Three hours into a six-hour fondo, a rider reaches for his ninth gel. He picked this brand for one reason: the label reads glucose and maltodextrin, nothing else. No fructose. He read somewhere, years ago, that fructose is the sugar endurance athletes are supposed to dodge — bad for the gut, bad for fat storage, bad news generally. He's been strict about it for three seasons, checking labels at the bike shop the way other people check nutrition panels at the supermarket.

At the base of the final climb, his legs go dead anyway. Not from an empty tank — he's fuelled on schedule, a gel every 30 minutes, exactly as planned, exactly as every article told him to. Something else is capping him. He is swallowing the carbohydrate. He isn't getting enough of it into his bloodstream fast enough to matter.

Section 01The ceiling nobody warns you about

Here's the part most fuelling advice leaves out. Your gut doesn't absorb carbohydrate through one generic, all-purpose door. Glucose — and the maltodextrin that most gels, chews, and drink mixes are built on, which digests down to glucose — crosses your intestinal wall through a specific transporter protein called SGLT1. That transporter has a ceiling. Push more glucose down your throat past roughly a gram a minute — somewhere in the region of 60 g/h — and the extra doesn't get absorbed any faster just because you swallowed it. It sits in your gut: undigested cargo, doing nothing for your legs and doing plenty for your risk of a mid-climb GI complaint.

Fructose crosses using a completely different transporter, GLUT5. It has its own ceiling, lower on its own than SGLT1's — but the two don't compete with each other for the same doorway. They stack. Take your carbohydrate in as a glucose-only drink and you're limited to whatever one transporter can move. Take the same total carbohydrate as a glucose-plus-fructose blend, and you've opened a second, independent pathway alongside the first.

Section 02What the actual trial found

This isn't theory or forum lore. Currell and Jeukendrup put it to a direct, controlled test in trained cyclists: 120 minutes of steady-state riding at 55% of max power, followed by a fixed-effort time trial (Currell & Jeukendrup, 2008). Two of the three drinks in the trial delivered carbohydrate at an identical rate — 1.8 g per minute, roughly 108 g/h. The only variable that changed between them was the sugar mix. One drink was glucose only. The other was glucose and fructose blended in a 2:1 ratio (Currell & Jeukendrup, 2008).

Same total carbohydrate delivered. Same ingestion rate. Same riders, same pre-loading protocol, same effort. The glucose-plus-fructose drink got the time trial done 8% faster than the glucose-only drink, at matched intake (Currell & Jeukendrup, 2008).

Worth being straight about the fine print, because the date matters here: this trial is from 2008, which predates the broader "push carbohydrate intake as high as your gut will tolerate" shift that reshaped mainstream sports-nutrition guidance from roughly 2010 onward. It's an older paper by the standard of a fast-moving field. It's also the foundational demonstration of the effect — the first study to show a clear, direct performance benefit from blending sugars rather than just modelling oxidation rates — and it has been replicated enough since that the practice is now a standard part of applied sports nutrition, not a fringe idea chasing a single result. Old, in this case, means "where the current default came from," not "outdated."

Section 03Why blending sugars works

The oxidation data explains why the performance gap exists, and it comes from the same research group running a cleaner, more isolated version of the comparison. Trained cyclists rode for 120 minutes at 50% of max power under three separate carbohydrate conditions. Two of those conditions delivered exactly 1.8 g/min of total carbohydrate — matched intake again — with the only difference being whether that 1.8 g/min arrived as glucose alone, or as 1.2 g/min glucose plus 0.6 g/min fructose (Jentjens et al., 2004).

Same total intake, both trials. The measured difference was in how much of what riders drank their bodies could actually put to work. Peak exogenous carbohydrate oxidation — the rate at which carbohydrate from the drink, specifically, was being burned as fuel — was 0.83 g/min on glucose alone, versus 1.26 g/min on the glucose-fructose blend, a jump the researchers reported as roughly 55% higher peak oxidation with the blend, from an otherwise identical carbohydrate intake (Jentjens et al., 2004).

That's the entire mechanism in one comparison. Nothing changed about how much carbohydrate the rider swallowed. What changed is how much of it the gut could get into circulation and out to working muscle, because the load was split across two transport systems instead of forced through one.

Section 04Why the fructose fear took hold in the first place

The distrust of fructose isn't irrational — it's just borrowed from the wrong context. Fructose consumed in excess as added sugar, sedentary, day after day, has a well-documented relationship with fat storage and metabolic strain; that's real, and it's where most of the "fructose is bad" messaging that reaches cyclists actually originates. None of that transfers cleanly to fructose consumed during hard exercise, when your liver and working muscles are burning through carbohydrate as fast as you can deliver it. The metabolic context is close to the opposite of a sedentary afternoon snack. Treating "has fructose" as a blanket red flag on a gel label means avoiding the one ingredient that helps you extract more out of everything else in the packet.

Section 05How to actually fuel with it

1. Pick your ratio

   Aim for roughly 0.8:1 to 2:1 glucose-to-fructose by weight. Currell and Jeukendrup's 2:1 blend is a reasonable, well-tested starting point (Currell & Jeukendrup, 2008).
2. Match the dose to the ride

   This only matters once you're fuelling at a meaningful rate for a meaningful duration. Rides under roughly 90 minutes rarely need multiple-transportable carbohydrate at all.
3. Read the ingredient panel, not the marketing

   Look for "fructose" or "fruit sugar" alongside glucose, maltodextrin, or dextrose. Plenty of popular gels and drink mixes are still glucose-only, whatever the front of the packet implies.
4. Gut-train it before race day

   A new blend and a new volume both ask something of your digestive system. Practice the exact product, at the exact dose, on training rides well before you lean on it in a race.
5. Don't oversolve a short effort

   Under about 2 hours, a single sugar source is fine on its own. Save the blend for the rides and races where the extra grams-per-hour are actually going to get used.

Section 06Where riders still get this wrong

The most common misread is stopping at "it's a bigger number, so more is always better." The benefit specifically shows up once total carbohydrate intake is already pushing past what a single sugar source can absorb — broadly, efforts approaching or exceeding 60–90 g/h. Below that rate, a single sugar source does the job fine, and a blend adds complexity without adding much real upside.

The second misread is skipping the gut training because the science checks out. A multiple-transportable-carbohydrate blend still asks a lot of a digestive system that hasn't practiced the volume. Riders who try a new high-carbohydrate blend for the first time on race day are swapping one limiter — an absorption ceiling — for another: a gut that hasn't rehearsed the load and complains about it at the worst possible moment.

The third is assuming the ratio is fixed and sacred. The 2:1 glucose-to-fructose split is a well-tested default, not a magic number — tolerance and preference vary rider to rider, and the broader literature supports a workable range rather than one exact figure.

Section 07Applying it with HELIOS

THRIVE's fuel plan already sets the carbohydrate target a given ride calls for — periodised off your actual training load, not a generic number pulled from a chart. Rest days get less. The big weekend session gets more. That's the FUEL layer of the plan doing its job on your real data.

What this research changes is what you do with the target once the app has set it. A 90+ g/h number delivered as a glucose-only product is asking your gut to do something the physiology doesn't fully support. The same number, hit with a glucose-fructose blend, is asking for something your gut can actually deliver. The app tells you how much carbohydrate the ride needs. This is about how you get it there.

Section 08Bottom line

A single carbohydrate source has a real, physiological absorption ceiling — roughly 60 g/h — no matter which gel brand you're loyal to. Blending in fructose, the sugar most cyclists have been quietly avoiding, opens a second, independent absorption pathway and lifts how much carbohydrate your body can actually use, not just how much you can force down. Currell and Jeukendrup's cyclists rode a time trial 8% faster on the blend at an identical carbohydrate intake, and the oxidation data explains exactly why: roughly half again as much of the ingested sugar actually made it into circulation. Nobody had to eat more to get there. They just had to stop avoiding half of the delivery system.