The Gel Bible: What Every Ultra Runner Needs to Know About Race-Day Fuel

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You're 40 miles into a 100-miler. The trail is dark. Your legs are hollow. And somewhere in your pack, there's a gel you've been staring at for the last three miles, talking yourself into taking.

This is ultra running. And gels — like it or not — are part of the deal.

But not all gels are created equal. And not all athletes know what they're actually eating. If you've ever grabbed a packet off a shelf because the flavor sounded good, this post is for you. Let's break down the science, the categories, and why a guy who just set a course record at Leadville was eating one gel every 20 minutes for 15 hours straight.

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First: Why Gels at All?

Your body stores roughly 1,500–2,000 calories of glycogen — enough to fuel about 90 minutes to two hours of hard effort before the tank empties. After that, performance crumbles. This is what athletes call "the bonk," and in a race that takes 15, 20, or 30+ hours, hitting it even once can end your day.

Gels exist to solve this problem: fast-absorbing, portable carbohydrates that bypass the complexity of whole food digestion and get energy into your blood quickly. But the mechanics of how that happens — and why some gels work better than others — depends entirely on what's inside the packet.

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The Carbohydrate Hierarchy: What's Actually in Your Gel

Glucose & Maltodextrin (The Workhorses)

Glucose is your body's primary fuel. Every carbohydrate you eat eventually becomes glucose. The question is how fast.

Maltodextrin is a chain of glucose molecules — typically 3 to 17 units long — that digests faster than free glucose because it passes through the stomach rapidly and releases energy quickly in the small intestine. It's essentially pre-processed glucose, and it's the backbone of nearly every mainstream gel on the market. It also has a low osmolality (fewer molecules per unit of volume), which means it doesn't pull water into your gut the way simple sugars do, reducing GI distress.

The limitation: your intestines can only absorb glucose through one transport pathway (SGLT1). Max throughput sits around 60 grams per hour. Push past that with glucose alone and the excess sits in your gut, ferments, and causes the kind of stomach problems that end races.

Fructose (The Dual-Channel Unlock)

Here's where things get interesting. Fructose is absorbed through a completely separate intestinal transporter (GLUT5) — meaning it doesn't compete with glucose for absorption. Pairing fructose with maltodextrin opens a second lane, allowing total carbohydrate absorption to exceed 60g/hr and approach 90–120g or higher in trained athletes.

The optimal research-backed ratio is somewhere between 2:1 (glucose:fructose) and 1:0.8, with newer studies leaning toward the higher fructose end for maximum absorption efficiency. This dual-source approach is now the standard for high-performance endurance gels.

Too much fructose on its own, though, is a problem. It absorbs more slowly and can cause gut distress when overdone. The ratio matters.

Hydrogel Technology (Maurten's Innovation)

Maurten popularized a different delivery system entirely. Their gels use a combination of sodium alginate and pectin — seaweed and fruit-based fibers — that remain liquid in the packet but form a gel when they hit the acid environment of your stomach. This hydrogel structure slows the rate at which carbohydrates empty from the stomach into the small intestine, reducing the traffic jam effect and allowing for smoother, steadier absorption with minimal GI distress. It's particularly useful at high intensities when blood flow to the gut is reduced.

Cluster Dextrin & Slow-Release Carbs (The Long Burn)

Cluster Dextrin (Highly Branched Cyclic Dextrin, or HBCD) is a modified form of amylopectin with a more complex, branched structure. It breaks down slowly and steadily — think of it as the slow-release version of maltodextrin. It has very low osmolality and is gentle on the stomach, making it appealing for athletes who struggle with GI issues. The tradeoff is that it doesn't deliver the same rapid energy spike as maltodextrin. It's better suited to lower-intensity efforts or as a sustained background fuel.

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The Major Gel Categories (and When to Use Them)

Traditional Maltodextrin Gels

Examples: GU Energy, Clif Shot, Koda

These are the originals. Simple maltodextrin base, small amount of fructose, sometimes with amino acids, electrolytes, and caffeine. GU delivers about 21–22g of carbs per packet. They're proven, widely available at aid stations, and easy to find on race courses. The carb load per packet is lower, which means you need to take them more frequently or supplement with a carb-rich drink mix to hit higher hourly targets.

Best for: athletes fueling in the 60g/hr range, those with sensitive stomachs who want a well-tested product.

Dual-Source Super Gels

Examples: Science in Sport Beta Fuel, Neversecond C30, Carbs Fuel

This is the new generation. These gels deliver 30–50g of carbs per packet using an optimized maltodextrin-to-fructose ratio (typically 1:0.8 to 2:1). The higher carb density per packet means you can carry fewer of them and still hit aggressive hourly targets. SiS Beta Fuel specifically delivers 40g per packet in a 1:0.8 ratio, which supports the 80–120g/hr intake range that elite athletes are now training toward.

Best for: athletes gut-trained for high carb intake, longer efforts where every gram of carried weight matters, anyone targeting 90g+/hr.

Hydrogel Gels

Examples: Maurten 100, Maurten 160

Maurten's hydrogel technology sets it apart from the rest of the market. Flavor-neutral (borderline tasteless), no artificial ingredients, and designed to pass through the gut with minimal friction. The 100 delivers 25g of carbs; the 160 delivers 40g. The texture is thick and almost jelly-like, which some athletes love and others hate. At $4–5 per packet, the cost adds up fast in a 100-miler.

Best for: athletes with sensitive stomachs, high-intensity segments where gut blood flow is compromised, anyone who struggles with flavor fatigue.

Natural / Real Food Gels

Examples: Huma Chia Energy Gel, Spring Energy

These lean into whole-food ingredients — chia seeds, rice, fruit — rather than processed carbohydrate chains. Huma uses brown rice syrup and cane sugar to create a natural 2:1 glucose-to-fructose profile. Spring Energy's Awesome Sauce is literally real food blended into a pouch. These tend to have lower carb density and slower digestion, but they're psychologically satisfying in a way that synthetic gels aren't — especially deep in a long race when the idea of another maltodextrin packet makes your stomach turn.

Best for: athletes who prefer whole food sources, lower-intensity efforts, or as a palate reset mid-race alongside traditional gels.

Caffeinated Gels

Examples: GU Roctane (35mg), Maurten with caffeine (100mg), Precision Fuel PF 30 Caffeine (100mg)

Caffeine is one of the most well-researched ergogenic aids in sport — it reduces perceived exertion, delays fatigue, and improves focus. Most gel brands offer caffeinated versions ranging from 25mg to 100mg per packet. The research is consistent: caffeine works. The strategy is the delivery. Most athletes use caffeine strategically — not every gel, but timed around hard climbs, the middle-of-the-night grind, or the final push to the finish.

The 100mg options (Maurten, Precision) are potent. Combined with caffeine from a drink mix, it's easy to overcaffeinate without realizing it. Track your total caffeine intake across all sources.

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Gel Comparison at a Glance

|Gel |Type |Carbs/Packet|Carb Sources |Sodium|Caffeine Option|Best For |
|--------------------|---------------------------|------------|---------------------------------------|------|---------------|-----------------------------------------|
|GU Energy |Traditional |21g |Maltodextrin + fructose |55mg |Yes (35mg) |General use, aid station staple |
|GU Roctane |Traditional + amino acids |21g |Maltodextrin + fructose |125mg |Yes (35mg) |Long efforts, high-sodium needs |
|Clif Shot |Traditional |24g |Maltodextrin + fructose |50mg |Yes (25–50mg) |Budget-friendly, widely available |
|Maurten 100 |Hydrogel |25g |Glucose + fructose |55mg |No |Sensitive stomachs, high intensity |
|Maurten 160 |Hydrogel |40g |Glucose + fructose |85mg |No |High carb + sensitive stomachs |
|SiS Beta Fuel |Dual-source super gel |40g |Maltodextrin + fructose (1:0.8) |82mg |Yes (100mg) |80–120g/hr targets, elite fueling |
|Precision Fuel PF 30|Dual-source |30g |Maltodextrin + fructose |30mg |Yes (100mg) |High-carb strategy, low sodium preference|
|Neversecond C30 |Isotonic dual-source |30g |Maltodextrin + fructose (2:1) |200mg |Yes (75mg) |High sodium needs, no-water consumption |
|Carbs Fuel |Dual-source super gel |50g |Maltodextrin + sucrose + fructose (2:1)|105mg |No |Maximum carb density, value |
|Huma Chia |Natural / real food |23g |Brown rice syrup + cane sugar (2:1) |50mg |Yes (25mg) |Whole food preference, palate reset |
|Spring Energy |Real food |18–22g |Real food blend (varies) |Varies|No |Late-race palatability, real food feel |
|Koda |Traditional (fructose-free)|25g |Maltodextrin only |75mg |Yes (80mg) |Fructose-sensitive athletes |

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What David Roche Did at Leadville — and Why It Matters

David Roche won the 2024 Leadville 100 in 15:12:30, setting a new course record. After the race, he published a detailed breakdown of his fueling plan — and it's become one of the most discussed nutrition case studies in ultrarunning.

Roche leaned hard into carbohydrates, consuming a staggering 152.5 grams per hour for the first ten hours. That meant a Science in Sport Beta Fuel gel every twenty minutes, with every sixth gel swapped for a 100mg caffeine hit.

Read that again. 152.5 grams of carbs per hour. That's roughly four to five standard GU packets worth of carbohydrate, every single hour, for ten hours.

In his bottles, he combined Skratch concentrated mix with Precision electrolytes, targeting about 24–36 ounces of fluid per hour in the early miles. "Whatever ultra powers I have come from carbs," he wrote, reflecting on the lessons carried over from Western States, where a fueling misstep led to what he described as a "mental shart visible from space."

The plan wasn't static. By the final stretch, his intake dropped to around 100 grams of carbs per hour, with hydration scaled back to 12–16 ounces. Roche believes the body's sweat rate naturally declines late in ultras, and that many athletes' stomach problems come from drinking as aggressively in hour 20 as they did in hour two.

He also did something unconventional before the gun: ninety minutes before the start, he took 4.5 grams of sodium bicarbonate — a supplement long associated with sprinters and middle-distance runners for buffering lactate. It's rarely seen in ultras, but Roche pointed to a 2022 case study on Kilian Jornet at UTMB that documented lactate spikes more typical of track racing.

What Roche's approach illustrates is the broader shift happening at the front of the field: where 60–90 grams of carbs per hour was once considered ambitious, elite athletes are now pushing into the 100–150g range, training their guts over months and years to handle the load.

This is the key insight. Roche didn't show up at Leadville and eat 150g of carbs an hour by accident. His gut was trained for it — methodically, over the course of a full training cycle.

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The Gut Is a Trainable Organ

This can't be overstated enough. Your intestines have a carbohydrate absorption ceiling, but that ceiling is not fixed. Research and elite athlete experience both show that consistent, high-carbohydrate fueling during training — actually practicing your race-day nutrition — gradually increases the gut's capacity to absorb and tolerate carbohydrates under exertion.

The athletes who can run 100 miles on 120g of carbs per hour didn't start there. They built up to it over months, the same way you build your long run mileage. Practice your nutrition on your long training days. Don't save your gel strategy for race day.

One of the most common mistakes at high-altitude races is going out too hard. Even at the pace needed to contend, athletes must still work at an intensity where the gut can receive blood flow and absorb nutrients. If the pace is too hard, no amount of carbohydrates will save you — the gut simply can't absorb them, and overcorrecting with a flood of gels will only create GI distress.

This is exactly why HARDN builds a dedicated Gut Training program directly into your training block. Starting at 30g of carbs per hour in the foundation phase and progressing systematically to your 90–120g race target, HARDN assigns specific long runs for gut practice, lets you log food type, quantity, and GI symptoms, and tracks your glucose-to-fructose ratio tolerance over time. The progression is rooted in SGLT1 transporter science — the same biology that explains why David Roche could stomach 152g of carbs per hour when most athletes would be doubled over.

The four-phase structure:

- Foundation — 30–40g/hr, Weeks 1–4. Establish your baseline tolerance.
- Build — 50–60g/hr, Weeks 5–8. Introduce dual-transport fueling.
- Peak — 70–80g/hr, Weeks 9–12. Race-pace gut stress testing.
- Race-Ready — 90–120g/hr, Weeks 13–16. Full race simulation fueling.

No other app does this. [See the Gut Training feature on HARDN →](https://hardn.app/features/nutrition)

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Building Your Gel Strategy

Here's a framework for thinking about your race-day gel plan:

Step 1 — Set your hourly carb target. If you're new to structured fueling, start at 60g/hr and build from there. Trained athletes who have practiced high-carb fueling can target 90–120g/hr. Be honest about where you actually are.

Step 2 — Choose a gel that matches your target. If you're shooting for 60g/hr, traditional gels work fine. If you're targeting 90g+/hr, you need dual-source gels with meaningful fructose content — or a combination of gels and a carb-rich drink mix.

Step 3 — Plan your caffeine. Decide when you want it, how much, and track it across all sources (gels, drink mix, coke at aid stations). A 100mg caffeinated gel plus a caffeine-spiked drink mix can deliver more than you expect.

Step 4 — Build in variety. Flavor fatigue is real and it will hit you. Rotate flavors, mix in real food at aid stations, and keep options in your drop bags for when the gel you loved at mile 20 sounds intolerable at mile 70.

Step 5 — Practice it in training. Every long run is an opportunity to rehearse. The worst time to discover that a gel destroys your stomach is at mile 40 of your A-race.

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The Bottom Line

The science of gel nutrition has evolved dramatically. The athletes pushing the limits — like David Roche at Leadville — are eating more carbohydrates per hour than most recreational runners consume in a full training day. They can do this because they've trained their guts, dialed in the right carbohydrate combinations, and treated nutrition as a discipline with the same seriousness as mileage.

You don't have to eat 152.5 grams of carbs an hour. But you do have to eat — consistently, deliberately, and starting earlier than you think you need to. The bonk doesn't announce itself. It just arrives, quietly, about 30 miles too late to fix.

So practice your fueling. Train your gut. Find what works for you. And when you're deep in the pain cave at mile 80, you'll be glad you did.

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Want a training plan that builds race-day nutrition practice into your long runs from day one? HARDN can help you structure it — including automated reminders for when to fuel during your workouts. [Start your free trial at hardn.app](https://hardn.app)