Fuelled or Fooled? Three Mechanisms that Might Explain Ultra-High Carbohydrate Fuelling

Back in 2023 I wrote two blogs on the 120 grams per hour question. The conclusion both times was the same. The evidence did not support it, the extra carbohydrate was not sparing glycogen, and for most triathletes it was not worth the gut risk.

That position has not changed. What has changed is the question.

Because the athletes at the front keep doing it. Aggressive fuelling has become standard across marathon, professional cycling and long course triathlon, and it has been associated with a run of record breaking performances. Worth being precise about that word. Associated. The trend is supported largely by anecdotal reports rather than by evidence, and nobody has shown these intakes caused those results. But it leaves two possibilities. Either those athletes are winning despite their nutrition strategy, or something is happening that our current models do not capture.

That is what we set out to explore in a new Current Opinion paper in Sports Medicine, written with Paul Booth, Taj Krieger and Ed Maunder (1). We define ultra-high carbohydrate, or U-HC, as anything above 90 grams per hour. The paper does two things. It shows why the usual explanations for U-HC do not survive contact with the data, and then it proposes three mechanisms that might. All three are speculative. None of them have been tested properly in the real world. But they are worth understanding, because they are the most plausible reasons U-HC could work, and they point to where the research needs to go next.

First, the usual explanations do not hold up

The standard rationale for taking carbohydrate during exercise is glycogen sparing. Burn more from the outside, burn less from the inside, arrive at the final hour with fuel left in the tank.

That rationale needs splitting in two, because muscle and liver are not the same story and they get collapsed together constantly.

Muscle glycogen sparing is the weaker of the two claims. A recent meta-analysis found that the muscle glycogen sparing effect of carbohydrate ingestion during exercise is small, and not dose dependent, at least across the range of intakes that have been studied (5). Not small above 90 grams per hour. Small, full stop.

The liver is where the real effect sits. Carbohydrate ingestion suppresses hepatic glucose output (6) and attenuates liver glycogen depletion during prolonged exercise (7), at moderate intake rates. Gonzalez and colleagues said it plainly in their title: ingesting glucose or sucrose prevents liver but not muscle glycogen depletion. On top of that, the performance benefit of carbohydrate during prolonged exercise now looks to be driven mainly by keeping blood glucose stable and avoiding hypoglycaemia, rather than by any dose dependent rise in oxidation (12). Blood glucose can be stabilised at moderate intakes.

So carbohydrate during exercise absolutely works. It just does not work the way most athletes think it does, and the part that does work is largely finished by the time you reach 90 grams an hour.

The ultra-high data says the same. Podlogar and colleagues gave athletes 90 and 120 grams per hour of a glucose-fructose mix during three hours of cycling (2). The higher dose did increase exogenous oxidation. It did not reduce endogenous oxidation at all. It simply raised total carbohydrate oxidation. Ravikanti and colleagues found the same in elite marathon runners, with no difference in endogenous oxidation across 60, 90 and 120 grams per hour, despite those runners hitting some of the highest exogenous oxidation rates ever recorded (3). King and colleagues went further and showed that 112.5 grams per hour actually produced greater muscle glycogen oxidation than lower doses (4).

Efficiency drops too. In Podlogar's study, oxidation efficiency fell from around 86 percent at 90 grams per hour to around 76 percent at 120 (2). That leaves roughly 30 grams an hour unoxidised and sitting somewhere it probably should not be.

Performance data tells the same story. Smith and colleagues had 51 cyclists complete trials across ingestion rates from 10 to 120 grams per hour, and their model predicted benefits up to around 78 grams per hour before diminishing returns set in (8). The relationship between intake and time trial performance was flat between roughly 30 and 90 grams per hour. Fell and colleagues showed a clear dose response across 0, 45 and 90 grams per hour, but nobody has extended that curve into U-HC territory and found it keeps climbing (9).

So if the extra carbohydrate is not sparing glycogen, and it is not reliably improving performance in the lab, what could it be doing?

Mechanism one: carbohydrate is a cheaper fuel per litre of oxygen

This is the one I find most interesting, because it flips a paradigm that endurance sport has held for thirty years.

Burning carbohydrate yields about 5.05 kcal per litre of oxygen consumed. Burning fat yields about 4.69 kcal per litre (10). Carbohydrate is the more oxygen-efficient fuel. If you can shift your metabolism further toward carbohydrate, you can produce the same power for slightly less oxygen.

For most of my career the goal was the opposite. Push the crossover point to the right, make the athlete better at using fat, delay the moment carbohydrate takes over. That made sense when substrate availability was the limiter. But if modern fuelling has largely removed the availability problem, the remaining gains might come from deliberately shifting the crossover point to the left and running the engine on the cheaper fuel, even at lower intensities.

There is supporting logic. We know the reverse works in reverse. Athletes adapted to low carbohydrate, high fat diets consistently show higher oxygen consumption at the same absolute workload (13). Noakes and colleagues describe a linear suppression of fat oxidation as carbohydrate ingestion increases, across a broad range of intake rates (12). When we ran a correlation across the studies in our Table 1, we found a large negative relationship between carbohydrate intake and fat oxidation (r = -0.58, 90 percent CI -0.73 to -0.37). More carbohydrate in, more of your energy coming from carbohydrate. Ravikanti's elite marathoners showed a 3.6 percent improvement in running economy at 120 grams per hour compared with 60 (3). Lukasiewicz and colleagues modelled the sub-two hour marathon and concluded it would require above 90 grams per hour for all runners, and up to 130 grams per hour for some female runners (11).

Now the honest part.

We pooled the oxygen consumption data from 16 studies comparing the highest and lowest carbohydrate intakes. The overall effect was 0.05 standardised mean difference, with a 95 percent confidence interval of -0.09 to 0.18. That is trivial and non-significant. There is no clear evidence that carbohydrate intake consistently changes VO2, at least across the intake ranges that have actually been studied, and most of those studies sat well below true U-HC rates.

There is a second problem. An apparent improvement in economy measured as absolute VO2 might just reflect the different oxygen cost of the fuel being burned, rather than any real improvement in mechanical or physiological efficiency. Those are not the same thing, and telling them apart requires more careful work than most studies have done.

And a third. The idea that U-HC intake deliberately shifts the crossover point leftward is speculative. The crossover point moves around a lot between people, and it is strongly influenced by habitual diet and training state, so a shift is often not unique to acute high carbohydrate availability. Whether any of this translates into a performance advantage for a lesser trained athlete, someone with a VO2max under 60 mL/kg/min whose absolute oxygen consumption is far lower to begin with, has not been established at all.

Mechanism two: fructose, lactate, and the trained athlete

The second idea involves lactate, and it may explain why elite athletes reach oxidation rates the rest of us do not.

Modern U-HC products lean heavily on fructose. Fructose taken up by the liver is rapidly converted to glucose and lactate, which raises the amount of lactate circulating in the blood and available to be burned (15). Lactate is no longer thought of as a waste product. It is a genuine oxidative fuel, shuttled to highly aerobic tissue like type I muscle fibres and cardiac muscle and used there (16). So a high fructose intake may not just be a way of getting more sugar across the gut wall. It may be a way of delivering a fuel that certain tissues are very good at using.

Here is the constraint. Fructose absorption across the gut runs through GLUT5, and that transporter is generally thought to saturate somewhere between 30 and 60 grams per hour (17). At U-HC rates with a 1:0.8 or 1:1 glucose to fructose ratio, fructose intake is already pushing against that ceiling. Anything beyond it never reaches the bloodstream, so it cannot contribute to liver lactate production or to the systemic lactate pool. It just sits in the gut.

Which means this mechanism is most relevant at intake rates where fructose absorption is still intact, and its contribution at genuinely ultra-high rates is uncertain.

The elite angle is where it gets interesting. Highly trained athletes express more MCT1 and MCT4, the transporters that move lactate in and out of cells, and they carry greater mitochondrial volume than untrained people (18). A better lactate shuttle means a greater capacity to take up and oxidise that fructose-derived lactate. It is plausible that the combination of high fructose delivery and an unusually good lactate handling system produces exogenous oxidation rates in elite athletes that simply do not appear in the rest of us. Plausible, but not demonstrated.

Mechanism three: the brain does not need the carbohydrate to be oxidised

The third mechanism does not require the carbohydrate to be metabolised at all.

We have known for years that swilling a carbohydrate drink around the mouth and spitting it out can improve performance (19). The effect is mediated through oral carbohydrate receptors, particularly the T1R2 and T1R3 sweet taste receptor complex, which sends afferent signals to reward and motor control centres in the brain. It works independently of whether any of that carbohydrate ever reaches a muscle.

The usual caveat is that mouth rinse effects are clearest when athletes are fasted or glycogen depleted, and in shorter or higher intensity efforts where metabolism is unlikely to be the limiter. In long events, the ergogenic effect of carbohydrate is mostly about maintaining blood glucose, with central effects playing a supporting role (12).

Our proposal is that continuous high carbohydrate feeding may keep those oral and gut receptors stimulated for the entire duration of an event, and that this sustained signalling could keep perceived exertion lower for longer. Perceived exertion is consistently reduced by carbohydrate ingestion. Learsi and colleagues reported significantly lower RPE across 105 minutes of steady exercise with carbohydrate, and those athletes then produced significantly faster 10 km time trials (20). Steiner and colleagues found a tendency toward reduced RPE during incremental cycling, though it did not reach statistical significance (p = 0.07), alongside a modest 2.1 percent increase in work output at a fixed RPE in a subsequent constant load trial (21).

If U-HC keeps that signal running harder and for longer, it could plausibly influence pacing and motivation over an Ironman or a Grand Tour stage. Whether more carbohydrate produces proportionally more of this effect is completely unknown.

Why this still is not a green light

Three mechanisms, all plausible, all untested in the field, and probably not mutually exclusive. If they are real, they may work together rather than separately.

But none of that changes what I would tell an age group athlete tomorrow.

The single biggest issue is individual variation. Hearris and colleagues gave athletes a standardised 120 grams per hour and measured oxidation rates ranging from about 1.3 to 1.9 grams per minute (14). At the top of that range, an athlete is oxidising around 114 grams per hour, so almost all of it is being used. At the bottom, that athlete is oxidising about 78 grams per hour, leaving a substantial share of what they swallowed unoxidised and unlikely to do anything useful. Same protocol, completely different outcome. Body size matters here too, with larger athletes showing greater absolute oxidation rates (22).

So a 120 gram per hour strategy may be fully usable by a small number of people at the very top of the oxidation distribution. For most athletes it exceeds their capacity, and the surplus becomes a gut problem rather than a fuel source.

Podlogar and colleagues recently showed what individualising this looks like. By setting intake based on each athlete's measured glucose oxidation rate, athletes achieved similar carbohydrate utilisation while consuming 28 percent less, with lower perceived exertion and less gastrointestinal fullness (23). Better outcome, less product, less discomfort. The catch is that measuring individual oxidation rates needs a lab, and that is out of reach for almost everyone reading this.

Two more things worth flagging. Gut training is often sold as the way to unlock these rates, but the evidence that it increases exogenous oxidation capacity is thin. Cox and colleagues showed an increase, most likely through upregulated intestinal SGLT1 transporters, but that finding has not been consistently replicated (25). Gut training seems to improve comfort and reduce malabsorption more than it raises the ceiling.

And there is a long term health question that nobody is really asking. Prins and colleagues found that moderately elevated chronic carbohydrate intake pushed around 30 percent of athletes into pre-diabetic fasting glucose ranges above 100 mg/dL within 31 days, an effect that reversed when intake came down (24). U-HC race fuelling and chronic high carbohydrate intake are technically different things, but in practice they are hard to separate, because gut training programmes require habitually high carbohydrate availability. If you are an age group athlete over-fuelling every session because that is what the pros do, this is worth thinking about.

Where I have landed

If you want a practical starting point, the averages in our Table 1 are useful. Glucose to fructose ratios of 1:0.8 to 1:1 consistently produced the highest average exogenous oxidation rates, around 88 plus or minus 9 grams per hour. Traditional 2:1 blends averaged around 65 plus or minus 7 grams per hour. So the ratio matters more than most people realise, and getting it right may do more for you than pushing the total higher.

Then there is the gap that should bother triathletes most. No study to date has examined U-HC intake across the full duration of an event like an Ironman or an ultramarathon. Every finding above comes from protocols measured in hours, not in the eight to seventeen hours where fuelling actually decides races.

U-HC does hold promise in one area, and it is not race day. High carbohydrate availability may help day to day recovery, particularly during intensified training blocks or multistage racing where you have to turn around fast. That is a different physiological goal from acute race performance and should be treated as one.

For everything else, the evidence still points to 60 to 90 grams per hour for most athletes, dialled in during training at race intensity, with a ratio closer to 1:0.8 than 2:1.

The three mechanisms in this paper might turn out to be real. If they are, the athletes at the front are onto something the rest of the field cannot yet copy, because they have the physiology to make it work. Until somebody tests that properly, copying the intake without the physiology just gives you an expensive habit and an unhappy gut.

Endure on!

Key takeaways

1. Carbohydrate during exercise works mainly by protecting liver glycogen and keeping blood glucose stable, not by sparing muscle glycogen. Muscle sparing is small and not dose dependent at any intake. Going above 90 grams per hour adds nothing to either, and oxidation efficiency falls as intake climbs.
2. Three untested mechanisms could explain a real benefit: a shift toward the more oxygen-efficient fuel, fructose-derived lactate feeding a well developed shuttle system in trained athletes, and sustained central signalling that lowers perceived exertion.
3. Individual oxidation capacity varies from roughly 1.3 to 1.9 grams per minute at the same intake, so 120 grams per hour is fully usable by very few people.
4. Glucose to fructose ratios of 1:0.8 to 1:1 produce far better average oxidation than 2:1 blends. Fix the ratio before you raise the total.
5. U-HC looks more promising for recovery and multi-day racing than for acute race day performance in a single event.

References

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