Walk down any sports-nutrition aisle today and you’ll see why many endurance athletes are confused. Bright packaging, bold claims, and a thousand “revolutionary” formulas all promising to be the missing piece. Strip away the marketing, and most athletes are really chasing one thing: fuel they can actually tolerate without blowing up their gut.

To make smart decisions, you need to understand what’s happening under the hood—how different carbohydrates behave, how the gut handles them under load, and why some products work while others turn a big day into a disaster. This isn’t about hype. It’s physiology.

Below is a clear breakdown of the primary carbohydrate sources endurance athletes rely on:
multiple transportable carbs, hydrogels, and highly branched cyclic dextrin (HBCD/Cluster Dextrin).
Understand the science and you can build a fueling plan that actually holds up under pressure.

1. Sugars, Stripped Down

Carbohydrates are simple: chains of carbon, hydrogen, and oxygen. Your body breaks every carb—simple or complex—down into simple sugars:

Glucose – primary fuel for almost every cell
Fructose – processed mainly in the liver
Galactose – converted into glucose

Complex carbs (maltodextrin, starches, glycogen) eventually end up as those same simple sugars.

In long, punishing efforts—ultras, mountain marathons, multi-pitch days, long ski tours—glucose availability becomes the limiter. When glucose drops, the lights go out. Research is consistent: fatigue correlates strongly with muscle glycogen depletion and falling blood glucose.

Modern guidelines recommend 90–120 g of carbohydrate per hour to sustain high-output endurance. That’s the target. The problem? Getting that much fuel into a moving body without the gut revolting.

2. The Athlete’s Gut: A System Under Stress

During long efforts, the gut takes a hit:

Blood flow gets redirected to working muscles
Digestion slows
Hormones shift
Repetitive impact (running, descending) adds mechanical stress

The result: nausea, cramping, vomiting, diarrhea—the usual suspects that ruin race days and big objectives alike.

The good news: the gut is trainable. Research is clear that repeated exposure to higher carbohydrate loads improves absorption, reduces symptoms, and increases carbohydrate availability. Mountain athletes train lungs and legs relentlessly, but often neglect training the system that keeps energy coming in.

3. The Carbs That Actually Matter

A. Multiple Transportable Carbohydrates (Glucose + Fructose)

Traditional gels and mixes (GU, Clif, Precision Fuel, etc.) use maltodextrin + fructose or glucose + fructose blends. This pairing matters because:

Glucose and maltodextrin use SGLT1 transporters
Fructose uses GLUT5

By splitting the load across two transport systems, the gut can absorb more total carbohydrate—exceeding the ~60 g/hr ceiling of glucose alone.
This is the backbone of most successful fueling strategies up to 120 g/hr.

Verdict:
Reliable, proven, and effective for most athletes.

B. Hydrogels (e.g., Maurten)

Hydrogels combine carbs with pectin and sodium alginate. In the acidic stomach, they form a gel-like matrix that:

Moves quickly through the stomach (faster gastric emptying)
Delivers carbs rapidly into the small intestine for absorption

Early research was inconclusive. Latest studies show clearer benefits:

Improved performance
Higher exogenous carbohydrate oxidation
Reduced GI distress compared to standard carb drinks

Performance gains over traditional solutions in studies: ~2% improvement—small on paper, meaningful on a race course.

Verdict:
Not magic, but very useful when pushing high carbohydrate loads (>90 g/hr) or when the stomach is prone to rebellion.

C. Highly Branched Cyclic Dextrin (HBCD / Cluster Dextrin)

Derived from corn starch, HBCD is built with:

High molecular weight
Low osmolality

This combination leads to:

Faster gastric emptying
Lower perceived GI strain
Smoother, sustained energy release

A single HBCD molecule contains 60–70 glucose units, far larger than maltodextrin’s typical 3–20-unit chains. Skratch’s Super High Carb Mix is a well-known example.

Verdict:
A strong option for long, steady efforts where digestive comfort and consistent energy release matter.

4. So What’s the Move?

Each category has its place. The “best” fuel is the one you can absorb at the rate your effort demands—without GI blowback.

Key Takeaways:

You need carbohydrates—more than most athletes think.
Modern endurance output requires 90–120 g/hr for optimal performance.
Your gut is a trainable system.
Don’t test fueling on race day. Train it like you train everything else.
Multiple transportable carbs, hydrogels, and HBCD each have strengths.
- Glucose + fructose: reliable and well-studied
- Hydrogels: better tolerance at high intake
- HBCD: smooth, steady fueling with low GI stress
There is no universal solution—only what works under your load, your terrain, and your physiology.
Field test your fueling the same way you test boots, ropes, or layering systems.
When in doubt: simplicity over gimmicks.
Evidence > advertising.

If you'd like, I can also produce:

A shorter, social-media-ready version for Blackthorn Strong
A slide deck summary for workshops or athlete briefings
A coach's handout with practical fueling protocols and gut-training guidelines

Fueling for Endurance: A No-Nonsense Breakdown for Mountain Athletes