The Science of Race Fueling: Why High-Carb Works and How to Get There

For decades, the standard recommendation was 30-60g of carbs per hour during endurance exercise. That number showed up in every coaching manual, every sports nutrition textbook, every “how to fuel your marathon” article online. And it wasn't wrong, exactly. It was a real physiological ceiling.

But it was a ceiling for glucose alone.

Your small intestine absorbs glucose through a transporter called SGLT1. That transporter maxes out at roughly 60g per hour no matter how much you eat. Dump 90g of pure glucose into your gut and you don't absorb 90g—you absorb 60g and the rest sits there causing bloating, nausea, and the kind of GI distress that turns mile 20 into a survival situation.

The breakthrough came when researchers discovered that fructose uses a completely different transporter—GLUT5. It's a separate doorway into the bloodstream. Glucose and fructose don't compete with each other. They absorb in parallel, through independent pathways, at the same time.

That single finding blew the ceiling off endurance fueling. Suddenly 90g per hour was achievable. Then 100. Then 120. The science didn't change what your body could do—it changed what we understood about how to deliver fuel to it.

I ran the 2025 NYC Marathon at roughly 90g carbs per hour using dual-source gels. That's 50% above what I would have been told to eat ten years ago. I finished in 3:04, never hit the wall, and had enough in the tank to close strong. That wasn't luck or genetics. It was the science working exactly as the research predicted.

I'm now training for sub-3:00 at the Copenhagen Marathon, pushing my intake to 100-105g/hr in long runs. Every tool on FuelCenter is built on this research. This article explains the science behind it.

How your gut actually absorbs carbs: the dual transporter model

Two transporters do most of the work.

SGLT1 (sodium-glucose co-transporter 1) sits in the lining of your small intestine and actively pumps glucose into your bloodstream. “Actively” means it requires energy—it uses a sodium gradient to pull glucose molecules across the intestinal wall, even against a concentration gradient. It's powerful, but it has a hard cap. Research on SGLT1 mechanics shows this transporter saturates at around 60g of glucose per hour. No amount of extra glucose will push past that limit.

GLUT5 (glucose transporter 5)handles fructose through a completely different mechanism—passive facilitated diffusion. It doesn't need sodium, doesn't require energy, and operates on its own independent pathway. A comprehensive review of intestinal sugar transporters confirms that GLUT5 functions alongside SGLT1 without competition.

This is the key insight: when you ingest glucose and fructose together, your intestine processes them simultaneously through separate channels. It's not one line—it's two lanes of traffic moving at the same time. The combined throughput reaches 90-120g per hour, a 50-100% increase over glucose alone.

The ratio matters.At 90g/hr total intake, a 2:1 glucose-to-fructose ratio works well because SGLT1 can handle its share (60g glucose) while GLUT5 handles the rest (30g fructose). But at higher intakes—120g/hr—you need more fructose in the mix. Hearris et al. (2022) established that a 1:0.8 maltodextrin-to-fructose ratio optimizes absorption at these higher rates. Podlogar et al. (2022) compared 120g/hr at 1:0.8 against 90g/hr at 2:1, demonstrating that the higher-intake strategy is viable when the ratio is dialed in.

This is why gel formulations matter. Maurten uses a 1:0.8 glucose-to-fructose ratio—optimized for high-intake athletes pushing 100g/hr or above. SiS Beta Fuel uses a similar formulation. GU and Clif Shot use a 2:1 ratio, which works well at moderate intakes (60-90g/hr) but becomes less efficient if you're trying to push past 100.

The practical takeaway: if your gel brand uses a single carb source, your ceiling is roughly 60g/hr no matter how disciplined your fueling is. Dual-source products unlock the higher ranges.

What the latest research says

The foundational work came from Asker Jeukendrup's lab between 2003 and 2014. His team proved that multiple transportable carbohydrates—glucose plus fructose—achieve 20-50% higher oxidation rates than glucose alone during prolonged exercise. These weren't small effects on small samples. The research was replicated across dozens of studies, across multiple exercise types, and the finding held consistently: dual-source carbs get burned for fuel faster and more completely than single-source.

That work laid the groundwork. The research since then has only strengthened the case.

A 2025 meta-analysis published in the Journal of Applied Physiology pooled 31 studies with 48 effect sizes comparing carbohydrate ingestion against placebo during prolonged exercise. The findings were unambiguous: carbohydrate supplementation improves endurance performance across all conditions tested. The effect size increases as exercise duration extends from 1 to 4 hours—exactly the window that matters for marathons and ultras. The longer you're out there, the more carbs matter.

A separate comprehensive review in Frontiers in Nutrition (April 2025) examined carbohydrate supplementation across the full race cycle—before, during, and after competition. The review addressed variables that earlier studies had ignored: individual GI differences, environmental conditions (heat significantly impairs gut absorption), interactions between carbs and other nutrients, and psychological factors. The takeaway: the science supports high-carb fueling, but execution depends on individual tolerance built through training.

One of the most striking recent papers is a 2023 modeling study in the Journal of Applied Physiology that estimated the exogenous carbohydrate needs for a sub-2-hour marathon. The numbers: 93 ± 26 g/hr for male runners and 108 ± 22 g/hr for female runners. Those aren't aspirational targets—they're the modeled physiological requirements. If you want to run at the absolute limit of human performance, you need to fuel at rates that would have been considered dangerous twenty years ago.

The current World Athletics nutrition guidelines (2024) recommend 60-90g/hr of multiple transportable carbohydrates as the standard for endurance events, with acknowledgment that trained athletes can safely push to 120g/hr. The ACSM's position is similar. The institutional consensus has caught up with the research.

The old 30-60g/hr ceiling wasn't wrong for its time. It was accurate for single-source glucose. But the science has moved on, and the guidelines have followed.

What the elites actually do

Eliud Kipchoge doesn't fuel at 60g per hour.

During his sub-2-hour marathon attempts and his Olympic and World Major performances, Kipchoge's fueling protocol targets 100+ grams of carbohydrates per hour. His primary fuel source is Maurten's hydrogel sports drink—delivered via bike-mounted bottles during the INEOS 1:59 Challenge and from tables during standard races—supplemented with Maurten Gel 100s. The formulation uses a 1:0.8 glucose-to-fructose ratio, optimized for maximum absorption at high intake rates.

Kipchoge isn't an outlier among elites. The top marathon runners in the world have moved to high-carb protocols almost universally. The 2023 modeling study estimated that sub-2-hour performance requires 93-108g/hr of exogenous carbs. That's not “nice to have”—it's the fuel requirement at that intensity and duration.

The gap between what elites do and what most recreational runners are told to do is striking. A first-time marathoner reading a generic training plan will see advice to “take a gel every 45 minutes”—which works out to roughly 30-40g/hr with a standard 22-25g gel. An elite is fueling at 2-3x that rate.

Pre-race carb loading is part of the equation too. World Athletics guidelines recommend 10-12g of carbs per kilogram of body weight per day for 36-48 hours before the race, then 1-4g/kg in the 3-4 hours before the start. For a 70kg (154 lb) runner, that's 700-840g of carbs per day in the loading phase—roughly 2,800-3,360 calories from carbs alone. The goal is maximizing muscle glycogen stores before you ever cross the start line.

Carb loading and in-race fueling work together. Higher glycogen stores at the start mean you have more onboard fuel. Higher in-race carb intake means you burn less glycogen per hour. The combination extends how long you can hold race pace before hitting the wall.

The science doesn't say every runner needs to fuel like Kipchoge. But it does say that 30-60g/hr is leaving performance on the table for anyone racing a marathon or longer.

Gut training: how to build tolerance

The science is clear: more carbs means better performance. But knowing you should eat 90g/hr and actually being able to eat 90g/hr without your stomach revolting are two different things.

This is where gut training comes in. And it's the piece most runners skip.

Your gut is adaptable. The transporter proteins that absorb carbs—SGLT1, GLUT5, GLUT2—aren't fixed in number. They respond to demand. Research on gut adaptation shows that a high-carbohydrate diet can double the density and activity of SGLT1 transporters in as little as 14 days. GLUT5 and GLUT2 also upregulate in response to increased carb intake. Your intestine literally builds more doorways when you give it more traffic.

The most cited gut training protocol comes from Hearris et al. (2022): 10 training sessions over two weeks, running at 60% VO2max for 60 minutes, ingesting 30g carbohydrate gels at 0, 20, and 40 minutes into each session (90g total per session). The result: significant reduction in GI symptoms and improved performance markers. Two weeks. Ten sessions. Measurable adaptation.

But most runners aren't starting from zero—they're trying to push from a moderate intake (40-60g/hr) to a high one (90-120g/hr). The progressive approach works better for that:

Weeks 1-2: Practice fueling at 40-60g/hr during easy long runs. Get your gut used to processing food while running. This is the baseline.

Weeks 3-4: Increase to 60-75g/hr. Start incorporating your race-day gel brand and timing. This is where you figure out what works for your stomach.

Weeks 5-8:Push to 75-90g/hr. This is the range where most runners doing a standard marathon (3:00-4:30) should land. Practice during workout sessions, not just easy runs—your gut behaves differently at higher intensities.

Weeks 8+:Experienced athletes with trained guts push to 100-120g/hr. This isn't necessary for everyone, but for runners targeting aggressive times or racing ultras, it's where the ceiling becomes a floor.

David and Megan Roche, whose 8-guideline fueling framework forms the basis of FuelCenter's calculator, put it directly: gut training is the most overlooked part of endurance performance. The sensation of fullness during fueling is itself trainable. Runners who feel like they “can't eat that much” during a run usually haven't practiced eating that much during a run. The gut adapts. You have to give it the stimulus.

Common mistakes that derail gut training

Trying race-day intake without building up. Jumping from 40g/hr in training to 90g/hr on race morning is asking for trouble. Your gut hasn't adapted, your transporter density hasn't increased, and you're layering race-day stress and adrenaline on top. Build progressively.

Testing new products on race day.Every gel has a different consistency, flavor, and carb composition. Maurten's hydrogel technology feels completely different from GU's syrup. SiS Beta Fuel is thicker than both. Your gut needs to practice with the specific product you'll race with. Flavor fatigue, texture aversion, and unexpected GI reactions all show up after 2+ hours—you want to discover these in training, not at mile 18.

Ignoring heat.Heat reduces gut blood flow (your body diverts blood to the skin for cooling), which impairs carb absorption. A perfectly trained gut at 55°F is an untrained gut at 85°F. If your race might be warm, practice fueling in warm conditions. Your thermal gut tolerance is a separate adaptation from your baseline gut tolerance.

How FuelCenter puts this into practice

The science is complex. The execution doesn't have to be.

FuelCenter's calculator is built on David and Megan Roche's 8-guideline framework, which translates the research above into a tiered system based on three variables: duration, intensity, and experience level.

The tiers range from 0g/hr (easy efforts under an hour—your glycogen stores handle it) up to 105-120g/hr (experienced high-carb athletes on race day). A first-time marathoner running 4:30 gets a different carb target than a sub-3 runner with two years of gut training. Both targets are grounded in the same science, adjusted for where you actually are.

Here's what the calculator does with this: you pick your gel brand (10 options, each with different carb counts and glucose-to-fructose ratios), enter your expected duration and intensity, optionally add drink mix, and set your experience level. It generates a minute-by-minute fueling timeline with exact gel counts and timing. No mental math. No iPhone calculator. No scribbling on your hand before a long run.

The race strategy builder goes further. Pick one of 14 races, enter your goal time, and get a mile-by-mile plan where the pace splits are adjusted for the actual course elevation using the Minetti energy cost formula, and the gel timing is layered on top. Your Boston splits are slower on the Newton Hills. Your NYC splits account for the Queensboro Bridge climb. The fueling plan is timed to the actual course, not a flat line across 26.2 miles.

Every target the calculator generates comes back to the dual transporter model, the progressive gut training approach, and the Roche framework. The science is baked into the tool so you can focus on the running.

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