomach instead of moving into the bloodstream, which leads to gut slosh, nausea, and early bonking. This catches many athletes by surprise because the legs often still feel capable while the gut is waving the white flag.

Once the gut quits, your race follows shortly after.

This mistake leads to predictable problems, which is why avoiding it matters:

• Decrease in power or output: Glucose-only fueling works early on, but after 90 minutes or so, your sustainable power begins to fade as your gut falls behind.
• Overloaded gut: When glucose piles up faster than your body can absorb it, the small intestine struggles, and discomfort builds quickly.
• Digestive upset: This can range from mild nausea to vomiting or bowel distress, all of which are show-stoppers on race day.

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The Reason This Happens

The reason this happens is many athletes rely on old information or outdated scientific studies.

This is especially true for athletes over 40, who often learned their fueling habits from high school coaches or early gym culture. Sports physiology has evolved dramatically over the past decade, driven by elite endurance labs and field data from cyclists, triathletes, and ultrarunners. The challenge is that this new knowledge rarely makes it into the hands of recreational athletes in a clear, usable form.

And honestly, none of this is your fault—most of us were never taught how endurance fueling actually works.

Here are some other specific reasons athletes haven’t adopted dual-carb fueling:

• Pro sports rarely discuss nutrition: You don’t hear Tour de France commentary explaining why Pogacar uses dual-carb fueling. Fans see performance, not the systems behind it.
• Old gels created a stigma: Chalky gels, syrupy mixes, and stomach-wrecking “sports drinks” gave mid-life athletes a kind of post-traumatic nutrition disorder. Many assume all modern products are the same. They aren’t.
• Limited access to current research: The best research is often behind paywalls, or buried in academic journals. The information exists—it just isn’t reaching the athletes who need it.

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How To Fix It

Fixing this begins with understanding that your body has more than one way to absorb carbohydrates, which leads to higher overall energy delivery.

Glucose uses the SGLT1 pathway, but fructose uses a different one called GLUT5, which means the gut can absorb more total fuel when both are present. This dual-transport system increases available energy while reducing the risk of backup and GI distress. The key is not to eat less, but to fuel smarter by opening both pathways.

And that starts with a few simple steps.

Here’s what you can do next:

• Test 2:1 glucose-to-fructose products: Pick up two or three products from your local running or cycling shop and see what you tolerate. My favorites: SiS Neutral gels and Skratch Labs Fruit Punch mix. (No sponsorships. Just stuff I like.)
• Ease into higher carbs: Most athletes tolerate 60g/hr fairly easily once they switch to dual-carb fueling. Don’t overdo it yet—we’ll go deeper in Day 3 when we talk about gut training.
• Avoid experimenting on long days: New shoes on race day? Bad idea. New fueling strategy? Even worse. And while we’re at it, don’t count on feed-station mystery food to fuel your fastest day.

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Don’t Forget To Check Your Inbox

We covered a lot today.

Understanding the limits of single-source sugars will help you avoid mid-race GI disasters and finally unlock energy that lasts deeper into your training sessions. These small adjustments today set the stage for more resilient, predictable fueling in the weeks ahead.
And tomorrow, we’ll build on this foundation.

So stay tuned for Mistake 2: Relying on Thirst Alone — Ignoring Your Electrolyte System.

Specifically:

• The reason thirst is an unreliable narrator
• Why this leads to dehydration or worse
• And how to fix it

See you tomorrow,
Coach Tony

Hey there!

Welcome to Day 3 of The Endurance Athlete’s Resilience System.

Today we’re covering Mistake #3: The Gut-Training Gap — Fueling Only on Race Day.

Here’s what we’ll cover:

• Why the gut must be trained like a muscle
• What happens when you only “practice” fueling on race day
• How to build a gut that can absorb more carbs with less distress

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The Mistake

Many endurance athletes treat fueling as something they “figure out on race day,” which is exactly when your gut is under the most stress and least capable of improvisation.

Your stomach isn’t a passive container; it’s a dynamic system that adapts to the demands you place on it. When you suddenly ask it to handle high carb loads during a hard effort, especially 80g, 100g, or even 130g per hour, it responds the only way an untrained system can: with resistance. Nausea, fullness, sloshiness, and emergency porta-potty stops are all signs that the gut was never prepared for the workload.

And just to be clear, this isn’t a discipline issue—your gut simply hasn’t been trained for the job yet.

Here’s what this mistake typically leads to:

• GI distress at the worst possible time: Race day intensity + unfamiliar carb loads = predictable disaster.
• Sudden bonking despite “eating enough”: Your stomach can’t keep up, so fuel never reaches the bloodstream in time.
• Inconsistent energy and pacing: When carb absorption fluctuates, so does your power, mood, and mental clarity.

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The Reason This Happens

The reason this happens is most athletes think of fueling as a last-minute add-on rather than a trainable physiological system.

Just like muscles get better at clearing lactate with practice, the gut gets better at absorbing carbohydrates when you gradually increase intake during training. Over time, your stomach actually improves gastric emptying, your intestines upregulate transporters like SGLT1 and GLUT5 (remember those from day 1?), and your overall tolerance for carbs goes up. But when fueling is only practiced occasionally, or worse yet, only on race day, you lose out on these adaptations.

This is why carb loads that should work end up backfiring.

Here are a few other reasons athletes fall into the gut-training trap:

• They separate fueling from training: Many athletes think “workouts are for fitness, race day is for nutrition,” which is backwards.
• They fear GI issues during training: Ironically, this fear creates more issues on race day because the gut never gets exposure.
• They underestimate how high carb needs really are: Many athletes assume 30–40g/hr is enough because it “feels fine,” until intensity spikes and the fuel gap shows up.

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How To Fix It

Fixing this begins with understanding that your gut is highly adaptable—and training it is the key to reliable race-day fueling.

Your digestive system responds to progressive overload just like your quads do. When you regularly fuel during training sessions, your gut improves its ability to absorb carbs, tolerate volume, and clear fuel quickly into the bloodstream. The result is smoother energy, more stable power, and far fewer gastrointestinal surprises on race day.

And it only takes a few structured tweaks.

Here’s what you can do to start:

• Begin with your current carb intake, then add 10–15g per week: If you’re doing 40g/hr now, move to 50–55g next week. Slow, steady increases work best. If you start to notice excessive gassiness post-workout, you have likely reached a threshold; don't increase until you're back to normal.
• Fuel during easy and moderate sessions, not just hard ones: Your gut adapts based on exposure, not intensity. I practice fueling even on Zone 2 days so my stomach stays tuned year-round. If I take four or more weeks off the bike, I back off about 15g/hr and begin the adaptation from there.
• Practice your race-day strategy during long workouts: Don’t just test flavors—test timing, volume, product combinations, and concentration. Your gut needs reps just like your legs do. In my most recent event, the 24-Hour World Championship Time Trial, I drank every 12 minutes and took in 40g gels every 45 minutes. I gut trained over six months to get there.

If you want to accelerate adaptation, jot down a few notes after today’s workout about how your stomach felt—you’ll begin spotting patterns quickly.

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Don’t Forget To Check Your Inbox

That was a lot to chew on—or maybe swallow—but hopefully it didn’t overload you too much.

Understanding the gut-training gap helps you build a digestive system that works with you, not against you, during long efforts. When your gut is trained, your fueling becomes predictable, your pacing becomes smoother, and your race-day confidence skyrockets.
And tomorrow, we take this a step further.

Stay tuned for Mistake 4: The 60-Minute Recovery Window Failure — Delaying the Rebuild Phase.

Specifically:

• Why the recovery window isn’t a myth
• How glycogen enzymes work after training
• And the simple timing trick that accelerates adaptation

See you tomorrow.